Deleterious Mutations and Selection for Sex in Finite Diploid Populations

Abstract

In diploid populations, indirect benefits of sex may stem from segregation and recombination. Although it has been recognized that finite population size is an important component of selection for recombination, its effects on selection for segregation have been somewhat less studied. In this article, we develop analytical two-and three-locus models to study the effect of recurrent deleterious mutations on a modifier gene increasing sex, in a finite diploid population. The model also incorporates effects of mitotic recombination, causing loss of heterozygosity (LOH). Predictions are tested using multilocus simulations representing deleterious mutations occurring at a large number of loci. The model and simulations show that excess of heterozygosity generated by finite population size is an important component of selection for sex, favoring segregation when deleterious alleles are nearly additive to dominant. Furthermore, sex tends to break correlations in homozygosity among selected loci, which disfavors sex when deleterious alleles are either recessive or dominant. As a result, we find that it is difficult to maintain costly sex when deleterious alleles are recessive. LOH tends to favor sex when deleterious mutations are recessive, but the effect is relatively weak for rates of LOH corresponding to current estimates (of the order 10 À4 À10 À5 ).

Meiosis as an Evolutionary Adaptation for DNA Repair

“…Otto (2003) showed that under this scenario high rates of outcrossing are favored only if deleterious alleles are weakly recessive (dominance close to 0.5). Another potential force pushing away from HW equilibrium considered by Roze and Michod (2010) is gene conversion which creates homozygosity. Gene conversion could result from mitotic HRR between sister chromosomes as discussed above.…”

“…In this case (and if deleterious alleles tend to be partially recessive) outcrossing is beneficial in the short term (because it masks deleterious alleles) but disadvantageous in the long term (because purging is less efficient). The magnitude of this force may be estimated from rates of loss of heterozygosity during development [discussed in Roze and Michod (2010)]. The few estimates which exist indicate that the loss of heterozygosity is low, and thus this selective force for outcrossing may be weak.…”

Relative effects of segregation and recombination on the evolution of sex in finite diploid populations

“…For instance, dominance within loci was not considered. Dominance of deleterious mutations in diploids has been investigated in other studies (Haag and Roze, 2007;Roze and Michod, 2010). However, little is known about the dominance of beneficial mutations, particularly the mode of interaction between deleterious and beneficial mutations (that is, beneficial and deleterious mutations coexist in the same locus).…”

“…Nevertheless, the mechanism by which drift influences the conditions favoring segregation remains elusive. Recently, Roze and Michod (2010) investigated the effects of recurrent deleterious mutations on the evolution of sex in finite diploids. The results have shown that excessive heterozygosity generated by a finite population size is the key component of selection to favor segregation when deleterious mutations are dominant (Roze and Michod, 2010).…”

Does the avoidance of sexual costs increase fitness in asexual invaders?

“…MR can lead to a loss of heterozygosity and may thus affect evolutionary rates in diploid asexual lineages. It has been demonstrated in some asexuals (41), even though the net effect of MR on the fitness of asexual lineages is still to be elucidated (42,43). A second set of assumptions is that the genotypes are adapted to their environment and that the environment of the offspring will be the same as that of the parents.…”