Selfing

Selfing or self-fertilization is the union of male and female gametes and/or nuclei from same haploid, diploid, or polyploid organism. It is an extreme degree of inbreeding.[1][2][3][4][5][6][7]

This page is about self-fertilization; see also self-pollination, also called selfing.

Selfing is widespread – from unicellular organisms to the most complex hermaphroditic plants and animals (especially invertebrates). In unicellular organisms such as Protozoa, selfing can occur when two individuals (or their cell nuclei) interbreed that were produced from a previous mitotic division of the same individual. About 10-15% of flowering plants are predominantly selfing.[8]

Among hermaphrodite animals there are some that regularly reproduce by self-fertilization. In others, it is a rare event; selfing in such species is more common in adverse environmental conditions, or in the absence of a partner.

Genetic consequences of selfing

Self-fertilization results in the loss of genetic variation within an individual (offspring), because many of the genetic loci that were heterozygous become homozygous. This can result in the expression of harmful recessive alleles, which can have serious consequences for the individual. The effects are most extreme when self-fertilization occurs in organisms that are usually out-crossing.[9] In plants, selfing can occur as autogamous or geitonogamous pollinations and can have varying fitness affects that show up as autogamy depression. After several generations, inbreeding depression is likely to purge the deleterious alleles from the population because the individuals carrying them have mostly died or failed to reproduce.

If no other effects interfere, the proportion of heterozygous loci is halved in each successive generation, as shown in the following table.

  • Parental : x (100%), and in
  • 1 generation gives: : : , which means that the frequency of heterozygotes now is 50% of the starting value.
  • By the 10 generation, heterozygotes have almost completely disappeared, and the population is polarized, with almost exclusively homozygous individuals ( and )

Illustration model of the decrease in genetic variation in a population of self-fertilized organisms derived from a heterozygous individual, assuming equal fitness

Generation AA
(%)		 Aa
(%)		 aa
(%)
P – 100 –
F1 25 50 25
F2 37.5 25 37.5
F3 43.75 12.5 43.75
F4 46.875 6.25 46.875
F5 48.4375 3.125 48.4375
F6 49.21875 1.5625 49.21875
F7 49.609375 0.78125 49.609375
F8 49.8046875 0.390625 49.8046875
F9 49.90234375 0.1953125 49.90234375
F10 49.995117187 ≈ 50.0 0.09765626 ≈ 0.0 49.995117187 ≈ 50.0

Fungi

There are basically two distinct types of sexual reproduction among fungi. The first is outcrossing (in heterothallic fungi). In this case, mating occurs between two different haploid individuals to form a diploid zygote, that can then undergo meiosis. The second type is self-fertilization or selfing (in homothallic fungi). In this case, two haploid nuclei derived from the same individual fuse to form a zygote than can then undergo meiosis. Examples of homothallic fungi that undergo selfing include species with an aspergillus-like asexual stage (anamorphs) occurring in many different genera.[10] several species of the ascomycete genus Cochliobolus.[11] and the ascomycete Pneumocystis jirovecii.[12] (for other examples, see Homothallism). A review of evidence on the evolution of sexual reproduction in the fungi led to the concept that the original mode of sexual reproduction in the last eukaryotic common ancestor was homothallic or self-fertile unisexual reproduction.[13]

See also

References
  1. "* Selfing (Biology) - Definition,meaning - Online Encyclopedia". en.mimi.hu. Retrieved 11 October 2018.
  2. Mayr E. (1963). Animal species and evolution (1st ed.). Cambridge: Belknap Press of Harvard University Press. ISBN 978-0-674-03750-2.
  3. Dobzhansky T. (1970). Genetics of the evolutionary process. Columbia, New York. ISBN 978-0-231-02837-0.
  4. Stebbins G. L., Jr. (1974). Flowering plants: evolution above the species level. Belknap Press. ISBN 978-0-674-30685-1.
  5. Mayr E . (1982). The growth of biological thought: diversity, evolution, and inheritance (1st ed.). Cambridge, Mass: Belknap Press. ISBN 978-0-674-36445-5.
  6. Hadžiselimović R. (2005). Bioantropology - diversity of recent man (in bosnian). Sarajevo: Institute for genetic engineering and biotechnology. ISBN 978-9958-9344-2-1.
  7. King R. C., Stransfield W. D. (1998). Dictionary of genetics. New York, Oxford: Oxford University Press. ISBN 978-0-19-50944-1-1.
  8. Wright SI, Kalisz S, Slotte T (June 2013). "Evolutionary consequences of self-fertilization in plants". Proc. Biol. Sci. 280 (1760): 20130133. doi:10.1098/rspb.2013.0133. PMC 3652455. PMID 23595268.
  9. Bernstein H, Byerly HC, Hopf FA, Michod RE (September 1985). "Genetic damage, mutation, and the evolution of sex". Science. 229 (4719): 1277–81. doi:10.1126/science.3898363. PMID 3898363.
  10. Dyer, Paul S.; O'Gorman, Céline M. (January 2012). "Sexual development and cryptic sexuality in fungi: insights from Aspergillus species". FEMS Microbiology Reviews. 36 (1): 165–192. doi:10.1111/j.1574-6976.2011.00308.x. PMID 22091779.
  11. Yun, S.-H.; Berbee, M. L.; Yoder, O. C.; Turgeon, B. G. (11 May 1999). "Evolution of the fungal self-fertile reproductive life style from self-sterile ancestors". Proceedings of the National Academy of Sciences. 96 (10): 5592–5597. doi:10.1073/pnas.96.10.5592. PMC 21905. PMID 10318929.
  12. Richard, S.; Almeida, J. M. G. C. F.; Cissé, O. H.; Luraschi, A.; Nielsen, O.; Pagni, M.; Hauser, P. M.; Weiss, Louis M. (20 February 2018). "Functional and Expression Analyses of the Pneumocystis MAT Genes Suggest Obligate Sexuality through Primary Homothallism within Host Lungs". mBio. 9 (1). doi:10.1128/mBio.02201-17. PMC 5821091. PMID 29463658.
  13. Heitman, Joseph (December 2015). "Evolution of sexual reproduction: A view from the fungal kingdom supports an evolutionary epoch with sex before sexes". Fungal Biology Reviews. 29 (3–4): 108–117. doi:10.1016/j.fbr.2015.08.002. PMC 4730888. PMID 26834823.
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