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Scientists who study a family of green algae that includes unicellular Chlamydomonas and multicellular Volvox are beginning to find answers to this question. Volvox is a spherical alga with about 2,000 cells and two cell types, into which the swimming and reproductive functions of the Chlamydomonas unicell have been segregated. Two things make Volvox and Chlamydomonas especially useful as a model system for studying the evolution of multicellularity. Volvox evolved multicellularity relatively recently, within the past 200 million years or so, and it and Chlamydomonas are excellent experimental organisms, with fully sequenced genomes, for which it is feasible to clone and test the functions of key genes. By comparing the Volvox and Chlamydomonas genomes, as well as cloning and then analyzing important developmental genes of Volvox, researchers have discovered that these two algae are actually very similar to each other at the genetic level. These findings are leading them to the idea that multicellularity can evolve through relatively small changes that alter the way preexisting pathways functioned in the unicellular ancestor." /> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <script type="text/plain" data-cc-script="C04"> var initScriptsList = ["", "/natedjsp/scripts/commonScripts.jsp", "/natedjsp/empty.jsp", "/natedjsp/scripts/readingScript.jsp"]; var jsLoadMethod = "topicRoomLoad"; </script> <script data-cc-script="C04" data-src="/scitable/nated/natedjsp/scripts/jqueryAjaxScripts.js" type="text/javascript"></script> <link href="/scitable/nated/natedjsp/scripts/commonStylesEmbeddedFonts.css" rel="stylesheet" type="text/css" /> <link href="/scitable/nated/natedjsp/scripts/topicRoomStyles.css" rel="stylesheet" type="text/css" /> <meta name="WT.cg_n" content="Scitable" /> <meta name="WT.cg_s" content="Cell Origins and Metabolism" /> <meta name="WT.z_cg_cat" content="Volvox, Chlamydomonas, and the Evolution of Multicellularity" /> <meta name="WT.z_cg_topic" content="Volvox, Chlamydomonas, and the Evolution of Multicellularity" /> <meta name="WT.z_cg_level" content="MED" /> <style type="text/css">#right_Nav_header {display:none;}</style><script type="text/plain" data-cc-script="C04">function openPageLink(B){var A=document.getElementById(B).value;if(A!=""){location.href=A}}</script></head> <body class="specific-scitable"><p style="background-color: #333; color: #fff; text-align: center; padding: 10px; font-weight: 600; margin: 0; border-bottom: 1px solid #000">This page has been archived and is no longer updated</p> <script type="text/plain" data-cc-script="C04"> $(document).ready(function() { if(document.getElementById('facebookLink')!=null){ var replacements = new Array(); replacements[0] = '/'; replacements[1] = '%2F'; var fjs = document.getElementById('facebookLink'); var locationHref = replaceall(location.href,replacements); replacements[0] = ':'; replacements[1] = '%3A'; locationHref = replaceall(locationHref,replacements) fjs.src = fjs.src.replace('link',locationHref); } }); function replaceall( str, replacements ) { if(str=='' || str==null || str==undefined) { return ''; } var idx = str.indexOf( replacements[0] ); while ( idx > -1 ) { str = str.replace( replacements[0], replacements[1] ); idx = str.indexOf( replacements[0] ); } return str; } var googletag = googletag || {}; googletag.cmd = googletag.cmd || []; (function() { var gads = document.createElement('script'); gads.async = true; gads.type = 'text/javascript'; var useSSL = 'https:' == document.location.protocol; gads.src = (useSSL ? 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Miller, Ph.D. (<span class="topicCitationItalics">Dept. of Biological Sciences, University of Maryland, Baltimore County</span>) © 2010 Nature Education </div><div class="fleft"></div></div><div class="clear"></div> <div class="clearfix posRelative"> <div class="fleft bold" style="width: 585px;">Citation: <span id="fullName" class="bold">Miller, S. M.</span><span id="publishDate" class="bold"> (2010)</span> Volvox, Chlamydomonas, and the Evolution of Multicellularity. <span id="publisher" class="topicCitationItalics">Nature Education</span> <span id="volume" class="bold">3(<span id="issue" class="bold">9</span>)</span>:65</div> <div class="fright w180" style="position: absolute; right: -20px; bottom: -2px;"> <div class="clearfix w100p hideForPrint"> <div class="fright padleft3px"> <a rel="nofollow" id="redditShareLink" href="http://www.reddit.com/submit?url=" onClick="shareWithReddit();return false;"><img class="nomargin borderNone pad2px" title="Share Using Reddit" src="/scitable/natedimages/reddit-share-icon.png"/></a> </div> <div class="fright padleft3px"> <a rel="nofollow" id="stumbleUponLink" href="#" onClick="shareWithStubmleUpon();return false;"><img class="nomargin borderNone pad2px" title="StumbleUpon" src="/scitable/natedimages/stumbleupon.png"/></a> </div> <div class="fright padleft3px"> <a rel="nofollow" id="googlePlusLink" href="https://plus.google.com/share?url=" onClick="shareWithGooglePlus();return false;"><img class="nomargin borderNone pad2px" title="Share with Google+" src="/scitable/natedimages/gplus-16.png"/></a> </div> <div class="fright padleft3px"> <a rel="nofollow" id="twitterShareLnk" href="http://twitter.com/share?url=" onClick="normalShareWithTwitter();return false;"><img class="nomargin borderNone pad2px" title="Share with Twitter" src="/scitable/natedimages/icon-twitter.jpg"/></a> </div> <div class="fright padleft3px"> <a rel="nofollow" id="faceBookShareLnk" href="http://www.facebook.com/share.php?u=" onClick="normalShareWithFaceBook();return false;"><img class="nomargin borderNone pad2px" title="Share with Facebook" src="/scitable/natedimages/icon-facebook.jpg" /></a> </div> <div class="fright padleft3px"> <a name="shareEMailSplashAnc" id="shareEMailSplashAnc" href="#" onClick="sendMailToShare('lnkLBMdlLIID', event);return false;"><img class="nomargin borderNone pad2px" src="/scitable/natedimages/email_icon.gif" alt="Email" /></a> </div> <div class="fright padleft3px"> <a href="javascript:printReadingPage()"><img title="Print" class="nomargin borderNone pad2px" src="/scitable/natedimages/print_15.gif"/></a> </div> <div class="fright padleft3px"> <a onMouseOver="return displayOnMouseOverText(this);" href="#" id="bookmarkTRArttAnc" name="bookmarkTRArttAnc" onclick="addToLocker(); return false;"><img title="Bookmark" class="nomargin borderNone pad2px" src="/scitable/natedimages/icon_bookmark.gif"/></a> </div> </div> </div> </div> </div> </div> <input type="hidden" name="isCoverPage" id="" value="N" /> </div> <div class="clear"></div> </div> <div class="clear"></div> </div> <div class="px18 normalFontWeight margintop10px readPgHdng" style="line-height: 26px ! important;">How does multicellularity evolve? Scientists who study a family of green algae that includes unicellular Chlamydomonas and multicellular Volvox are beginning to find answers to this question.</div> <div class="fright padright5px hideForPrint"> <a href="javascript:resizeFontScitable('fontSize11')" class="padright10px"><span class="px11 mousePointerHand lh22" id="sizeNo1">Aa</span></a> <a href="javascript:resizeFontScitable('fontSize15')" class="padright10px"><span class="px15 mousePointerHand lh22" id="sizeNo2">Aa</span></a> <a href="javascript:resizeFontScitable('fontSize20')" class="padright10px"><span class="mousePointerHand lh22 px20" id="sizeNo3">Aa</span></a> </div> <div class="clear"></div> <div class="articleHR w98p"></div> <div id="orgCnt"> <div id="trOutLine"></div> <div id="fontSizeChange" class="fontSizeChange"><div id="mainCntArtPageDiv"><div id="trPage"> <div class="articleContent"> <div class="clear"></div> <div class="contentSection"> <div class="sectionTitle"><h2 style="display:none;"> </h2></div> <div class="clear"></div> <div class="sectionParas"> <div class="paraArticle"> <div> <p> Life is very good at reinventing itself over time, and one of its most important innovations has been multicellularity, the capacity to make multiple cells and cell types that carry out specialized functions. Without the <span class="ontologyTermLink" onmouseover="showDescription("78",this);return false;">evolution</span> of multicellularity, our planet would be a very different place — a world without plants or animals of any kind, and of course without humans. Yet even though multicellular <span class="ontologyTermLink" onmouseover="showDescription("312",this);return false;">species</span> have evolved independently in most major lineages of <span class="glossaryTermLink" onmouseover="showGloDescription("503",this);return false;">eukaryotic</span> organisms — including not only those to which plants and animals belong, but also green algae, brown algae, red algae, ciliates, slime molds, and fungi — we know surprisingly little about how this evolution came about (Figure 1). Do certain properties predispose a unicellular lineage to make the leap to multicellularity? Are certain types of genes/gene families, or genetic mechanisms especially important for this sort of transition to occur? Does the evolution of multicellularity require big steps involving major increases in <span class="ontologyTermLink" onmouseover="showDescription("43",this);return false;">genome</span> size and/or expansions in gene families, or even many new kinds of genes? Or might the <span class='glossaryTermLink' onmouseover='showGloDescription("1620",this);return false;'>transition</span> to a multicellular form possibly take place in smaller steps, involving only subtle changes? Scientists who study a family of green algae that includes unicellular <i>Chlamydomonas</i> and multicellular <i>Volvox</i> are beginning to find answers to some of these questions.