Parasites and the evolution of self-fertilization

Assuming all else is equal, an allele for selling should spread when rare i n an outcrossing population and rapidly reach fixation. Such an allele will not spread, however, if self-fertilization results in inbreeding depressio n so severe that the fitness of selfed offspring is less that half that of outcrossed offspring. Here we consider an ecological force that may also co unter the spread of a selfing allele: coevolution with parasites. Computer simulations were conducted for four different genetic models governing the details of infection. Within each of these models, we varied both the level of selfing in the parasite and the level of male-gamete discounting in the host (i.e., the reduction in outcrossing fitness through male function due to the selfing allele). We then sought the equilibrium level of host selfi ng under the different conditions. The results show that, over;l wide range of conditions, parasites can select for host reproductive strategies in wh ich both selfed and outcrossed progeny are produced (mixed mating). In addi tion, mixed mating, where it exits, tends to be biased toward selfing.