Assortative Mating in the Tropical Sea Urchin Lytechinus Variegatus

Non-random mating is presumed to be an important mechanism that allows for the maintenance of genetic variation. Assortative mating has been studied extensively in organisms that possess defined ways in which sperm is transferred to eggs (e.g. via copulation, courtship or vector assisted pollination in plants), but rarely in broadcast spawners. Broadcast spawning is perceived as a mating event that allows for mixing of gametes and promotes random mating. However, there are multiple pathways in which spawning adults can affect fertilization of gametes in non-random ways. For example, positive assortative mating can occur in broadcast spawners if similar phenotypes spawn closer together in space or time, or possess similar gamete recognition proteins that expedite fertilization. Here, I propose to examine assortative fertilization, patterns of aggregation and gamete recognition protein genotype of the sperm bindin gene as a function of spine color in the sea urchin Lytechinus variegatus as well as evaluating deviations from Hardy-Weinberg Equilibrium (HWE) based on color. Results indicate that laboratory crosses of urchins within color morphs yielded higher fertilization success than crosses between color morphs. Field surveys determined that these sea urchins are aggregating by color at times of their reproductive season when they are more likely to spawn. Tests for HWE using field data of urchin phenotypes suggest strong deviations from HWE. However, DNA sequences of regions of the sperm bindin gene for sea urchins of different color do not show evidence of genetic structure of the population. Paternal success in broadcast spawners is largely determined by the proximity of males to spawning females and the compatibility between them at the time they release their gametes. Selection is predicted to favor traits and behaviors that increase the likelihood of spawning near a more compatible neighbor. These results provide strong evidence for assortative mating and an explanation for the maintenance of color variation in this species.