The evolution of condition-dependent self-fertilisation

Self-fertilisation is common in cosexuals, but selfing rates vary among species, among populations and among individuals within populations. Most evolutionary theory seeking to explain this variation assumes genetically determined selfing rates. Here we study the evolution and consequences of condition-dependent selfing, where individuals adjust their selfing rate in response to their deleterious mutation load. We analyse a two-locus population-genetic model in which one locus determines condition and the other is a modifier locus that determines condition-dependent selfing rates, and then extend the analysis to a polygenic background in which condition is determined by many loci. Our results show that selection favours positive condition dependence: high-condition individuals self-fertilise whereas low-condition individuals outcross. The resulting reaction norm generates stable within-population variation in realised selfing rates at evolutionary equilibrium and reduces the mutation load. We further show that it persists under environmental heterogeneity, and that pollen discounting favours a gradual increase in selfing with condition, leading to a continuum of selfing phenotypes. Altogether, our results indicate that condition-dependent selfing can generate substantial within-population variation in selfing rates. It may therefore contribute to mating-system diversity, and in particular to the maintenance of mixed mating.