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class="firstHeading mw-first-heading"><span class="mw-page-title-main">Evolution of sex-determining mechanisms</span></h1> <div id="p-lang-btn" class="vector-dropdown mw-portlet mw-portlet-lang" > <input type="checkbox" id="p-lang-btn-checkbox" role="button" aria-haspopup="true" data-event-name="ui.dropdown-p-lang-btn" class="vector-dropdown-checkbox mw-interlanguage-selector" aria-label="This article exist only in this language. 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id="mw-content-subtitle"></div></div> <div id="mw-content-text" class="mw-body-content"><div class="mw-content-ltr mw-parser-output" lang="en" dir="ltr"><p>The <b>evolution of sex-determining mechanisms</b>, characterized by the evolutionary transition to genetic sex determination or temperature-dependent sex determination from the opposite mechanism, has frequently and readily occurred among multiple taxa across a transitionary continuum. </p> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Pogonavitticeps.jpg" class="mw-file-description"><img alt="Bearded dragon lizard close-up." src="//upload.wikimedia.org/wikipedia/commons/thumb/c/c4/Pogonavitticeps.jpg/220px-Pogonavitticeps.jpg" decoding="async" width="220" height="165" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/c/c4/Pogonavitticeps.jpg/330px-Pogonavitticeps.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/c/c4/Pogonavitticeps.jpg/440px-Pogonavitticeps.jpg 2x" data-file-width="1024" data-file-height="768" /></a><figcaption>The <a href="/wiki/Central_bearded_dragon" title="Central bearded dragon">central bearded dragon</a> is a model organism for sex reversal studies and research on transitions from genetic sex determination to temperature-dependent sex determination.</figcaption></figure> <p>Sex-determining mechanisms include genetic sex determination, where sex is determined by genes on <a href="/wiki/Sex_chromosome" title="Sex chromosome">sex chromosomes</a>, and <a href="/wiki/Environmental_sex_determination" title="Environmental sex determination">environmental sex determination</a>/<a href="/wiki/Temperature-dependent_sex_determination" title="Temperature-dependent sex determination">temperature-dependent sex determination</a>, where sex is permanently fixed by environmental cues after <a href="/wiki/Fertilisation" title="Fertilisation">fertilization</a>.<sup id="cite_ref-:0_1-0" class="reference"><a href="#cite_note-:0-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-:3_2-0" class="reference"><a href="#cite_note-:3-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> Evolutionary transitions between these mechanisms are frequently driven by <a href="/wiki/Sex_reversal" title="Sex reversal">sex reversal</a>, a phenomenon where environmental overrides produce organisms with discordant genotypic and phenotypic sex.<sup id="cite_ref-:0_1-1" class="reference"><a href="#cite_note-:0-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> </p> <meta property="mw:PageProp/toc" /> <div class="mw-heading mw-heading2"><h2 id="Evolutionary_transitions">Evolutionary transitions</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Evolution_of_sex-determining_mechanisms&action=edit&section=1" title="Edit section: Evolutionary transitions"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Technological advances in <a href="/wiki/Comparative_genomics" title="Comparative genomics">comparative chromosome mapping</a> and <a href="/wiki/Molecular_cytogenetics" title="Molecular cytogenetics">molecular cytogenetics</a> have advanced understanding of the many transitions between sex-determining modes.<sup id="cite_ref-:4_3-0" class="reference"><a href="#cite_note-:4-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> Threshold changes in gene expression for either male- or female-determining factors is enough to change modes of sex determination, as these thresholds are heritable, and more labile sex-determining mechanisms can be advantageous in unpredictable or changing environments.<sup id="cite_ref-:1_4-0" class="reference"><a href="#cite_note-:1-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> </p><p>Evolutionary transitions from genetic sex determination to temperature-dependent sex determination are possible as long as there is temperature sensitivity in the genetic system (on the <a href="/wiki/Sex_chromosome" title="Sex chromosome">sex chromosomes</a>) and selection occurs on those sensitivity levels.