</p> <p><a name="anchEmbedImg_1284056171278-3126020789_1" ><input type="hidden" name="hidEmbedImg" id="hidEmbedImg_1284056171278-3126020789_1" value="/scitable"/><input type="hidden" name="hidEmbedPlayerState" id="hidEmbedPlayerState_1284056171278-3126020789_1" value="uninitedaudio"/></a><div style="width:632px;" class="imageSet horzAlignCenter"><div style="width:632px;"><IMG src="/scitable/content/ne0000/ne0000/ne0000/ne0000/14458921/f1_miller_fig1_1_2.jpg" title="" alt="An unrooted phylogenetic tree shows how multicellularity evolved independently in several eukaryotic lineages. The lineages are represented by branching lines. All of the lines are connected and radiate outward from the middle of the diagram. The taxa are represented by circles at the ends of the lines. The color of each circle indicates the taxon’s degree of multicellularity." /><br/><div class="imageCaption pipeblack bold" style="padding:10px;" ><div class="imageCaption pipeblack bold nopad">Figure 1: Multicellularity evolved from multiple independent origins.</div><div class="imageLegend pipeblack nopad">Shown are representatives of all major lineages of eukaryotic organisms, color coded for occurrence of multicellularity. Solid black circles indicate major lineages composed entirely of unicellular species. Other groups shown contain only multicellular species (solid red), some multicellular and some unicellular species (red and black circles), or some unicellular and some colonial species (yellow and black circles). Colonial species are defined as those that possess multiple cells of the same type. Some lineages shown here that contain some multicellular species, such as the chlorophycean algae (of which the volvocine algae are members) also include several colonial species. For simplicity, many unicellular lineages that are closely related to the multicellular lineages depicted here are not shown. The existence of such groups provides evidence that multicellularity evolved independently in the sister lineages.<br/></div><div class="creditLine bold nopad fleft padright4px">© 2006 <a target="_blank" href="http://www.nature.com/nature_education" class="inlineLinks">Nature Education</a> Modified from King <i>et al.</i> (2004). All rights reserved. <a href='javascript:show_inform("Terms of Use", "You may reproduce this material, without modifications, in print or electronic form for your personal, non-commercial purposes or for non-commercial use in an educational environment.");' class="inlineLinks"><img class="nomargin infoImg nofloat" title="View Terms of Use" alt="View Terms of Use" height="13px" width="13px" src="/scitable/natedimages/info_icon.png" /></a></div><div class="clear"></div><div class="nomargin padleft4px pipeblack bold fright textalignleft"><a title="The evolutionary relationships between unicellular, multicellular, and colonial taxa are shown in an unrooted phylogenetic tree. Major eukaryotic taxa are represented by circles at the ends of branched lines. The lines represent lineages and meet at common ancestors. Shorter lines and fewer branches between taxa indicate that the taxa are more closely related. Some taxa are completely unicellular, but most branches contain both unicellular and multicellular species. One of the major branches contains plants and algae. Land plants are multicellular, and charophycean algae, chlorophycean algae, and red algae are taxa that contain both multicellular and unicellular species. Another major branch contains diatoms, ciliates, and brown algae. Diatoms are either unicellular or colonial. Ciliates are primarily unicellular with some rare multicellular species. Brown algae are either multicellular or unicellular. Another branch contains excavates, which are strictly unicellular, and acrasid slime molds, which include both multicellular and unicellular species. One of the branches splits to form two major lineages. One of these lineages splits again and contains dictyostelid slime molds and plasmodial slime molds, which are both exclusively multicellular. The other lineage contains fungi, which include multicellular and unicellular species, animals, which are entirely multicellular, and choanoflagellates, which are unicellular or colonial. One taxon, rhizaria, is distant from all other branches and contains only unicellular species. The existence of multicellular species in different branches indicates that multicellularity evolved independently." href="javascript:void(0)" onclick="callNewShowInformAfterPublish("true","true","Y","/scitable/content/ne0000/ne0000/ne0000/ne0000/14458921/f1_miller_fig1.jpg", "Multicellularity evolved from multiple independent origins.", "Figure 1", "Shown are representatives of all major lineages of eukaryotic organisms, color coded for occurrence of multicellularity. Solid black circles indicate major lineages composed entirely of unicellular species. Other groups shown contain only multicellular species (solid red), some multicellular and some unicellular species (red and black circles), or some unicellular and some colonial species (yellow and black circles). Colonial species are defined as those that possess multiple cells of the same type. Some lineages shown here that contain some multicellular species, such as the chlorophycean algae (of which the volvocine algae are members) also include several colonial species. For simplicity, many unicellular lineages that are closely related to the multicellular lineages depicted here are not shown. The existence of such groups provides evidence that multicellularity evolved independently in the sister lineages.<br/>", '600','http://www.nature.com/nature_education', "The evolutionary relationships between unicellular, multicellular, and colonial taxa are shown in an unrooted phylogenetic tree. Major eukaryotic taxa are represented by circles at the ends of branched lines. The lines represent lineages and meet at common ancestors. Shorter lines and fewer branches between taxa indicate that the taxa are more closely related. Some taxa are completely unicellular, but most branches contain both unicellular and multicellular species. One of the major branches contains plants and algae. Land plants are multicellular, and charophycean algae, chlorophycean algae, and red algae are taxa that contain both multicellular and unicellular species. Another major branch contains diatoms, ciliates, and brown algae. Diatoms are either unicellular or colonial. Ciliates are primarily unicellular with some rare multicellular species. Brown algae are either multicellular or unicellular. Another branch contains excavates, which are strictly unicellular, and acrasid slime molds, which include both multicellular and unicellular species. One of the branches splits to form two major lineages. One of these lineages splits again and contains dictyostelid slime molds and plasmodial slime molds, which are both exclusively multicellular. The other lineage contains fungi, which include multicellular and unicellular species, animals, which are entirely multicellular, and choanoflagellates, which are unicellular or colonial. One taxon, rhizaria, is distant from all other branches and contains only unicellular species. The existence of multicellular species in different branches indicates that multicellularity evolved independently.")" class="inlineLinks"> Figure Detail </a></div><div class="clear"></div></div></div></div></p> </div> </div> <div class="clear"></div> </div> </div> <div class="clear"></div> <div class="contentSection"> <div class="sectionTitle"> <h2> <SECTIONTITLE-1>What Is Multicellularity?</SECTIONTITLE-1> </h2> </div> <div class="clear"></div> <div class="sectionParas"> <div class="paraArticle"> <div> <p>Before we delve into these questions, note that not all multicellular lifestyles are the same. Many species of multicellular organisms differ greatly from each other with respect to the types of developmental mechanisms and traits they have evolved. For instance, by definition every multicellular organism possesses multiple cells that remain associated following <span class="ontologyTermLink" onmouseover="showDescription("47",this);return false;">cell division</span>. But while plant and animal species generate at least a dozen different types of cells, with groups of cells organized into tissues and/or organs, some multicellular organisms, such as slime molds and at least one ciliate species, possess very few cell types and do not produce tissues or organs that are themselves composed of multiple cell types. Furthermore, animal embryos undergo gastrulation, a process by which groups of cells migrate or change position with respect to each other, but plant embryos do not. It turns out there are many different ways to be multicellular.