Masking of Mutations and the Evolution of Sex

Abstract

Outcrossing and recombination are basic aspects of sex; however, selfing maintains many of the aspects of sex, such as recombination, yet abandons outcrossing. The role of deleterious recessive alleles in the evolution of selfing and outcrossing is studied by mathematical analysis of genetic models in which reproductive system is determined by a single locus. If viability is determined by heterozygote superiority at a second locus, partial outcrossing can increase when rare in a population of selfers even though inbreeding depression is less than 1/2, and mixed mating systems that are polymorphic for both outcrossing and selfing exist and are stable. In another model, the masking model, outcrossers are assumed to mask mutations by keeping them heterozygous. There are three basic parameters in this model: one measuring the difference between outcrossers and selfers in the heterozygosity of their offspring, that is, masking ability; one measuring the fitness difference between homozygous and heterozygous offspring; and a third measuring the probability that a selfer is able to purge its genome of mutations. In most cases, outcrossing is stable when common so long as heterozygous offspring are more than twice as fit as mutant-homozygote offspring, mutations are nearly recessive, and the values for the masking and purging parameters are in a realistic range implied by three or more segregating mutations. The conditions for outcrossing to increase when rare are also generally favorable to outcrossing. For most combinations of parameters, a low probability of purging, along with a high mutation rate, will allow outcrossing to invade. The implication of these results for the evolution of sex is discussed.