Editing Selfish genetic element (section)

=== Selfish mitochondria ===
Genomic conflicts often arise because not all genes are inherited in the same way. Probably the best example of this is the conflict between [[Uniparental inheritance|uniparentally]] (usually but not always, maternally) inherited mitochondrial and biparentally inherited nuclear genes. Indeed, one of the earliest clear statements about the possibility of genomic conflict was made by the English botanist Dan Lewis in reference to the conflict between maternally inherited mitochondrial and biparentally inherited nuclear genes over sex allocation in [[Hermaphrodite|hermaphroditic]] plants.<ref name=":7" />

A single cell typically contains multiple mitochondria, creating a situation for competition over transmission. Uniparental inheritance has been suggested to be a way to reduce the opportunity for selfish mitochondria to spread, as it ensures all mitochondria share the same genome, thus removing the opportunity for competition.<ref name=":20" /><ref>{{cite journal | vauthors = Hadjivasiliou Z, Lane N, Seymour RM, Pomiankowski A | title = Dynamics of mitochondrial inheritance in the evolution of binary mating types and two sexes | journal = Proceedings. Biological Sciences | volume = 280 | issue = 1769 | pages = 20131920 | date = October 2013 | pmid = 23986113 | pmc = 3768323 | doi = 10.1098/rspb.2013.1920 }}</ref><ref>{{cite journal | vauthors = Law R, Hutson V | title = Intracellular symbionts and the evolution of uniparental cytoplasmic inheritance | journal = Proceedings. Biological Sciences | volume = 248 | issue = 1321 | pages = 69–77 | date = April 1992 | pmid = 1355912 | doi = 10.1098/rspb.1992.0044 | bibcode = 1992RSPSB.248...69L | s2cid = 45755461 }}</ref> This view remains widely held, but has been challenged.<ref>{{cite journal | vauthors = Christie JR, Schaerf TM, Beekman M | title = Selection against heteroplasmy explains the evolution of uniparental inheritance of mitochondria | journal = PLOS Genetics | volume = 11 | issue = 4 | pages = e1005112 | date = April 2015 | pmid = 25880558 | pmc = 4400020 | doi = 10.1371/journal.pgen.1005112 | doi-access = free }}</ref> Why inheritance ended up being maternal, rather than paternal, is also much debated, but one key hypothesis is that the mutation rate is lower in female compared to male gametes.<ref>{{cite journal | vauthors = Greiner S, Sobanski J, Bock R | title = Why are most organelle genomes transmitted maternally? | journal = BioEssays | volume = 37 | issue = 1 | pages = 80–94 | date = January 2015 | pmid = 25302405 | pmc = 4305268 | doi = 10.1002/bies.201400110 }}</ref>

The conflict between mitochondrial and nuclear genes is especially easy to study in flowering plants.<ref>{{cite journal | vauthors = Liu XQ, Xu X, Tan YP, Li SQ, Hu J, Huang JY, Yang DC, Li YS, Zhu YG | title = Inheritance and molecular mapping of two fertility-restoring loci for Honglian gametophytic cytoplasmic male sterility in rice (Oryza sativaL.) | journal = Molecular Genetics and Genomics | volume = 271 | issue = 5 | pages = 586–94 | date = June 2004 | pmid = 15057557 | doi = 10.1007/s00438-004-1005-9 | s2cid = 1898106 }}</ref><ref>{{cite journal | vauthors = Schnable PS, Wise RP | title = The molecular basis of cytoplasmic male sterility and fertility restoration. | journal = Trends Plant Sci. | date = 1998 | volume = 3 | issue = 5 | pages = 175–180 | doi = 10.1016/S1360-1385(98)01235-7 }}</ref> Flowering plants are typically hermaphrodites,<ref>Barrett SCH. The evolution of plant sexual diversity. Nat Rev Genet. 2002;3: 274–284.</ref> and the conflict thus occurs within a single individual. Mitochondrial genes are typically only transmitted through female gametes, and therefore from their point of view the production of pollen leads to an evolutionary dead end. Any mitochondrial mutation that can affect the amount of resources the plant invests in the female reproductive functions at the expense of the male reproductive functions improves its own chance of transmission. [[Cytoplasmic male sterility]] is the loss of male fertility, typically through loss of functional pollen production, resulting from a mitochondrial mutation.<ref>{{cite journal | vauthors = Hanson MR, Bentolila S | title = Interactions of mitochondrial and nuclear genes that affect male gametophyte development | journal = The Plant Cell | volume = 16 | issue = Suppl | pages = S154–69 | date = 2004 | pmid = 15131248 | pmc = 2643387 | doi = 10.1105/tpc.015966 }}</ref> In many species where cytoplasmic male sterility occurs, the nuclear genome has evolved so-called restorer genes, which repress the effects of the cytoplasmic male sterility genes and restore the male function, making the plant a hermaphrodite again.<ref>{{cite journal | vauthors = Budar F, Pelletier G | title = Male sterility in plants: occurrence, determinism, significance and use | journal = Comptes Rendus de l'Académie des Sciences, Série III | volume = 324 | issue = 6 | pages = 543–50 | date = June 2001 | pmid = 11455877 | doi = 10.1016/S0764-4469(01)01324-5}}</ref><ref>{{cite journal | vauthors = Budar F, Touzet P, De Paepe R | title = The nucleo-mitochondrial conflict in cytoplasmic male sterilities revisited | journal = Genetica | volume = 117 | issue = 1 | pages = 3–16 | date = January 2003 | pmid = 12656568 | doi = 10.1023/A:1022381016145 | s2cid = 20114356 }}</ref>