</p> </div> </div> <div class="clear"></div> </div> </div> <div class="clear"></div> <div class="contentSection"> <div class="sectionTitle"> <h2> <SECTIONTITLE-1>Good Model Systems for Multicellularity Are Rare</SECTIONTITLE-1> </h2> </div> <div class="clear"></div> <div class="sectionParas"> <div class="paraArticle"> <div> <p>There is a very simple reason we know so little about how multicellularity arises: The phenomenon is very difficult to investigate. One complicating factor is that most transitions to multicellularity, such as the ones that gave rise to plants and animals, occurred deep in the past, approaching a billion years ago (Sanderson 2003; Peterson & Butterfield 2005). Why does it matter how long ago multicellularity evolved? The longer ago <span class="ontologyTermLink" onmouseover="showDescription("196",this);return false;">divergence</span> from a common unicellular ancestor of existing multicellular and unicellular species occurred, the more genetic changes have accumulated that are irrelevant to multicellularity. These accumulated changes make it harder to sift though and determine exactly which genetic changes account for the transition to multicellularity.</p> <p> </p> <p>Another thing that makes evolution of multicellularity difficult to study is the challenge in finding a good set of existing organisms to compare — a good <span class="glossaryTermLink" onmouseover="showGloDescription("960",this);return false;">model</span> system. Few unicellular-multicellular species sets are suitable for this type of comparison, as a true system for experimental testing, because the two sets must be closely related. The reason why the best-known animal model systems (fruit flies, nematodes, zebrafish, mice, etc.) and plant model systems (<i>Arabidopsis</i>, corn, rice, tobacco, etc.) are not well suited is that their closest unicellular relatives — the choanoflagellates (animals) and charophycean algae (plants) — diverged from the multicellular <span class="glossaryTermLink" onmouseover="showGloDescription("863",this);return false;">lineage</span> too long ago, so they have relatively few genetic similarities to animals and plants. </p> <p> </p> </div> </div> <div class="clear"></div> </div> </div> <div class="clear"></div> <div class="contentSection"> <div class="sectionTitle"> <h2> <SECTIONTITLE-1>An Ideal Model System: <i>Volvox</i>, <i>Chlamydomonas</i>, and the Volvocine Green Algae</SECTIONTITLE-1> </h2> </div> <div class="clear"></div> <div class="sectionParas"> <div class="paraArticle"> <div> <p><a name="anchEmbedImg_1284056471846-3003425087_1" ><input type="hidden" name="hidEmbedImg" id="hidEmbedImg_1284056471846-3003425087_1" value="/scitable"/><input type="hidden" name="hidEmbedPlayerState" id="hidEmbedPlayerState_1284056471846-3003425087_1" value="uninitedaudio"/></a><div style="width:341px;" class="imageSet Right"><div style="width:341px;"><A href="#" onclick="dispOrigImg("/scitable/content/ne0000/ne0000/ne0000/ne0000/14458956/f2_miller_fig2.jpg", "Volvox carteri and Chlamydomonas reinhardtii", "true", "Figure 2", "(A) Young Volvox adult, with about 2,000 small somatic cells in a monolayer at the surface, and nineteen large gonidia embedded in the extracellular matrix (ECM), just under the somatic cell layer. (B) The adult is about a day further along in development than the one shown in panel A; its gonidia have completed embryogenesis to produce young (juvenile) spheroids that each contain about 2,000 somatic cells and about sixteen gonidia. These juveniles are about one day younger than the adult shown in panel A. (C) Close-up, side-on view of two adult (parental) somatic cells (arrowheads point to the orange photoreceptor-containing organelle called the eyespot). Note the two flagella just below the arrowhead pointing to the topmost somatic cell. Visible above this somatic cell is part of a young juvenile (arrow) with several immature somatic cells and part of one gonidium in the field of view. (D) Head-on view of the field of parental somatic cells (flagella almost completely out of the plane of focus). (E) Gonidium of a young adult. (F) Chlamydomonas unicell, with apical (flagellar) end oriented up. Note that scale bars are unequal. ", "true", "All rights reserved.", '600', '800', 'http://www.nature.com/nature_education');"><IMG src="/scitable/content/ne0000/ne0000/ne0000/ne0000/14458956/miller_fig2_2_1.jpg" title="" alt="A series of six photomicrographs shows the multicellular Volvox species and the unicellular Chlamydomonas species. Both types of algae are semi-transparent with light-green, white, and orange structures. The background is gray." /></A><br/><div class="imageCaption pipeblack bold" style="padding:10px;" ><div class="imageCaption pipeblack bold nopad"><A class="pipeblack bold" href="#" onclick="dispOrigImg("/scitable/content/ne0000/ne0000/ne0000/ne0000/14458956/f2_miller_fig2.jpg", "Volvox carteri and Chlamydomonas reinhardtii", "true", "Figure 2", "(A) Young Volvox adult, with about 2,000 small somatic cells in a monolayer at the surface, and nineteen large gonidia embedded in the extracellular matrix (ECM), just under the somatic cell layer. (B) The adult is about a day further along in development than the one shown in panel A; its gonidia have completed embryogenesis to produce young (juvenile) spheroids that each contain about 2,000 somatic cells and about sixteen gonidia. These juveniles are about one day younger than the adult shown in panel A. (C) Close-up, side-on view of two adult (parental) somatic cells (arrowheads point to the orange photoreceptor-containing organelle called the eyespot). Note the two flagella just below the arrowhead pointing to the topmost somatic cell. Visible above this somatic cell is part of a young juvenile (arrow) with several immature somatic cells and part of one gonidium in the field of view. (D) Head-on view of the field of parental somatic cells (flagella almost completely out of the plane of focus). (E) Gonidium of a young adult. (F) Chlamydomonas unicell, with apical (flagellar) end oriented up. Note that scale bars are unequal. ", "true", "All rights reserved.", '600', '800', 'http://www.nature.com/nature_education');"><img class="fleft borderNone" alt="View Full-Size Image" src="/scitable/natedimages/plus_box.gif"/>Figure 2: Volvox carteri and Chlamydomonas reinhardtii</A><div class="clear nomargin"></div></div><div class="imageLegend pipeblack nopad"><A style="font-weight: normal !important;" class="pipeblack" href="#" onclick="dispOrigImg("/scitable/content/ne0000/ne0000/ne0000/ne0000/14458956/f2_miller_fig2.jpg", "Volvox carteri and Chlamydomonas reinhardtii", "true", "Figure 2", "(A) Young Volvox adult, with about 2,000 small somatic cells in a monolayer at the surface, and nineteen large gonidia embedded in the extracellular matrix (ECM), just under the somatic cell layer. (B) The adult is about a day further along in development than the one shown in panel A; its gonidia have completed embryogenesis to produce young (juvenile) spheroids that each contain about 2,000 somatic cells and about sixteen gonidia. These juveniles are about one day younger than the adult shown in panel A. (C) Close-up, side-on view of two adult (parental) somatic cells (arrowheads point to the orange photoreceptor-containing organelle called the eyespot). Note the two flagella just below the arrowhead pointing to the topmost somatic cell. Visible above this somatic cell is part of a young juvenile (arrow) with several immature somatic cells and part of one gonidium in the field of view. (D) Head-on view of the field of parental somatic cells (flagella almost completely out of the plane of focus). (E) Gonidium of a young adult. (F) Chlamydomonas unicell, with apical (flagellar) end oriented up. Note that scale bars are unequal. ", "true", "All rights reserved.", '600', '800', 'http://www.nature.com/nature_education');">(A) Young Volvox adult, with about 2,000 small somatic cells in a monolayer at the surface, and nineteen large gonidia embedded in the extracellular matrix (ECM), just under the somatic cell layer. (B) The adult is about a day further along in development than the one shown in panel A; its gonidia have completed embryogenesis to produce young (juvenile) spheroids that each contain about 2,000 somatic cells and about sixteen gonidia. These juveniles are about one day younger than the adult shown in panel A. (C) Close-up, side-on view of two adult (parental) somatic cells (arrowheads point to the orange photoreceptor-containing organelle called the eyespot). Note the two flagella just below the arrowhead pointing to the topmost somatic cell. Visible above this somatic cell is part of a young juvenile (arrow) with several immature somatic cells and part of one gonidium in the field of view. (D) Head-on view of the field of parental somatic cells (flagella almost completely out of the plane of focus). (E) Gonidium of a young adult. (F) Chlamydomonas unicell, with apical (flagellar) end oriented up. Note that scale bars are unequal. </A></div><div class="creditLine bold nopad fleft padright4px">© 2010 <a target="_blank" href="http://www.nature.com/nature_education" class="inlineLinks">Nature Education</a> Courtesy of Ichiro Nishii. All rights reserved. <a href='javascript:show_inform("Terms of Use", "You may reproduce this material, without modifications, in print or electronic form for your personal, non-commercial purposes or for non-commercial use in an educational environment.");' class="inlineLinks"><img class="nomargin infoImg nofloat" title="View Terms of Use" alt="View Terms of Use" height="13px" width="13px" src="/scitable/natedimages/info_icon.png" /></a></div><div class="clear"></div><div class="nomargin padleft4px pipeblack bold fright textalignleft"><a title="Multicellular Volvox carteri are shown in five photomicrographs (panels A through E), and a sixth