Genetic load may increase or decrease with selfing depending upon the recombination environment

The ability of natural selection to remove deleterious mutations from a population is a function of the effective population size. Increases in selfing rate, and concomitant increases in population-level homozygosity, can increase or decrease the efficacy of selection, depending on the dominance and selection coefficients of the deleterious mutations. Most theory has focused on how (partial) selfing affects the efficacy of selection for mutations of a given dominance and fitness effect in isolation. It remains unclear how selfing affects the purging of deleterious mutations in a genome-wide context where mutations with different selection and dominance coefficients co-segregate. Here, we use computer simulations to investigate how mutation, selection and recombination interact with selfing rate to shape genome-wide patterns of genetic load. We recover various mechanisms previously described for how (partial) selfing affects the efficacy of selection against mutations of a given dominance class. However, we find that the interaction of purifying selection against mutations of different dominance classes changes with selfing rate. In particular, as outcrossing populations transition from purifying selection to pseudo-overdominance they experience a dramatic increase in the genetic load caused by additive, mildly deleterious mutations. We describe the threshold selfing rate that prevents pseudo-overdominance and decreases genetic load.