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]}

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.

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. 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 $(P)$: $Aa$ x $Aa$ (100%), and in
• $F$₁ generation gives: ${\displaystyle 1AA}$ : ${\displaystyle 2Aa}$: ${\displaystyle 1aa}$, which means that the frequency of heterozygotes now is 50% of the starting value.
• By the $F$₁₀ generation, heterozygotes have almost completely disappeared, and the population is polarized, with almost exclusively homozygous individuals ($AA$ and $aa$)

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	0.01953125	49.90234375
F10	49.995117187 ≈ 50.0	0.009765626 ≈ 0.0	49.995117187 ≈ 50.0

See also

• Effective selfing model
• Inbreeding
• Outbreeding
• Inbreeding depression
• Outbreeding depression
• Sequential hermaphroditism; the organism spends part of its life as a female and part as a male; self-fertilization is not possible.

References

1. http://en.mimi.hu/biology/selfing.html
2. Mayr E. (1963). Animal species and evolution (1st ed.). Cambridge: Belknap Press of Harvard University Press. ISBN 0-674-03750-2.
3. Dobzhansky T. (1970). Genetics of the evolutionary process. Columbia, New York. ISBN 0-231-02837-7.
4. Stebbins G. L., Jr. (1974). Flowering plants: evolution above the species level. Belknap Press. ISBN 0-674-30685-6.
5. Mayr E . (1982). The growth of biological thought: diversity, evolution, and inheritance (1st ed.). Cambridge, Mass: Belknap Press. ISBN 0-674-36445-7.
6. Hadžiselimović R. (2005). Bioantropology - diversity of recent man (in bosnian). Sarajevo: Institute for genetic engineering and biotechnology. ISBN 9958-9344-2-6.
7. King R. C., Stransfield W. D. (1998). Dictionary of genetics. New York, Oxford: Oxford University Press. ISBN 0-19-50944-1-7.