photomicrograph (panel F) shows a Chlamydomonas reinhardtii cell. Panel A shows a spherical, multicellular Volvox adult, with a diameter of approximately 350 um. The exterior of the Volvox contains many evenly-spaced, small somatic cells, which look like light-green dots. The interior of the organism contains nineteen large, spherical, green gonidia. At this stage of embryogenesis, the gonidia do not have interior and exterior cell layers, but appear uniformly green. Each gonidium is approximately 50 um in diameter. Panel B shows an older Volvox adult. The diameter of the Volvox is about 650 um, and the somatic cells on the exterior are spaced farther apart than those in panel A. Eighteen large, circular gonidia are shown inside the organism. The gonidia have completed embryogenesis and are now juveniles with smaller cells on the exterior and larger cells inside. Each juvenile is approximately 100 um in diameter. Panel C shows a magnified view of part of an adult Volvox. Arrowheads point to two somatic cells, which are circular and approximately 7 um in diameter. Each of these somatic cells contains a single, small, orange eyespot and two long, rope-like flagella that point toward the exterior of the Volvox. In the upper left hand corner of the photograph, an arrow points to part of a young juvenile. The juvenile somatic cells are approximately 3 um in diameter, and are close to one another. Part of a gonidium is also visible within the young juvenile. Panel D shows nine somatic Volvox cells, which are circular and about 5-10 um in diameter. Each somatic cell contains a small orange eyespot. The flagella are not visible. Panel E shows a single Volvox gonidium. The circular, green gonidium is approximately 60 um in diameter. The gonidium is from a young adult, and roughly a dozen cells of similar size are visible in the focal plane shown in the photograph. Although multiple cells are visible, there are far fewer than in the juvenile. Panel F shows a single Chlamydomonas cell, which is circular and approximately 7 um in diameter. Two flagella are present at the top of the cell and resemble insect antennae. The left flagellum curves to the left, and the right flagellum curves to the right. A small, orange eyespot is visible in the bottom left region of the cell. The Chlamydomonas cell looks very similar to the somatic Volvox cells." href="javascript:void(0)" onclick="callNewShowInformAfterPublish("true","true","Y","/scitable/content/ne0000/ne0000/ne0000/ne0000/14458956/f2_miller_fig2.jpg", "Volvox carteri and Chlamydomonas reinhardtii", "Figure 2", "(A) Young Volvox adult, with about 2,000 small somatic cells in a monolayer at the surface, and nineteen large gonidia embedded in the extracellular matrix (ECM), just under the somatic cell layer. (B) The adult is about a day further along in development than the one shown in panel A; its gonidia have completed embryogenesis to produce young (juvenile) spheroids that each contain about 2,000 somatic cells and about sixteen gonidia. These juveniles are about one day younger than the adult shown in panel A. (C) Close-up, side-on view of two adult (parental) somatic cells (arrowheads point to the orange photoreceptor-containing organelle called the eyespot). Note the two flagella just below the arrowhead pointing to the topmost somatic cell. Visible above this somatic cell is part of a young juvenile (arrow) with several immature somatic cells and part of one gonidium in the field of view. (D) Head-on view of the field of parental somatic cells (flagella almost completely out of the plane of focus). (E) Gonidium of a young adult. (F) Chlamydomonas unicell, with apical (flagellar) end oriented up. Note that scale bars are unequal. ", '600','http://www.nature.com/nature_education', "Multicellular <i>Volvox carteri</i> are shown in five photomicrographs (panels A through E), and a sixth photomicrograph (panel F) shows a <i>Chlamydomonas</i><i> </i><i>reinhardtii</i> cell. Panel A shows a spherical, multicellular <i>Volvox</i> adult, with a diameter of approximately 350 um. The exterior of the <i>Volvox </i>contains many evenly-spaced, small somatic cells, which look like light-green dots. The interior of the organism contains nineteen large, spherical, green gonidia. At this stage of embryogenesis, the gonidia do not have interior and exterior cell layers, but appear uniformly green. Each gonidium is approximately 50 um in diameter. Panel B shows an older <i>Volvox</i> adult. The diameter of the <i>Volvox</i> is about 650 um, and the somatic cells on the exterior are spaced farther apart than those in panel A. Eighteen large, circular gonidia are shown inside the organism. The gonidia have completed embryogenesis and are now juveniles with smaller cells on the exterior and larger cells inside. Each juvenile is approximately 100 um in diameter. Panel C shows a magnified view of part of an adult <i>Volvox</i>. Arrowheads point to two somatic cells, which are circular and approximately 7 um in diameter. Each of these somatic cells contains a single, small, orange eyespot and two long, rope-like flagella that point toward the exterior of the <i>Volvox</i>. In the upper left hand corner of the photograph, an arrow points to part of a young juvenile. The juvenile somatic cells are approximately 3 um in diameter, and are close to one another. Part of a gonidium is also visible within the young juvenile. Panel D shows nine somatic <i>Volvox</i> cells, which are circular and about 5-10 um in diameter. Each somatic cell contains a small orange eyespot. The flagella are not visible. Panel E shows a single <i>Volvox</i> gonidium. The circular, green gonidium is approximately 60 um in diameter. The gonidium is from a young adult, and roughly a dozen cells of similar size are visible in the focal plane shown in the photograph. Although multiple cells are visible, there are far fewer than in the juvenile. Panel F shows a single <i>Chlamydomonas</i> cell, which is circular and approximately 7 um in diameter. Two flagella are present at the top of the cell and resemble insect antennae. The left flagellum curves to the left, and the right flagellum curves to the right. A small, orange eyespot is visible in the bottom left region of the cell. The <i>Chlamydomonas</i> cell looks very similar to the somatic <i>Volvox</i> cells.")" class="inlineLinks"> Figure Detail </a></div><div class="clear"></div></div></div></div>Fortunately, the family of volvocine green algae is remarkably well suited to studying the evolution of multicellularity. The volvocine algae include both unicellular and multicellular organisms that are closely related and exist today (Kirk 1998). The unicellular species in this group is named <i>Chlamydomonas</i> <i>reinhardtii </i>(hereafter <i>Chlamydomonas</i>), and its best-studied, close multicellular relative is a species named <i>Volvox carteri </i>(hereafter <i>Volvox</i>). What are these organisms like? <i>Chlamydomonas</i> are single-celled organisms with two apical flagella, which they use for sensory <span class="glossaryTermLink" onmouseover="showGloDescription("1607",this);return false;">transduction</span> and for moving around in a wet <span class="ontologyTermLink" onmouseover="showDescription("254",this);return false;">environment</span> (Figure 2F). But <i>Chlamydomonas</i> unicells don't always have these flagella. They resorb them in preparation for cell division, so the <i>Chlamydomonas</i> life cycle consists of alternation between a swimming phase during which the cells grow, and an aflagellate/immotile reproductive phase during which they replicate their <span class="ontologyTermLink" onmouseover="showDescription("107",this);return false;">DNA</span> and divide. When <i>Chlamydomonas</i> cells divide, they use what is called a multiple fission mode of division: They usually undergo sequential rounds of DNA <span class="ontologyTermLink" onmouseover="showDescription("33",this);return false;">replication</span> and <a href='http://www.nature.com/scitable/topicpage/Mitosis-and-Cell-Division-205' title="mitosis" ><span class="ontologyTermLink" onmouseover="showDescription("159",this);return false;">mitosis</span></a>, and produce four, eight, or sixteen unicellular, <span class="ontologyTermLink" onmouseover="showDescription("221",this);return false;">asexual</span> daughter cells. In <i>Volvox</i>, these two functions — swimming and <span class="ontologyTermLink" onmouseover="showDescription("27",this);return false;">reproduction</span> — have been segregated into distinct <span class="ontologyTermLink" onmouseover="showDescription("93",this);return false;">cell</span> types (Figure 2A-E; Kirk 1998).