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.
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.
Illustration model of the decrease in genetic variation in a population of self-fertilized organisms derived from a heterozygous individual, assuming equal fitness
GenerationAA
Aa
aa
P100
F1255025
F237.52537.5
F343.7512.543.75
F446.8756.2546.875
F548.43753.12548.4375
F649.218751.562549.21875
F749.6093750.7812549.609375
F849.80468750.39062549.8046875
F949.902343750.195312549.90234375
F1049.995117187 ≈ 50.00.09765626 ≈ 0.049.995117187 ≈ 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 sexual reproduction in the last eukaryotic common ancestor was homothallic or self-fertile unisexual reproduction.