California Sea Grant College Program

Chemical communication between sperm and eggs is ubiquitous in organisms with widely divergent reproductive strategies. Sperm attractants are found in marine taxa that broadcast their gametes into the sea, as well as terrestrial organisms with internal fertilization, such as humans. Chemosensory-mediated behavior thus is a key component in sperm-egg interactions and fertilization. Despite nearly a century of research, the mechanisms controlling fertilization remains one of the least understood biological processes. For example, waterborne egg-factors that attract sperm are thought to be ecologically meaningful for increasing gamete encounters, or evolutionary significant by maintaining species barriers, but the contributions remains unknown. In this study, I (i) developed analytical and behavioral techniques to establish the reproductive consequences of sperm attractants, and (ii) examined the biological processes controlling fertilization success in the red abalone, Haliotis rufescens. Using RP and size-exclusion chromatographic teclmiques, the amino acid L-tryptophan, was found to be the natural sperm attractant. The production of tryptophan from live abalone eggs elicited sperm chemotaxis, effectively doubling the egg size. When the enzyme tryptophanase, which selectively degrades tryptophan, was added to the seawater around eggs, sperm no longer were able to locate the eggs, gamete encounters dropped, and fertilization levels dramatically lowered. Thus, the natural tryptophan gradient is critical for gamete encounters. To establish the evolutionary consequences of sperm attractants, red and green (Haliotis fulgens) abalone gametes were crossed, and fertilization assays conducted. Although long theorized as baniers to hybridization, species-specific sperm attractants were only minor contributors to reproductive isolation. When the influence of sperm and egg concentration, sperm-to-egg ratio, and gamete contact times on reproductive success was established, egg concentration had the same effect as sperm concentration on fertilization rates, and eggs, not sperm, were the limiting factor controlling abalone fecundity. These results contrast widely with the theory that sperm is the primary determinant for fe1iilization success. Given the fact that eggs, rather then sperm, effectively control reproduction, chemoattractants may have evolved thru sperm competition.