</p> <p> </p> <p>Cells of one type, called somatic cells, number about 2,000 and closely resemble <i>Chlamydomonas</i> unicells. Somatic cells are small, have two flagella, and reside in a monolayer at the surface of a sphere of gelatinous extracellular matrix (ECM). Their job is to swim and keep <i>Volvox </i>in the light so that it can photosynthesize. Unlike <i>Chlamydomonas</i> unicells, <i>Volvox</i> somatic cells cannot divide, and this distinction is very important — <i>Volvox </i>has multicellularity with division of labor because its somatic cells lost the capacity for reproduction. Reproduction is carried out by a second type of specialized cell, called the gonidium. Gonidia are large and do not have flagella (see Figure 2E), so they cannot swim (and must therefore rely on somatic cells for motility), but they can divide. Each of the approximately sixteen gonidia has the capacity to generate a new individual through a series of ten to eleven embryonic cell divisions that generate all the cells present in the next generation. Is this sort of division of labor unique to <i>Volvox</i>? Probably not. Some scientists believe that the <span class="glossaryTermLink" onmouseover="showGloDescription("1408",this);return false;">segregation</span> of somatic functions (like swimming) and reproduction into distinct cell types was one of the first key steps in the evolution of multicellularity in animals as well (Buss 1987; King 2004). </p> <p>Researchers believe that the last common ancestor of the present-day volvocine algae was a unicellular species closely resembling modern-day <i>Chlamydomonas</i> and that <i>Chlamydomonas</i> may not have changed much at the genetic level, with respect to that ancestor (Kirk 2005; Herron & Michod 2008). Researchers also know, on the basis of information from plant and algal fossils and from <a href='http://www.nature.com/scitable/topicpage/The-Molecular-Clock-and-Estimating-Species-Divergence-41971' title="molecular clock analyses" ><span class="ontologyTermLink" onmouseover="showDescription("166",this);return false;">molecular clock</span> analyses</a> that members of the volvocine family have been diverging from each other for only about 200 million years (Herron <i>et al.</i> 2009). Combined with the fact that both <i>Volvox </i>and <i>Chlamydomonas</i> are good experimental organisms that can be manipulated at both the genetic and molecular levels, this means it should be feasible to discover the genetic innovations that made multicellularity possible within these species. </p> </div> </div> <div class="clear"></div> </div> </div> <div class="clear"></div> <div class="contentSection"> <div class="sectionTitle"> <h2> <SECTIONTITLE-1>Multicellularity in the Volvocine Algae</SECTIONTITLE-1> </h2> </div> <div class="clear"></div> <div class="sectionParas"> <div class="paraArticle"> <div> <p>How does <i>Volvox</i> compare to plants, animals, and other multicellular organisms with respect to the sorts of processes it has evolved? In a way, <i>Volvox </i>exhibits a relatively streamlined type of multicellularity. It possesses just two cell types, and these cells are not organized into tissues or organs. Nonetheless it has evolved an impressive degree of developmental and morphological novelty. Indeed, David Kirk compared the developmental programs of <i>Volvox</i>, <i>Chlamydomonas</i>, and several other volvocine algae and inferred that twelve new developmental traits evolved in <i>Volvox</i> that its unicellular ancestor did not possess (Kirk 2005). For instance, <i>Volvox</i> exhibits <span class="glossaryTermLink" onmouseover="showGloDescription("463",this);return false;">embryo</span> <span class="ontologyTermLink" onmouseover="showDescription("246",this);return false;">inversion</span>, a morphogenetic process that is analogous to animal gastrulation and that positions the flagellar ends of somatic cells correctly following cell division (Kirk & Nishii 2001). In addition, <i>Volvox </i>embryos execute a specialized type of cell division that generates cells of different sizes and types, called asymmetric division (Kirk 2001). And, as described above, <i>Volvox</i> makes specialized cell types: terminally differentiated somatic cells and immortal, stem cell-like gonidia. As we will see shortly, researchers have already discovered some clues about how each of these traits evolved. </p> </div> </div> <div class="clear"></div> </div> </div> <div class="clear"></div> <div class="contentSection"> <div class="sectionTitle"> <h2> <SECTIONTITLE-1>Strategies for Investigating the Evolution of Multicellularity</SECTIONTITLE-1> </h2> </div> <div class="clear"></div> <div class="sectionParas"> <div class="paraArticle"> <div> <p>How does one go about learning how multicellularity evolves? Most approaches that researchers have used to study the genetic basis of multicellularity fall into one of two strategies — <span class="ontologyTermLink" onmouseover="showDescription("165",this);return false;">comparative genomics</span> and mutational/functional genetics — or a combination of the two. A third approach, involving molecular taxonomy studies that tell us about the <span class="ontologyTermLink" onmouseover="showDescription("123",this);return false;">relatedness</span> of species, has also been very important. In fact, this third approach forms the foundation on which the other two approaches are built. However, it will not be discussed here, because it is beyond the scope of the current discussion.</p> <p>With comparative genomic approaches, researchers compare fully sequenced genomes of close multicellular-unicellular cousins to determine which <span class="glossaryTermLink" onmouseover="showGloDescription("1947",this);return false;">genes</span> are unique to either genome, and to determine how the proteins encoded by the genomes differ. The idea with this type of analysis is that any genes or gene families present in the multicellular species but not the unicellular one might have been important for the evolution of multicellularity. That is, certain "special" genes for multicellularity might be found only in multicellular organisms. Or if the multicellular species contains significantly more copies of a certain kind of gene than does its unicellular cousin, or if the proteins encoded by certain related genes have changed a great deal in the two species, then those extra copies or changed proteins might be important for multicellularity. </p> <p>It is important to keep in mind here that large-scale comparative genomic studies typically uncover only big differences in <span class="ontologyTermLink" onmouseover="showDescription("29",this);return false;">gene</span> families, or differences in well-known genes and gene families. Such studies might not uncover subtle differences, such as small changes in the sizes of gene families that occur when a gene is duplicated (or lost) in one species but not the other. Evolutionary biologists think that <a href='http://www.nature.com/scitable/topicpage/origins-of-new-genes-and-pseudogenes-835' title="gene duplication" >gene <span class="ontologyTermLink" onmouseover="showDescription("70",this);return false;">duplication</span></a> events could be extremely important for the evolution of new traits, because the new genes are free to change over time and subsequently <span class="glossaryTermLink" onmouseover="showGloDescription("563",this);return false;">function</span> somewhat differently from the genes they were copied from. </p> <p>Researchers also use mutational/functional approaches, which start by identifying <span class="glossaryTermLink" onmouseover="showGloDescription("996",this);return false;">mutant</span> versions of the multicellular species that are defective for key developmental processes that don't occur in the unicellular species (such as the ability to make different cell types). These mutants are then used to <span class="glossaryTermLink" onmouseover="showGloDescription("248",this);return false;">clone</span> the affected genes. After that, researchers analyze the unicellular species genome to determine whether the same (<span class="glossaryTermLink" onmouseover="showGloDescription("1096",this);return false;">orthologous</span>) genes exists and, if so, whether or how they differ from the multicellular versions. These types of investigations using current, living organisms are very powerful. They reveal valuable clues to which genes were important for evolving novel abilities, and how those genes were shuffled around and/or changed during the evolutionary past. On the whole, sorting out the differences between multicellular and unicellular organisms lends clues to how multicellularity may have evolved.</p> </div> </div> <div class="clear"></div> </div> </div> <div class="clear"></div> <div class="contentSection"> <div class="sectionTitle"> <h2> <SECTIONTITLE-1>Lessons Learned from the Volvocine Algae: Comparative Genomics</SECTIONTITLE-1> </h2> </div> <div class="clear"></div> <div class="sectionParas"> <div class="paraArticle"> <div> <p>What have the volvocine algae taught us about how multicellularity evolves? Recently researchers sequenced and compared the <i>Chlamydomonas</i> and <i>Volvox</i> genomes and found them to be remarkably similar (Prochnik <i>et al.