<sup id="cite_ref-:5_5-0" class="reference"><a href="#cite_note-:5-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-:4_3-1" class="reference"><a href="#cite_note-:4-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-:3_2-1" class="reference"><a href="#cite_note-:3-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> All of these drivers of transitions between sex-determining mechanisms are due to a suggested novel locus changing <a href="/wiki/Fitness_(biology)" title="Fitness (biology)">fitness</a>, which selection then acts upon. Current sex-determining systems must be destabilized in order to drive the evolution of a new system. When evolving from a genetic <a href="/wiki/XY_sex-determination_system" title="XY sex-determination system">XX/XY</a> or <a href="/wiki/ZW_sex-determination_system" title="ZW sex-determination system">ZZ/ZW</a> system to a temperature-dependent system, temperature-dependent sex determination naturally avoids nonviable YY or WW genotypes.<sup id="cite_ref-:5_5-1" class="reference"><a href="#cite_note-:5-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> The evolution of temperature-dependent sex determination is considered to be adaptive in most hypotheses, with a few suggest neutral or quasi-neutral evolution.<sup id="cite_ref-:0_1-2" class="reference"><a href="#cite_note-:0-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> </p><p>Originally, these transitions were believed to require the crossing of a <a href="/wiki/Fitness_landscape" title="Fitness landscape">fitness valley</a> (a barrier to adaptation that a population must pass through in order to reach an advantageous stage<sup id="cite_ref-6" class="reference"><a href="#cite_note-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup>), where the current sex-determining system was ineffective, degenerated, or had reduced viability.<sup id="cite_ref-:5_5-2" class="reference"><a href="#cite_note-:5-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> However, recent<sup class="noprint Inline-Template" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Manual_of_Style/Dates_and_numbers#Chronological_items" title="Wikipedia:Manual of Style/Dates and numbers"><span title="The time period mentioned near this tag is ambiguous. (December 2022)">when?</span></a></i>]</sup> studies have found that these transitions can occur rapidly and easily without crossing a fitness valley, and stable, mixed-mechanism endpoints in sex-determining mechanisms that incorporate both genetic and temperature dependence can occur. Research has suggested sex-determining systems to be a continuum rather than a dichotomy, with placement for a particular system on that continuum being based on the value of the male-determining threshold.<sup id="cite_ref-:5_5-3" class="reference"><a href="#cite_note-:5-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-:4_3-2" class="reference"><a href="#cite_note-:4-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-:6_7-0" class="reference"><a href="#cite_note-:6-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup> </p><p><a href="/wiki/Chromosomal_inversion" title="Chromosomal inversion">Chromosomal inversion</a> is another suggested driver of evolution between mechanisms, as inversions are known to be associated with hybrid incompatibility.<sup id="cite_ref-:0_1-3" class="reference"><a href="#cite_note-:0-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Ancestral_form">Ancestral form</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Evolution_of_sex-determining_mechanisms&action=edit&section=2" title="Edit section: Ancestral form"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Phylogenetic analyses have determined that both genetic sex determination and temperature-dependent sex determination have <a href="/wiki/Convergent_evolution" title="Convergent evolution">evolved multiple times</a>, with multiple independent origins, and these transitions are not as conserved and linear as once thought.<sup id="cite_ref-:2_8-0" class="reference"><a href="#cite_note-:2-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup> Temperature-dependent sex determination is believed by many to have been the ancestral sex determining mechanism.<sup id="cite_ref-:6_7-1" class="reference"><a href="#cite_note-:6-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup> In turtles, one study suggests that genetic sex determination has originated at least six times: in (1) <a href="/wiki/Trionychidae" title="Trionychidae">trionychids</a>, (2) <a href="/wiki/Chelidae" title="Chelidae">chelids</a>, (3) <a href="/w/index.php?title=Staurotypids&action=edit&redlink=1" class="new" title="Staurotypids (page does not exist)">staurotypids</a>, (4) <a href="/wiki/Emydidae" title="Emydidae">emydids</a>, and (5 and 6) twice in <a href="/wiki/Bataguridae" class="mw-redirect" title="Bataguridae">batagurids</a>.<sup id="cite_ref-:0_1-4" class="reference"><a href="#cite_note-:0-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> Temperature-dependent sex determination is believed to have evolved at least five times in lizards.