Modeling how reproductive ecology can drive protein diversification and result in linkage disequilibrium between sperm and egg proteins.

Gamete-recognition proteins determine whether sperm and eggs are compatible at fertilization, and they often evolve rapidly. The source of selection driving the evolution of these proteins is still debated. It has been suggested that sexual conflict can result in proliferation of genetic variation and possibly linkage disequilibrium between sperm and egg proteins. Empirical evidence suggests that both male and female reproductive success can be predicted by their sperm ligand genotype, but why female success can be predicted by a protein expressed only in males is unknown. Here we use mathematical modeling to investigate the interaction between reproductive behavior and sperm availability on the evolution of sperm ligands and egg receptors. We consider haploid and diploid expression in gametes in two possible ecological scenarios, monogamous spawning and competitive spawning. Reproductive behavior plays an important role in determining possible outcomes resulting from sexual conflict. Sperm limitation selects for common genotypes regardless of mating behavior. Under conditions of sperm abundance, competitive spawning provides conditions for the persistence of allelic variation and gametic disequilibrium. With monogamous spawning, such conditions are more restrictive.