</i> 2010). By almost every measure — overall <span class="glossaryTermLink" onmouseover="showGloDescription("1952",this);return false;">genome size</span>, number of protein-coding genes, number of different kinds of protein domains encoded, and distribution of <span class="glossaryTermLink" onmouseover="showGloDescription("594",this);return false;">gene family</span> sizes — the two organisms are very much the same. When these investigators looked carefully at certain families of genes, especially those known to be involved in regulating the sorts of developmental processes that occur in <i>Volvox </i>but not <i>Chlamydomonas</i>, they again found only similarities, for the most part. They did find one very obvious and important difference, however: Compared to <i>Chlamydomonas</i>, <i>Volvox</i> has many more genes that encode cell wall/ECM proteins, and many of the extra genes are quite different from the ones <i>Chlamydomonas</i> has. Here it is important to point out that the cell wall surrounding <i>Chlamydomonas</i> has two parts: an inner layer and an outer one. <i>Volvox</i> has versions of both, but the inner layer is greatly expanded compared to the <i>Chlamydomonas</i> inner layer. It makes up the bulk of the ECM that is not present in <i>Chlamydomonas</i>, and it helps cement the <i>Volvox</i> cells together. Researchers believe that the explosion in cell wall genes, and the morphing of some of those genes into different kinds of cell wall genes, is what drove the creation of ECM in <i>Volvox</i>. </p> </div> </div> <div class="clear"></div> </div> </div> <div class="clear"></div> <div class="contentSection"> <div class="sectionTitle"> <h2> <SECTIONTITLE-1>Lessons Learned from the Volvocine Algae: Mutational/Functional Genetics</SECTIONTITLE-1> </h2> </div> <div class="clear"></div> <div class="sectionParas"> <div class="paraArticle"> <div> <p>Clearly, pure comparative genomic approaches have their limitations; they cannot tell us everything there is to know about how developmental processes and multicellularity evolve. But genetic screens are possible for <i>Volvox</i> and <i>Chlamydomonas</i>. What insights have these screens provided into how multicellularity evolved in the volvocine lineage? </p> <p>Researchers have used genetic screens for developmental defects in <i>Volvox</i> to identify one gene that is essential for asymmetric division (<i>glsA</i>) and three others that are required for embryo inversion (<i>invA</i>, <i>invB</i>, and <i>invC</i>; Miller & Kirk 1999; Nishii <i>et al.</i> 2003; Ueki & Nishii 2008, 2009). All four genes have easily recognizable orthologs in <i>Chlamydomonas</i> that are very similar to their <i>Volvox</i> counterparts. Researchers have cloned <i>Chlamydomonas</i> orthologs corresponding to two of the <i>Volvox</i> developmental genes. One set of investigators showed that the <i>GAR1</i> gene of <i>Chlamydomonas</i>, which is orthologous to <i>glsA</i>, is able to function just like <i>glsA</i>: When transformed into <i>glsA</i> mutants, it repaired, or rescued, their asymmetric division defect (Cheng <i>et al.</i> 2003). Likewise, another set of researchers found that <i>IAR1</i> (orthologous to <i>invA</i>) can rescue the inversion defect of <i>invA</i> mutants (Nishii <i>et al.</i> 2003). These results tell us that the <i>glsA/GAR1</i> and <i>invA/IAR1</i> genes have not changed in important ways since the time that <i>Volvox</i> and <i>Chlamydomonas</i> diverged from a common ancestor. </p> <p><a name="anchEmbedImg_1284056959823-2623507203_1" ><input type="hidden" name="hidEmbedImg" id="hidEmbedImg_1284056959823-2623507203_1" value="/scitable"/><input type="hidden" name="hidEmbedPlayerState" id="hidEmbedPlayerState_1284056959823-2623507203_1" value="uninitedaudio"/></a><div style="width:341px;" class="imageSet Right"><div style="width:341px;"><A href="#" onclick="dispOrigImg("/scitable/content/ne0000/ne0000/ne0000/ne0000/14458976/f3_miller_fig3.jpg", "Gene and pathway co-option and the origins of asymmetric cell division and cellular differentiation in Volvox", "true", "Figure 3", "(A) The function of glsA appears to have been co-opted without change from an unknown function in the unicellular ancestor of Volvox, so that it is now part of a pathway (shaded green) that is required for asymmetric cell division. This may have happened because some not yet identified gene (X) that acted in the same pathway (shaded gray) as the ancestor of glsA (proto-glsA) changed to take on a new function, generating the new asymmetric division pathway. The dashed arrow indicates that the ancestral pathway may or may not exist in Volvox. (B) The evolution of the somatic cell fate appears to have involved gene duplication and then change (divergence) of one of the gene copies, regA. Scientists hypothesize that the ancestor of regA, proto-regA, acted in a stress-activated pathway (shaded gray) that led to the repression of growth and cell division. Duplication of proto-regA produced regA* and regA, both of which may initially have functioned in that same pathway, in an intermediate species. In Volvox, regA acts in a pathway (shaded green) that represses growth and cell division in response to developmental cues; while the descendent of regA*, known in Volvox as rlsD, might act in the stress-activated pathway that represses growth and cell division. Thus, regA could have gained its cell fate function because it changed in a way that permitted it to co-opt an existing pathway that repressed growth and cell division.", "true", "All rights reserved.", '650', '503', 'http://www.nature.com/nature_education');"><IMG src="/scitable/content/ne0000/ne0000/ne0000/ne0000/14458976/miller_fig3_2_1.jpg" title="" alt="A two-part schematic shows how physiological pathways in a unicellular ancestor could have been altered to perform different developmental functions in Volvox. Panel A shows the evolution of the gls-A pathway. Panel B shows the evolution of the regA pathway. In both panels, beige ovals represent the original pathways, which are present in both the unicellular ancestor and in Volvox. Green ovals represent the new pathways that are present in Volvox. The basic steps of the pathways are superimposed over the ovals." /></A><br/><div class="imageCaption pipeblack bold" style="padding:10px;" ><div class="imageCaption pipeblack bold nopad"><A class="pipeblack bold" href="#" onclick="dispOrigImg("/scitable/content/ne0000/ne0000/ne0000/ne0000/14458976/f3_miller_fig3.jpg", "Gene and pathway co-option and the origins of asymmetric cell division and cellular differentiation in Volvox", "true", "Figure 3", "(A) The function of glsA appears to have been co-opted without change from an unknown function in the unicellular ancestor of Volvox, so that it is now part of a pathway (shaded green) that is required for asymmetric cell division. This may have happened because some not yet identified gene (X) that acted in the same pathway (shaded gray) as the ancestor of glsA (proto-glsA) changed to take on a new function, generating the new asymmetric division pathway. The dashed arrow indicates that the ancestral pathway may or may not exist in Volvox. (B) The evolution of the somatic cell fate appears to have involved gene duplication and then change (divergence) of one of the gene copies, regA. Scientists hypothesize that the ancestor of regA, proto-regA, acted in a stress-activated pathway (shaded gray) that led to the repression of growth and cell division. Duplication of proto-regA produced regA* and regA, both of which may initially have functioned in that same pathway, in an intermediate species. In Volvox, regA acts in a pathway (shaded green) that represses growth and cell division in response to developmental cues; while the descendent of regA*, known in Volvox as rlsD, might act in the stress-activated pathway that represses growth and cell division. Thus, regA could have gained its cell fate function because it changed in a way that permitted it to co-opt an existing pathway that repressed growth and cell division.", "true", "All rights reserved.", '650', '503', 'http://www.nature.com/nature_education');"><img class="fleft borderNone" alt="View Full-Size Image" src="/scitable/natedimages/plus_box.gif"/>Figure 3: Gene and pathway co-option and the origins of asymmetric cell division and cellular differentiation in Volvox</A><div class="clear nomargin"></div></div><div class="imageLegend pipeblack nopad"><A style="font-weight: normal !important;" class="pipeblack" href="#" onclick="dispOrigImg("/scitable/content/ne0000/ne0000/ne0000/ne0000/14458976/f3_miller_fig3.jpg", "Gene and pathway co-option and the origins of asymmetric cell division and cellular differentiation in Volvox", "true", "Figure 3", "(A) The function of glsA appears to have been co-opted without change from an unknown function in the unicellular ancestor of Volvox, so that it is now part of a pathway (shaded green) that is required for asymmetric cell division. This may have happened because some not yet identified gene (X) that acted in the same pathway (shaded gray) as the ancestor of glsA (proto-glsA) changed to take on a new function, generating the new asymmetric division pathway. The dashed arrow indicates that the ancestral pathway may or may not exist in Volvox. (B) The evolution of the somatic cell fate appears to have involved gene duplication and then change (divergence) of one of the gene copies, regA. Scientists hypothesize that the ancestor of regA, proto-regA, acted in a stress-activated pathway (shaded gray) that led to the repression of growth and cell division. Duplication of proto-regA produced regA* and regA, both of which may initially have functioned in that same pathway, in an intermediate species. In Volvox, regA acts in a pathway (shaded green) that represses growth and cell division in response to developmental cues; while the descendent of regA*, known in Volvox as rlsD, might act in the stress-activated pathway that represses growth and cell division. Thus, regA could have gained its cell fate function because it changed in a way that permitted it to co-opt an existing pathway that repressed growth and cell division.", "true", "All rights reserved.", '650', '503', 'http://www.nature.com/nature_education');">(A) The function of glsA appears to have been co-opted without change from an unknown function in the unicellular ancestor of Volvox, so that it is now part of a pathway (shaded green) that is required for asymmetric cell division. This may have happened because some not yet identified gene (X) that acted in the same pathway (shaded gray) as the ancestor of glsA (proto-glsA) changed to take on a new function, generating the new asymmetric division pathway. The dashed arrow indicates that the ancestral pathway may or may not exist in Volvox. (B) The evolution of the somatic cell fate appears to have involved gene duplication and then change (divergence) of one of the gene copies, regA. Scientists hypothesize that the ancestor of regA, proto-regA, acted in a stress-activated pathway (shaded gray) that led to the repression of growth and cell division. Duplication of proto-regA produced regA* and regA, both of which may initially have functioned in that same pathway, in an intermediate species. In Volvox, regA acts in a pathway (shaded green) that represses growth and cell division in response to developmental cues; while the descendent of regA*, known in Volvox as rlsD, might act in the stress-activated pathway that represses growth and cell division. Thus, regA could have gained its cell fate function because it changed in a way that permitted it to co-opt an existing pathway that repressed growth and cell division.