Host-parasite coevolution and selection on sex through the effects of segregation.

The advantage of producing novel variation to keep apace of coevolving species has been invoked as a major explanation for the evolution and maintenance of sex (the Red Queen hypothesis). Recent theoretical investigations of the Red Queen hypothesis have focused on the effects of recombination in haploid species, finding that species interactions rarely favor the evolution of sex unless selection is strong. Yet by focusing on haploids, these studies have ignored a potential advantage of sex in diploids: generating novel combinations of alleles at a particular locus through segregation. Here we investigate models of host-parasite coevolution in diploid species to determine whether the advantages of segregation might rescue the Red Queen hypothesis as a more general explanation for the evolution of sex. We find that the effects of segregation can favor the evolution of sex but only under some models of infection and some parameter combinations, almost always requiring inbreeding. In all other cases, the effects of segregation on selected loci favor reductions in the frequency of sex. In cases where segregation and recombination act in opposite directions, we found that the effects of segregation dominate as an evolutionary force acting on sex in diploids.