Evolutionary Dynamics of the Interacting Gamete Recognition Proteins Sperm Bindin and Its Egg Receptor EBR1 in the Purple Sea Urchin (Strongylocentrotus Purpuratus)

Gamete recognition proteins (GRPs) mediate the fusion of gametes by determining whether sperm and eggs are compatible during fertilization and are key players involved in reproduction, reproductive isolation and speciation in diverse taxa. Gamete recognition proteins have been widely described as highly variable and under positive selection in divergent taxa; however, the mechanisms of selection have remained largely undetermined in both inter- and intraspecific studies. This dissertation focuses in the interacting GRPs sperm bindin and its egg receptor Ebr1 that mediate the fusion and compatibility of gametes in the purple urchin Stongylocentrotus purpuratus and closely related taxa. A potential mechanism for favoring the high diversity in GRPs is that of balancing selection in which several genotypes are maintained due sexual conflict between male and female gametes. Sexual conflict can occur under high sperm concentrations due to polyspermy, the condition in which a single female egg are fertilized by several males leading to zygote failure. This conflict can be resolved by female GRPs expressed in eggs evolving fast to avoid being fertilized easily by males with common genotypes, while males evolve as they compete with each other to become the best fertilizers, generating an evolutionary arms race. Gamete specificity due to compatibility between sperm and egg GRPs can lead to assortative fertilization between compatible matched gametes. Theory predicts that interacting gamete recognition proteins from sperm and eggs could be under linkage disequilibrium due to assortative fertilization, however the empirical evidence has been elusive. Here I show that in the purple sea urchin S. purpuratus the sperm bindin gene is highly variable, and under selection, and that specific bindin genotypes influence fertilization success. I furthermore demonstrate that the egg receptor for bindin Ebr1 is also highly variable and under selection. Finally, I show that sperm bindin and Ebr1 are under linkage disequilibrium, and are coevolving likely due to assortative fertilization.