The co-evolutionary arms race between selfish mitochondrial genes and nuclear compensatory alleles can often be detected by crossing individuals from different species that have different combinations of male sterility genes and nuclear restorers, resulting in hybrids with a mismatch.<ref>{{cite journal | vauthors = Case AL, Finseth FR, Barr CM, Fishman L | title = Selfish evolution of cytonuclear hybrid incompatibility in Mimulus | journal = Proceedings. Biological Sciences | volume = 283 | issue = 1838 | pages = 20161493| date = September 2016 | pmid = 27629037 | pmc = 5031664 | doi = 10.1098/rspb.2016.1493 }}</ref>

Another consequence of the maternal inheritance of the mitochondrial genome is the so-called [[Mother's curse|Mother's Curse]].<ref>{{cite journal | vauthors = Gemmell NJ, Metcalf VJ, Allendorf FW | title = Mother's curse: the effect of mtDNA on individual fitness and population viability | journal = Trends in Ecology & Evolution | volume = 19 | issue = 5 | pages = 238–44 | date = May 2004 | pmid = 16701262 | doi = 10.1016/j.tree.2004.02.002 }}</ref> Because genes in the mitochondrial genome are strictly maternally inherited, mutations that are beneficial in females can spread in a population even if they are deleterious in males.<ref>{{cite journal | vauthors = Frank SA, Hurst LD | title = Mitochondria and male disease | journal = Nature | volume = 383 | issue = 6597 | pages = 224 | date = September 1996 | pmid = 8805695 | doi = 10.1038/383224a0 | bibcode = 1996Natur.383..224F | s2cid = 4337540 | doi-access = free }}</ref> Explicit screens in fruit flies have successfully identified such female-neutral but male-harming mtDNA mutations.<ref>{{cite journal | vauthors = Camus MF, Clancy DJ, Dowling DK | title = Mitochondria, maternal inheritance, and male aging | journal = Current Biology | volume = 22 | issue = 18 | pages = 1717–21 | date = September 2012 | pmid = 22863313 | doi = 10.1016/j.cub.2012.07.018 | doi-access = free | bibcode = 2012CBio...22.1717C }}</ref><ref>{{cite journal | vauthors = Patel MR, Miriyala GK, Littleton AJ, Yang H, Trinh K, Young JM, Kennedy SR, Yamashita YM, Pallanck LJ, Malik HS | title = A mitochondrial DNA hypomorph of cytochrome oxidase specifically impairs male fertility in Drosophila melanogaster | journal = eLife | volume = 5 | date = August 2016 | pmid = 27481326 | pmc = 4970871 | doi = 10.7554/eLife.16923 | doi-access = free }}</ref> Furthermore, a 2017 paper showed how a mitochondrial mutation causing [[Leber's hereditary optic neuropathy]], a male-biased eye disease, was brought over by one of the [[King's Daughters|''Filles du roi'']] that arrived in Quebec, Canada, in the 17th century and subsequently spread among many descendants.<ref>{{cite journal | vauthors = Milot E, Moreau C, Gagnon A, Cohen AA, Brais B, Labuda D | title = Mother's curse neutralizes natural selection against a human genetic disease over three centuries | journal = Nature Ecology & Evolution | volume = 1 | issue = 9 | pages = 1400–1406 | date = September 2017 | pmid = 29046555 | doi = 10.1038/s41559-017-0276-6 | bibcode = 2017NatEE...1.1400M | s2cid = 4183585 }}</ref>