<sup id="cite_ref-:5_5-4" class="reference"><a href="#cite_note-:5-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> Nine-spined sticklebacks (<i><a href="/wiki/Ninespine_stickleback" title="Ninespine stickleback">Pungitius pungitius</a></i>) displayed an ancestral ZW system, but chromosome incompatibility and inversion due to hybrids led to the evolution of their current XY system.<sup id="cite_ref-:2_8-1" class="reference"><a href="#cite_note-:2-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup> The haphazard distribution of both genetic sex determination and temperature-dependent sex determination among reptiles suggests frequent transitions that are likely easy to achieve. The ability for these frequent transitions between diverse sex determining mechanisms is believed to be driven by an even greater diversity of transitional stages.<sup id="cite_ref-:7_9-0" class="reference"><a href="#cite_note-:7-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup> </p><p>In other vertebrates, genetic sex determination has been found to be the ancestral condition, and there is documentation of temperature-dependent sex determination origin in fishes.<sup id="cite_ref-:0_1-5" class="reference"><a href="#cite_note-:0-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> Temperature-dependent sex determination is proposed to be an adaptation to particular life histories,<sup id="cite_ref-:6_7-2" class="reference"><a href="#cite_note-:6-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup> allowing embryonic lability according to the surrounding environment.<sup id="cite_ref-:8_10-0" class="reference"><a href="#cite_note-:8-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup> Most current evidence suggests ancestral temperature-dependent sex determination, though ancestral genetic sex determination in reptiles is possible.<sup id="cite_ref-:0_1-6" class="reference"><a href="#cite_note-:0-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> Some scientists do not recommend ordering sex-determining mechanisms, as gains and losses of either mechanism are equally plausible. In the absence of conclusive information, parsimony is suggested to be the best approach for determining ancestral forms.<sup id="cite_ref-:0_1-7" class="reference"><a href="#cite_note-:0-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Sex_reversal">Sex reversal</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Evolution_of_sex-determining_mechanisms&action=edit&section=3" title="Edit section: Sex reversal"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p><a href="/wiki/Sex_reversal" title="Sex reversal">Sex reversal</a>, an environmental override where phenotypic sex is discordant to genotypic sex, is a powerful and rapid driver of transitions between these mechanisms.<sup id="cite_ref-:1_4-1" class="reference"><a href="#cite_note-:1-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-:3_2-2" class="reference"><a href="#cite_note-:3-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> Sex reversal is heritable and variable, indicating evolution of the threshold can and does occur. This is thought to be a response to reduce extreme sex ratio bias. Rapidly changing environments can lead to unbalanced sex ratios and possible extinction if the threshold is not allowed to evolve over multiple generations.<sup id="cite_ref-:1_4-2" class="reference"><a href="#cite_note-:1-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-:6_7-3" class="reference"><a href="#cite_note-:6-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup> </p><p>Reptiles have experienced numerous transitions between genetic sex determination and temperature-dependent sex determination, even in closely related species, with sex reversal being a large driver of those transitions.<sup id="cite_ref-:0_1-8" class="reference"><a href="#cite_note-:0-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> In species with a ZW sex chromosome system, such as central bearded dragons (<i><a href="/wiki/Central_bearded_dragon" title="Central bearded dragon">Pogona vitticeps</a></i>), sex reversal produces ZZf females. That subsequent loss of the W chromosome in those individuals eventually leads to the loss of the W chromosome in the population, shifting the species towards temperature-dependent sex determination. When exposed to the appropriate temperatures, females no longer need a W chromosome, which allows specific, natural, and wild shifts in sex determining mechanisms.