</A></div><div class="creditLine bold nopad fleft padright4px">© 2010 <a target="_blank" href="http://www.nature.com/nature_education" class="inlineLinks">Nature Education</a> All rights reserved. <a href='javascript:show_inform("Terms of Use", "You may reproduce this material, without modifications, in print or electronic form for your personal, non-commercial purposes or for non-commercial use in an educational environment.");' class="inlineLinks"><img class="nomargin infoImg nofloat" title="View Terms of Use" alt="View Terms of Use" height="13px" width="13px" src="/scitable/natedimages/info_icon.png" /></a></div><div class="clear"></div><div class="nomargin padleft4px pipeblack bold fright textalignleft"><a title="Panel A shows how the gls-A pathway in a unicellular ancestor might have been co-opted to be used in asymmetric division in Volvox. A beige oval at the top of panel A shows the gls-A pathway in a unicellular ancestor. The pathway is written from left to right as proto-glsA followed by an arrow, an \"X,\" another arrow, and, finally, the words \"unknown function.\" This original pathway can be interpreted as proto-glsA acting on an unidentified gene \"X,\" which then increases some unknown function. A bold arrow aimed downward points to the altered pathway in Volvox. The original pathway is still present in Volvox and is shown in a beige oval. The pathway is shown here as glsA followed by an arrow, an \"X*,\" a dashed arrow, and, finally, the words \"unknown function.\" A dashed arrow is used because it is not clear whether or not this ancestral pathway still exists in Volvox. In addition to this ancestral pathway, Volvox has a new pathway, which is shown in a green oval that overlaps with the beige oval and slopes downward to the right. This new pathway includes the first part of the ancestral pathway, glsA and \"X*,\" but a solid arrow leads from the \"X*\" to a new function, \"asymmetric division.\" Thus, glsA and \"X*\" are used in both the ancestral pathway and the new \"asymmetric division\" pathway. Panel B shows how duplication of an ancestral regA could lead to a new pathway in Volvox. A beige oval at the top of panel B shows the ancestral regA pathway. A stress stimulus acts on proto-regA, which inhibits growth and cell division. A bold arrow leads to an intermediate pathway in which regA is duplicated. This intermediate pathway is also shown in a beige oval. Two arrows lead from the stress signal to regA and regA*, both of which converge and inhibit growth and cell division. A second bold arrow points downward to the Volvox pathway. Volvox has the unicellular ancestral pathway, which is shown in a beige oval. Stress acts on rlsD (a descendant of regA*), which inhibits growth and cell division. A second pathway, which is shown in green below the first pathway and points upward to the right where it intersects with the first pathway, partially overlaps the end of the original pathway. In this second pathway, a developmental cue acts on regA, which inhibits growth and cell division. Thus, the duplication of regA has allowed two different stimuli (stress and a developmental cue) to lead to the inhibition of growth and cell division." href="javascript:void(0)" onclick="callNewShowInformAfterPublish("true","true","Y","/scitable/content/ne0000/ne0000/ne0000/ne0000/14458976/f3_miller_fig3.jpg", "Gene and pathway co-option and the origins of asymmetric cell division and cellular differentiation in Volvox", "Figure 3", "(A) The function of glsA appears to have been co-opted without change from an unknown function in the unicellular ancestor of Volvox, so that it is now part of a pathway (shaded green) that is required for asymmetric cell division. This may have happened because some not yet identified gene (X) that acted in the same pathway (shaded gray) as the ancestor of glsA (proto-glsA) changed to take on a new function, generating the new asymmetric division pathway. The dashed arrow indicates that the ancestral pathway may or may not exist in Volvox. (B) The evolution of the somatic cell fate appears to have involved gene duplication and then change (divergence) of one of the gene copies, regA. Scientists hypothesize that the ancestor of regA, proto-regA, acted in a stress-activated pathway (shaded gray) that led to the repression of growth and cell division. Duplication of proto-regA produced regA* and regA, both of which may initially have functioned in that same pathway, in an intermediate species. In Volvox, regA acts in a pathway (shaded green) that represses growth and cell division in response to developmental cues; while the descendent of regA*, known in Volvox as rlsD, might act in the stress-activated pathway that represses growth and cell division. Thus, regA could have gained its cell fate function because it changed in a way that permitted it to co-opt an existing pathway that repressed growth and cell division.", '650','http://www.nature.com/nature_education', "Panel A shows how the gls-A pathway in a unicellular ancestor might have been co-opted to be used in asymmetric division in <i>Volvox</i>. A beige oval at the top of panel A shows the gls-A pathway in a unicellular ancestor. The pathway is written from left to right as proto-glsA followed by an arrow, an \"X,\" another arrow, and, finally, the words \"unknown function.\" This original pathway can be interpreted as proto-glsA acting on an unidentified gene \"X,\" which then increases some unknown function. A bold arrow aimed downward points to the altered pathway in <i>Volvox</i>. The original pathway is still present in Volvox and is shown in a beige oval. The pathway is shown here as glsA followed by an arrow, an \"X*,\" a dashed arrow, and, finally, the words \"unknown function.\" A dashed arrow is used because it is not clear whether or not this ancestral pathway still exists in <i>Volvox</i>. In addition to this ancestral pathway, Volvox has a new pathway, which is shown in a green oval that overlaps with the beige oval and slopes downward to the right. This new pathway includes the first part of the ancestral pathway, glsA and \"X*,\" but a solid arrow leads from the \"X*\" to a new function, \"asymmetric division.\" Thus, glsA and \"X*\" are used in both the ancestral pathway and the new \"asymmetric division\" pathway. Panel B shows how duplication of an ancestral regA could lead to a new pathway in <i>Volvox</i>. A beige oval at the top of panel B shows the ancestral regA pathway. A stress stimulus acts on proto-regA, which inhibits growth and cell division. A bold arrow leads to an intermediate pathway in which regA is duplicated. This intermediate pathway is also shown in a beige oval. Two arrows lead from the stress signal to regA and regA*, both of which converge and inhibit growth and cell division. A second bold arrow points downward to the <i>Volvox </i>pathway. Volvox has the unicellular ancestral pathway, which is shown in a beige oval. Stress acts on rlsD (a descendant of regA*), which inhibits growth and cell division. A second pathway, which is shown in green below the first pathway and points upward to the right where it intersects with the first pathway, partially overlaps the end of the original pathway. In this second pathway, a developmental cue acts on regA, which inhibits growth and cell division. Thus, the duplication of regA has allowed two different stimuli (stress and a developmental cue) to lead to the inhibition of growth and cell division.")" class="inlineLinks"> Figure Detail </a></div><div class="clear"></div></div></div></div>But how can this be, when these genes play such critical roles in <i>Volvox</i> development? One way to think about how existing genes (like <i>glsA </i>and<i> invA</i>) might be incorporated or co-opted without change into a new developmental pathway is to consider the analogy of the gas-electric hybrid car. All cars have brakes. Hybrids are engineered to convert the potential energy generated during braking into electricity. The brakes on hybrids still function as brakes, but they have also been co-opted into a new "pathway" that generates electricity. Take away the brakes from a hybrid car and it no longer produces