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Caenorhabditis elegans
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=== Sex determination === ''C. elegans'' are mostly hermaphroditic organisms, producing both sperms and [[oocyte]]s.<ref name=":7">{{cite journal | vauthors = Starostina NG, Lim JM, Schvarzstein M, Wells L, Spence AM, Kipreos ET | title = A CUL-2 ubiquitin ligase containing three FEM proteins degrades TRA-1 to regulate C. elegans sex determination | journal = Developmental Cell | volume = 13 | issue = 1 | pages = 127–39 | date = July 2007 | pmid = 17609115 | doi = 10.1016/j.devcel.2007.05.008 | pmc = 2064902 }}</ref> Males do occur in the population in a rate of approximately 1 in 200 hermaphrodites, but the two sexes are highly differentiated.<ref name="Haag_2005" /><ref name=":4">{{Cite web|title=Handbook - Male Introduction|url=https://www.wormatlas.org/male/introduction/mainframe.htm|access-date=2021-03-30|website=www.wormatlas.org}}</ref> Males differ from their hermaphroditic counterparts in that they are smaller and can be identified through the shape of their tail.<ref name=":4" /> ''C.elegans'' reproduce through a process called [[androdioecy]]. This means that they can reproduce in two ways: either through self-fertilization in hermaphrodites or through hermaphrodites breeding with males. Males are produced through [[Nondisjunction|non-disjunction]] of the X chromosomes during meiosis. The worms that reproduce through self-fertilization are at risk for high [[linkage disequilibrium]], which leads to lower genetic diversity in populations and an increase in accumulation of [[Mutation|deleterious]] alleles.<ref name=":3">{{cite journal | vauthors = Frézal L, Félix MA | title = C. elegans outside the Petri dish | journal = eLife | volume = 4 | pages = e05849 | date = March 2015 | pmid = 25822066 | doi = 10.7554/eLife.05849 | pmc = 4373675 | doi-access = free }}</ref> In ''C. elegans'', somatic sex determination is attributed to the ''[[TRA (gene)|tra-1]]'' gene.<ref name=":5">{{Cite journal|date=1999-08-06|title=The TRA-1A Sex Determination Protein of C. elegans Regulates Sexually Dimorphic Cell Deaths by Repressing the egl-1 Cell Death Activator Gene|journal=Cell|language=en|volume=98|issue=3|pages=317–327|doi=10.1016/S0092-8674(00)81961-3|issn=0092-8674|last1=Conradt|first1=Barbara|last2=Horvitz|first2=H.Robert|pmid=10458607|s2cid=14951719|doi-access=free}}</ref> The ''tra-1'' is a gene within the TRA-1 transcription factor sex determination pathway that is regulated post-transcriptionally and works by promoting female development.<ref name=":5" /> In hermaphrodites (XX), there are high levels of ''tra-1'' activity, which produces the female reproductive system and inhibits male development.<ref name=":7" /><ref name=":5" /> At a certain time in their life cycle, one day before adulthood, hermaphrodites can be identified through the addition of a vulva near their tail. In males (XO), there are low levels of ''tra-1'' activity, resulting in a male reproductive system.<ref name=":5" /> Recent research has shown that there are three other genes, ''fem-1, fem-2, and fem-3,'' that negatively regulate the TRA-1 pathway and act as the final determiner of sex in ''C. elegans''.<ref name=":7" /> ==== Evolution ==== The sex determination system in ''C. elegans'' is a topic that has been of interest to scientists for years.<ref name="Haag_2005">{{cite journal | vauthors = Haag ES | title = The evolution of nematode sex determination: C. elegans as a reference point for comparative biology | journal = WormBook: The Online Review of C. Elegans Biology | volume = | issue = | pages = 1–14 | date = December 2005 | pmid = 18050417 | pmc = 4781019 | doi = 10.1895/wormbook.1.120.1 }}</ref> Since they are used as a model organism, any information discovered about the way their sex determination system might have evolved could further the same evolutionary biology research in other organisms. After almost 30 years of research, scientists have begun to put together the pieces in the evolution of such a system.<ref name="Haag_2005" /> What they have discovered is that there is a complex pathway involved that has several layers of regulation.<ref name="Haag_2005" /> The closely related organism ''[[Caenorhabditis briggsae]]'' has been studied extensively and its whole genome sequence has helped put together the missing pieces in the evolution of ''C. elegans'' sex determination.<ref name="Haag_2005" /> It has been discovered that two genes have assimilated, leading to the proteins XOL-1 and MIX-1 having an effect on sex determination in ''C. elegans'' as well.<ref name="Haag_2005" /> Mutations in the XOL-1 pathway leads to feminization in ''C. elegans .''<ref>{{cite journal | vauthors = Miller LM, Plenefisch JD, Casson LP, Meyer BJ | title = xol-1: a gene that controls the male modes of both sex determination and X chromosome dosage compensation in C. elegans | language = English | journal = Cell | volume = 55 | issue = 1 | pages = 167–83 | date = October 1988 | pmid = 3167975 | doi = 10.1016/0092-8674(88)90019-0 | s2cid = 5005906 }}</ref> The ''mix-1'' gene is known to hypoactivate the X chromosome and regulates the morphology of the male tail in ''C. elegans.''<ref>{{Cite web|title=mix-1 (gene) - WormBase : Nematode Information Resource|url=https://wormbase.org/species/c_elegans/gene/WBGene00003367#0-9f-10|access-date=2021-04-23|website=wormbase.org}}</ref> Looking at the nematode as a whole, the male and hermaphrodite sex likely evolved from parallel evolution.<ref name="Haag_2005" /> Parallel evolution is defined as similar traits evolving from an ancestor in similar conditions; simply put, the two species evolve in similar ways over time. An example of this would be [[marsupial]] and [[Placentalia|placental]] mammals. Scientists have also hypothesized that hermaphrodite [[Asexual reproduction|asexual]] reproduction, or "selfing", could have evolved convergently by studying species similar to ''C. elegans''<ref name="Haag_2005" /> Other studies on the sex determination evolution suggest that genes involving sperm evolve at the faster rate than female genes.<ref name=":9">{{cite journal | vauthors = Cutter AD, Ward S | title = Sexual and temporal dynamics of molecular evolution in C. elegans development | journal = Molecular Biology and Evolution | volume = 22 | issue = 1 | pages = 178–88 | date = January 2005 | pmid = 15371532 | doi = 10.1093/molbev/msh267 | doi-access = free }}</ref> However, sperm genes on the X chromosome have reduced evolution rates. Sperm genes have short coding sequences, high codon bias, and disproportionate representation among [[orphan gene]]s.<ref name=":9" /> These characteristics of sperm genes may be the reason for their high rates of evolution and may also suggest how sperm genes evolved out of hermaphrodite worms. Overall, scientists have a general idea of the sex determination pathway in ''C. elegans'', however, the evolution of how this pathway came to be is not yet well defined.
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