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. Selfing is widespread – from unicellular organisms to the most complex hermaphroditic plants and animals. In unicellular organisms such as Protozoa, selfing can occur when two individuals interbreed that were produced from a previous mitotic division of the same individual. About 10-15% of flowering plants are predominantly selfing. 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, 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. 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 , 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
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 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 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 occurring in many different genera. several species of the ascomycete genus Cochliobolus. and the ascomycete Pneumocystis jirovecii.. A review of evidence on the evolution of sexual reproduction in the fungi led to the concept that the original mode of sexualreproduction in the last eukaryotic common ancestor was homothallic or self-fertile unisexual reproduction.
