ABSTRACT
Cytological data from flowering plants suggest that the evolution of recombination rates is affected by the mating system of organisms, as higher chiasma frequencies are often observed in self-fertilizing species compared with their outcrossing relatives. Understanding the evolutionary cause of this effect is of particular interest, as it may shed light on the selective forces favoring recombination in natural populations. While previous models showed that inbreeding may have important effects on selection for recombination, existing analytical treatments are restricted to the case of loosely linked loci and weak selfing rates, and ignore the effect of genetic interference (Hill-Robertson effect), known to be an important component of selection for recombination in randomly mating populations. In this article, we derive general expressions quantifying the stochastic and deterministic components of selection acting on a mutation affecting the genetic map length of a whole chromosome along which deleterious mutations occur, valid for arbitrary selfing rates. The results show that selfing generally increases selection for recombination caused by interference among mutations as long as selection against deleterious alleles is sufficiently weak. While interference is often the main driver of selection for recombination under tight linkage or high selfing rates, deterministic effects can play a stronger role under intermediate selfing rates and high recombination, selecting against recombination in the absence of epistasis, but favoring recombination when epistasis is negative. Individual-based simulation results indicate that our analytical model often provides accurate predictions for the strength of selection on recombination under partial selfing.
AUTHOR SUMMARY Understanding the evolutionary advantage of sex and the associated process of meiotic recombination remains a major challenge in evolutionary biology. Useful insights on this question may possibly be gained from exploring the causes of genetic variation for recombination rates within and between species. In particular, a remarkable pattern observed in flowering plants is that self-fertilizing species tend to have higher rates of meiotic crossovers than their outcrossing relatives. In this article, we use analytical and simulation methods to investigate how selfing affects the sign and magnitude of stochastic and deterministic components of selection for recombination in hermaphroditic organisms. The results show that, while selection for recombination vanishes under complete selfing (as recombination has no genetic effect in fully homozygous individuals), it is often maximized for selfing rates slightly below one. This is primarily due to the fact that selfing increases the strength of selective interference among loci in finite populations, and may explain the higher crossover rates observed in inbred plant species.
Competing Interest Statement
The authors have declared no competing interest.
