Effects of Sperm Environment on the Evolution of Gamete Traits in Ciona Robusta

Fertilization is a complex process, and gamete traits can affect the rate at which sperm and egg collide and fuse, making them prime targets for selection. This is particularly true for broadcast spawners, whose fertilization success mainly depends on interactions between gametes. Gamete traits can modify collision and fusion rates affecting fertilization success, but sperm availability can affect the rate at which gametes can interact. Because of this the strength and direction of selection on gamete traits to optimize fertilization success is dependent upon sperm availability. While many studies have examined differences in selection pressures due to sperm availability on individual traits, rarely has the effect of interactions between traits been examined. Yet, interactions between traits may have an important impact on selection by modifying the focal trait's effect on fertilization success, and that modification can be contingent on the sperm environment. For instance, eggs that increase collisions and are quicker to fuse with sperm may be more prone to polyspermy (reproductive failure due to multiple sperm fusions) at lower sperm availabilities than eggs that increase collisions but are slower to fuse with sperm. Therefore, determining how interactions between traits can affect fertilization success, and how that effect can change across different sperm environments, is important for understanding the selective pressures a particular trait may face for a given sperm environment. Additionally, while many studies have postulated that interactions between different male and female gamete recognition protein (GRP) variants can affect fertilization success by altering fusion rates, it has yet to be examined. In this dissertation, I present the results of manipulative and observational experiments designed to determine how interactions between a suite of gamete traits may affect fertilization success. I conducted a series of no-choice fertilization assays over a range of sperm availabilities, in order to determine whether collision rates, genetic variability in GRPs (which mediate compatibility), or interactions between the two were the most important in determining fertilization success. I also attempted to determine if there was a difference in compatibility between different male and female GRP variants by examining whether individuals garnered a greater share of paternity based on their respective genotypes. This was accomplished by conducting fertilization assays, in which the eggs were offered a mix of two males' sperm. I also examined whether there might be a functional link between genetic variation in GRPs and chemoattractant-mediated differences in sperm behavior. This was accomplished by examining whether there was a difference in sperm chemotaxis or chemokinesis based on the respective GRP genotypes of the eggs and sperm using video analysis and dichotomous chambers. Finally, I looked at whether the trends seen in the laboratory were found in a natural population. I correlated changes in settler density (as a proxy for sperm availability) with changes in collision trait values, as well as examining for increases in assortative mating based on GRP identity with increasing settler density. I found that interactions between traits tended to explain most of the variance in fertilization success for most sperm environments. While I was unable to determine differences in compatibility, my results suggest that sperm that have the same female GRP genotype as the eggs they were exposed to tended to garner a higher share of the paternity. Additionally, my results also suggest that sperm will aggregate around eggs when they both share the same GRP genotype at the receptor locus, offering a mechanism by which assortative mating between gametes based on GRP genotype could occur. I then found that assortative mating based on the female receptor genotype does occur in a natural population, as more homozygous settlers were produced than expected under random mating as settler density increased. Additionally, while I found that collision rate traits changed in the directions predicted from previous studies based on increasing settler density, that relationship could be modified by GRP genotype. This suggests that assortative mating based on multiple trait values can occur, most likely due to the fact that different combinations of traits can maximize fertilization success. Overall, interactions of gamete traits with compatibility played a large role in fertilization success, particularly as sperm density increased. These results highlight the importance of examining the interplay of multiple traits under differing spawning conditions, in order to truly understand how they can affect fitness, and shape trait evolution.