Editing Selfish genetic element (section)

=== Segregation distorters ===
[[File:Segregation distorters.png|thumb|Segregation distorters (here shown in red) get transmitted to >50% of the gametes.]]
Some selfish genetic elements manipulate the [[W:Transmission (genetics)|genetic transmission process]]  to their own advantage, and so end up being overrepresented in the gametes. Such distortion can occur in various ways, and the umbrella term that encompasses all of them is segregation distortion. Some elements can preferentially be transmitted in egg cells as opposed to [[polar body|polar bodies]] during meiosis, where only the former will be fertilized and transmitted to the next generation. Any gene that can manipulate the odds of ending up in the egg rather than the polar body will have a transmission advantage, and will increase in frequency in a population.<ref name=":21" />

Segregation distortion can happen in several ways. When this process occurs during meiosis it is referred to as [[W:meiotic drive|meiotic drive]]. Many forms of segregation distortion occur in male gamete formation, where there is differential mortality of spermatids during the process of sperm maturation or [[W:Spermiogenesis|spermiogenesis]]. The segregation distorter (SD) in ''Drosophila melanogaster'' is the best studied example, and it involves a nuclear envelope protein Ran-GAP and the X-linked repeat array called Responder (Rsp), where the SD allele of Ran-GAP favors its own transmission only in the presence of a Rsp<sup>sensitive</sup> allele on the homologous chromosome.<ref>{{cite journal | vauthors = Brittnacher JG, Ganetzky B | title = On the components of segregation distortion in Drosophila melanogaster. III. Nature of enhancer of SD | journal = Genetics | volume = 107 | issue = 3 | pages = 423–34 | date = July 1984 | doi = 10.1093/genetics/107.3.423 | pmid = 6428976 | pmc = 1202333 }}</ref><ref name=":26">{{cite journal | vauthors = Brittnacher JG, Ganetzky B | title = On the Components of Segregation Distortion in Drosophila melanogaster. II. Deletion Mapping and Dosage Analysis of the SD Locus | journal = Genetics | volume = 103 | issue = 4 | pages = 659–73 | date = April 1983 | doi = 10.1093/genetics/103.4.659 | pmid = 17246120 | pmc = 1202047 }}</ref><ref>{{cite journal | vauthors = Brittnacher JG, Ganetzky B | title = On the components of segregation distortion in Drosophila melanogaster. IV. Construction and analysis of free duplications for the Responder locus | journal = Genetics | volume = 121 | issue = 4 | pages = 739–50 | date = April 1989 | doi = 10.1093/genetics/121.4.739 | pmid = 2498160 | pmc = 1203657 }}</ref><ref>{{cite journal | vauthors = Powers PA, Ganetzky B | title = On the components of segregation distortion in Drosophila melanogaster. V. Molecular analysis of the Sd locus | journal = Genetics | volume = 129 | issue = 1 | pages = 133–44 | date = September 1991 | doi = 10.1093/genetics/129.1.133 | pmid = 1936954 | pmc = 1204561 }}</ref><ref>{{cite journal | vauthors = Larracuente AM, Presgraves DC | title = The selfish Segregation Distorter gene complex of Drosophila melanogaster | journal = Genetics | volume = 192 | issue = 1 | pages = 33–53 | date = September 2012 | pmid = 22964836 | pmc = 3430544 | doi = 10.1534/genetics.112.141390 }}</ref> SD acts to kill RSP<sup>sensitive</sup> sperm, in a post-meiotic process (hence it is not strictly speaking meiotic drive). Systems like this can have interesting rock-paper-scissors dynamics, oscillating between the SD-RSP<sup>insensitive</sup>, SD+-RSP<sup>insensitive</sup> and SD+-RSP<sup>sensitive</sup> haplotypes. The SD-RSP<sup>sensitive</sup> haplotype is not seen because it essentially commits suicide.<ref name=":26" />

When segregation distortion acts on sex chromosomes, they can skew the sex ratio. The SR system in ''Drosophila pseudoobscura'', for example, is on the X chromosome, and XSR/Y males produce only daughters, whereas females undergo normal meiosis with Mendelian proportions of gametes.<ref name=":11">{{cite journal | vauthors = Curtsinger JW, Feldman MW | title = Experimental and Theoretical Analysis of the "Sex-Ratio" Polymorphism in Drosophila pseudoobscura | journal = Genetics | volume = 94 | issue = 2 | pages = 445–66 | date = February 1980 | doi = 10.1093/genetics/94.2.445 | pmid = 17249004 | pmc = 1214151 }}</ref><ref>{{cite journal | vauthors = Curtsinger JW | title = Artificial selection on the sex ratio in Drosophila pseudoobscura | journal = Journal of Heredity | date = 1981 | volume = 72 | issue = 6 | pages = 377–381 | doi = 10.1093/oxfordjournals.jhered.a109535 }}</ref> Segregation distortion systems would drive the favored allele to fixation, except that most of the cases where these systems have been identified have the driven allele opposed by some other selective force. One example is the lethality of the t-haplotype in mice,<ref name=":14">{{cite journal | vauthors = Lyon MF | title = Transmission ratio distortion in mice | journal = Annual Review of Genetics | volume = 37 | pages = 393–408 | date = 2003 | pmid = 14616067 | doi = 10.1146/annurev.genet.37.110801.143030 }}</ref> another is the effect on male fertility of the Sex Ratio system in ''D. pseudoobscura''.<ref name=":11" />