electricity. Think of <i>glsA</i> and <i>invA</i> as the brakes in this analogy; they likely have the same function they had in the unicellular ancestor of <i>Volvox</i>, but take them away and <i>Volvox</i> can no longer do asymmetric division or inversion (Figure 3A). </p> <p> </p> <p>Additional insights of a different sort have come from analysis of the somatic regenerator, or <i>regA</i>, gene. This gene is required for maintenance of the <span class="glossaryTermLink" onmouseover="showGloDescription("1473",this);return false;">somatic cell</span> fate in <i>Volvox</i>; <i>regA</i> mutant somatic cells develop normally at first, but instead of remaining somatic cells their entire lives and then eventually dying, as somatic cells usually do, they enlarge and regenerate as gonidia that eventually divide to produce new spheroids (Kirk 1998). Therefore <i>regA</i> somehow prevents somatic cells from growing and dividing, and keeps them from having the stem cell-like potential that gonidia possess. Think of <i>regA</i> as a <span class="ontologyTermLink" onmouseover="showDescription("184",this);return false;">tumor suppressor</span> gene that prevents the sort of uncontrolled growth that <span class="ontologyTermLink" onmouseover="showDescription("272",this);return false;">cancer</span> cells exhibit. On analyzing the <i>Volvox</i> and <i>Chlamydomonas</i> genomes to determine how many <i>regA</i>-like genes they have, investigators discovered that both algae have a large family of <span class="glossaryTermLink" onmouseover="showGloDescription("1121",this);return false;">paralogous</span> genes that encode proteins resembling the <i>regA</i> product. But using phylogenetic analyses and other methods, they also found that <i>Chlamydomonas</i> does not have a <i>regA</i> gene (Duncan <i>et al.</i> 2007). Why not? In addition, where did <i>regA</i> come from in the first place, and how did it come to take on its role as a master regulator of the somatic cell fate? </p> <p>Researchers found answers to some of these questions through further archaeological analysis of the <i>Chlamydomonas</i> and <i>Volvox</i> genomes. Their analyses revealed that <i>regA</i> likely was generated when a progenitor gene in the ancestor of <i>Chlamydomonas</i> and <i>Volvox</i> was inadvertently copied to produce two paralogous genes: one that eventually gave rise to <i>regA</i>, and one that gave rise to a related gene. While <i>Volvox</i> retained both <i>regA</i> and the other gene (a paralog), <i>Chlamydomonas</i> lost <i>regA</i>. In terms of how the <i>regA</i> function evolved, the modern-day versions of that other gene offer the best place to look for clues. Investigators studying this question found that<i> </i>the <i>Chlamydomonas</i> version of that <i>regA</i>-like gene, named <i>RLS1</i>, is turned on when <i>Chlamydomonas</i> is deprived of light or certain nutrients (Nedelcu 2009). This <span class="glossaryTermLink" onmouseover="showGloDescription("320",this);return false;">correlation</span> suggests that perhaps <i>RLS1</i> functions when cells are deprived of energy or nutrients. Since <i>regA</i> represses reproduction, it seems logical that <i>RLS1</i> probably does too. If this is the case, then the capacity to make different cell types may have evolved from a pathway that repressed growth and cell division in response to energy/nutrient deprivation. This could have happened when the gene that controls that pathway was copied and then used to co-opt the entire pathway to repress growth and division in a developmental context (Figure 3B). Think of the hybrid car analogy again, except in this case the entire stress response pathway is the brake system. Something like this — the <span class="glossaryTermLink" onmouseover="showGloDescription("252",this);return false;">co-option</span> of an existing genetic pathway so that it causes a cell to do something it would ordinarily do only under different circumstances — might explain, in general, how organisms evolve new cell types. </p> </div> </div> <div class="clear"></div> </div> </div> <div class="clear"></div> <div class="contentSection"> <div class="sectionTitle"> <h2> <SECTIONTITLE-1>Summary</SECTIONTITLE-1> </h2> </div> <div class="clear"></div> <div class="sectionParas"> <div class="paraArticle"> <div> <p>What <i>Volvox</i> and <i>Chlamydomonas</i> have taught us so far is that multicellularity, at least certain aspects of it, can evolve through relatively minor modifications of the unicellular blueprint (see Figure 3). Presumably not just any unicellular blueprint will do; no doubt the unicellular ancestor of <i>Volvox</i> already had many of the requisite genetic and cell biological raw materials for multicellularity: a multiple fission cell division program, a cell wall that could be modified into ECM, and possibly a stress response pathway that could be adapted to repress growth and division of a subset of cells, causing them to lose the ability to reproduce. But there is still much to learn. What new gene functions evolved to permit the evolution of asymmetric division and inversion? How did the other novel developmental traits of <i>Volvox</i> evolve? And are there similarities between the way multicellularity evolved in the volvocine algae and the way it evolved in other kinds of organisms? With the rate of recent progress in this field, answers to these questions, and more, should be on their way soon.</p> </div> </div> <div class="clear"></div> </div> </div> <div class="clear"></div> </div> <div style="display:none" id="tykChk"></div> <div id="divSources"> <h2 class="sourcesHead nopad">References and Recommended Reading</h2> <hr class="topFivebotTen"> <div class="articleSourcesList articleSourcesListBig nopad" id="sourcesDisp"> <p>Buss, L. <i>The Evolution of Individuality</i>. Princeton, NJ: Princeton University Press, 1987.</p> <p>Cheng, Q. <i>et al.</i> The role of GlsA in the evolution of asymmetric cell division in the green alga <i>Volvox carteri</i>. <i>Development Genes and Evolution</i> <b>213</b>, 328–335 (2003).</p> <p>Duncan, L. <i>et al.</i> The VARL gene family and the evolutionary origins of the master cell-type regulatory gene, <i>regA</i>, in <i>Volvox carteri</i>. <i>Journal of Molecular Evolution</i> <b>65</b>, 1–11 (2007).</p> <p>Herron, M. D. <i>et al.</i> Triassic origin and early radiation of multicellular volvocine algae. <i>PNAS</i> <b>106</b>, 3254–3258 (2009).</p> <p>Herron, M. D. & Michod, R. E. Evolution of complexity in the volvocine algae: Transitions in individuality through Darwin's eye. <i>Evolution</i> <b>62</b>, 436–451 (2008).</p> <p>King, N. The unicellular ancestry of animal development. <i>Developmental Cell</i> <b>7</b>, 313–325 (2004).</p> <p>Kirk, D. L. Germ-soma differentiation in <i>Volvox</i>. <i>Developmental Biology</i> <b>238</b>, 213–223 (2001).</p> <p>Kirk, D. L. A twelve-step program for evolving multicellularity and a division of labor. <i>BioEssays</i> <b>27</b>, 299–310 (2005).</p> <p>Kirk, D. L. <i>Volvox: The Molecular Genetic Origins of Multicellularity and Cellular Differentiation</i>. Cambridge: Cambridge University Press, 1998.</p> <p>Kirk, D. L. & Nishii, I. <i>Volvox carteri</i> as a model for studying the genetic and cytological control of morphogenesis. <i>Development, Growth & Differentiation</i> <b>43</b>, 621–631 (2001).</p> <p>Miller, S. M. & Kirk, D. L. <i>glsA</i>, a <i>Volvox</i> gene required for asymmetric division and germ cell specification, encodes a chaperone-like protein. <i>Development</i> <b>126</b>, 649–658 (1999).</p> <p>Nedelcu, A. M. Environmentally induced responses co-opted for reproductive altruism. <i>Biology Letters</i> <b>5</b>, 805–808 (2009).</p> <p>Nishii, I. <i>et al.</i> A kinesin, <i>invA</i>, plays an essential role in <i>Volvox </i>morphogenesis. <i>Cell</i> <b>113</b>, 743–753 (2003).</p> <p>Peterson, K. J. & Butterfield, N. J. Origin of the Eumetazoa: Testing ecological predictions of molecular clocks against the Proterozoic fossil record. <i>PNAS</i> <b>102</b>, 9547–9552 (2005).</p> <p>Prochnik, S. E. <i>et al.</i> Genomic analysis of organismal complexity in the multicellular green alga <i>Volvox carteri</i>. <i>Science</i> <b>329</b>, 223–226 (2010.)</p> <p>Sanderson, M. J. Molecular data from 27 proteins do not support a Precambrian origin of land plants. <i>American Journal of Botany</i> <b>90</b>, 954–956 (2003).</p> <p>Ueki, N. & Nishii, I. Controlled enlargement of the glycoprotein vesicle surrounding a <i>Volvox</i> embryo requires the InvB nucleotide-sugar transporter and is required for normal morphogenesis. <i>Plant Cell</i> <b>21</b>, 1166–1181 (2009).</p> <p>Ueki, N. & Nishii, I. <i>Idaten</i> is a new cold-inducible transposon of <i>Volvox</i> <i>carteri</i> that can be used for tagging developmentally important genes. <i>Genetics</i> <b>180</b>, 1343–1353 (2008).</p> </div> </div> </div> </div></div> </div> <div class="hideForPrint"> <div class="clear"></div> <ul class="topicDetailsLeft hideForPrint "> <li><a href="#TB_inline?height=300&width=400&inlineId=trOutLine" title="Outline of this Article" class="thickbox inlineLinks">Outline</a></li> <li id="bar1">|</li> <li id="keywordHide"><a onMouseOver="return displayOnMouseOverText(this);" href="#url" id="keywordLnkTR" name="keywordLnkTR" onClick="loadContentTagsOnStartUp(event, 'keywordLnkTR'); return false;" class="inlineLinks">Keywords</a></li> <li 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