Inverted meiosis and the evolution of sex by loss of complementation.

Inverted meiosis, in which sister chromatids segregate before homologous chromosomes, is a common aberration of conventional meiosis (in which sister chromatids segregate after homologous chromosomes) and is routinely observed in certain species. This raises an evolutionary mystery: what is the adaptive advantage of the more common, conventional order of segregation in meiosis? I use a population genetic model to show that asexual mutants arising from inverted meiosis are relatively immune from the deleterious effects of loss of complementation (heterozygosity), unlike the asexual mutants arising from conventional meiosis, in which loss of complementation can outweigh the two-fold cost of meiosis. Hence, asexual reproduction can replace sexual reproduction with inverted meiosis, but not with conventional meiosis. The results are in line with analogous considerations on other alternative types of reproduction and support the idea that amphimixis is stable in spite of the two-fold cost of meiosis because loss of complementation in mutant asexuals outweigh the two-fold cost.