Mating Systems and the Efficacy of Selection at the Molecular Level

Mating systems are thought to play a key role in molecular evolution through their effects on effective population size (N(e)) and effective recombination rate. Because of reduced N(e), selection in self-fertilizing species is supposed to be less efficient, allowing fixation of weakly deleterious alleles or lowering adaptation, which may jeopardize their long-term evolution. Relaxed selection pressures in selfers should be detectable at the molecular level through the analyses of the ratio of nonsynonymous and synonymous divergence, D(n)/D(s), or the ratio of nonsynonymous and synonymous polymorphism, pi(n)/pi(s). On the other hand, selfing reveals recessive alleles to selection (homozygosity effect), which may counterbalance the reduction in N(e). Through population genetics models, this study investigates which process may prevail in natural populations and which conditions are necessary to detect evidence for relaxed selection signature at the molecular level in selfers. Under a wide range of plausible population and mutation parameters, relaxed selection against deleterious mutations should be detectable, but the differences between the two mating systems can be weak. At equilibrium, differences between outcrossers and selfers should be more pronounced using divergence measures (D(n)/D(s) ratio) than using polymorphism data (pi(n)/pi(s) ratio). The difference in adaptive substitution rates between outcrossers and selfers is much less predictable because it critically depends on the dominance levels of new advantageous mutations, which are poorly known. Different ways of testing these predictions are suggested, and implications of these results for the evolution of self-fertilizing species are also discussed.