<sup id="cite_ref-:0_1-9" class="reference"><a href="#cite_note-:0-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> This process can result in skewed sex ratios, but selection for rarer sex tends to occur,<sup id="cite_ref-:8_10-1" class="reference"><a href="#cite_note-:8-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup> with some species and individuals resisting sex reversal despite being at the threshold temperature. Resistance to the process of sex reversal can lead to evolutionary changes in thermal threshold in future populations.<sup id="cite_ref-11" class="reference"><a href="#cite_note-11"><span class="cite-bracket">[</span>11<span class="cite-bracket">]</span></a></sup> This evolution in the threshold, both increases and decreases, is often what promotes transitions between both sex-determining systems. These threshold shifts can also occur due to <a href="/wiki/Genetic_drift" title="Genetic drift">genetic drift</a>, <a href="/wiki/Pleiotropy" title="Pleiotropy">pleiotropic benefits</a>, sex ratio selection, <a href="/wiki/Sexual_antagonism" class="mw-redirect" title="Sexual antagonism">sexual antagonism</a>, or <a href="/wiki/Genetic_hitchhiking" title="Genetic hitchhiking">hitchhiking genes</a>.<sup id="cite_ref-:6_7-4" class="reference"><a href="#cite_note-:6-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-:2_8-2" class="reference"><a href="#cite_note-:2-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup> Sex-reversed female bearded dragons can lay twice as many eggs per year as genetic ZW females, suggesting an immediate fitness advantage to sex reversal that could be another driver of evolutionary transitions.<sup id="cite_ref-12" class="reference"><a href="#cite_note-12"><span class="cite-bracket">[</span>12<span class="cite-bracket">]</span></a></sup> </p><p>Evolutionary transitions from ZZ/ZW to temperature-dependent sex determination involve an increase in the thermal threshold, which results in sex reversal of genetic males and ultimately the loss of the W allele, which prevents the nonviable WW combination from occurring. Transitioning from XX/XY to temperature-dependent sex determination requires lowering of the threshold. Only XX embryos can be sex-reversed, resulting in the loss of the Y allele and preventing the nonviable YY combination. Transitions move rapidly through intermediate states, and research has shown that evolutionary transitions from genetic sex determination to temperature-dependent sex determination should be simple under the right selection pressures and varied temperatures.<sup id="cite_ref-:5_5-5" class="reference"><a href="#cite_note-:5-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> However, most transitions from genetic sex determination to temperature-dependent sex determination occur rapidly through intermediate forms rather that persisting in those states, with the discrete separations representing the stable states of sex determining mechanisms.<sup id="cite_ref-:5_5-6" class="reference"><a href="#cite_note-:5-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> Sex reversal, in particular, is considered to be a common transitional step along this continuum.<sup id="cite_ref-:7_9-1" class="reference"><a href="#cite_note-:7-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="References">References</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Evolution_of_sex-determining_mechanisms&action=edit&section=4" title="Edit section: References"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1239543626">.mw-parser-output .reflist{margin-bottom:0.5em;list-style-type:decimal}@media screen{.mw-parser-output .reflist{font-size:90%}}.mw-parser-output .reflist .references{font-size:100%;margin-bottom:0;list-style-type:inherit}.mw-parser-output .reflist-columns-2{column-width:30em}.mw-parser-output .reflist-columns-3{column-width:25em}.mw-parser-output .reflist-columns{margin-top:0.3em}.mw-parser-output .reflist-columns ol{margin-top:0}.mw-parser-output .reflist-columns li{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .reflist-upper-alpha{list-style-type:upper-alpha}.mw-parser-output .reflist-upper-roman{list-style-type:upper-roman}.mw-parser-output .reflist-lower-alpha{list-style-type:lower-alpha}.mw-parser-output .reflist-lower-greek{list-style-type:lower-greek}.mw-parser-output .reflist-lower-roman{list-style-type:lower-roman}</style><div class="reflist"> <div class="mw-references-wrap mw-references-columns"><ol class="references"> <li id="cite_note-:0-1"><span class="mw-cite-backlink">^ <a href="#cite_ref-:0_1-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-:0_1-1"><sup><i><b>b</b></i></sup></a> <a href="#cite_ref-:0_1-2"><sup><i><b>c</b></i></sup></a> <a href="#cite_ref-:0_1-3"><sup><i><b>d</b></i></sup></a> <a href="#cite_ref-:0_1-4"><sup><i><b>e</b></i></sup></a> <a href="#cite_ref-:0_1-5"><sup><i><b>f</b></i></sup></a> <a href="#cite_ref-:0_1-6"><sup><i><b>g</b></i></sup></a> <a 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