RIFFELL, J.A.; KRUG, P.J.; ZIMMER, R.K.; Univ. of California, Los Angeles; California State Univ., Los Angeles; Univ. of California, Los Angeles: Ecological and evolutionary consequences of sperm chemoattraction for fertilization success

Chemical communication between sperm and egg is a critical factor mediating sexual reproduction. Sperm attractants may be significant evolutionarily for maintaining species barriers, and important ecologically for increasing gamete encounters. Still unresolved, however, are the functional consequences of these dissolved signal molecules. Here, we provide the first evidence that sperm chemoattraction directly affects the magnitude of fertilization success. The recent discovery of L-tryptophan as a potent attractant to red abalone (Haliotis rufescens) sperm offered the rare opportunity to quantify how navigation affects gamete interactions. Sperm behavioral responses to manipulations of the natural tryptophan gradient around individual eggs revealed that both chemotaxis and chemokinesis significantly promote contacts. Our results showed further that attractant release via diffusion effectively doubles the target size of red abalone eggs which, in turn, significantly increases fertilization success. Although long theorized as potential barriers to hybridization, species-specific sperm attractants in red and green (H. fulgens) abalone are not responsible for maintaining reproductive isolation. For abalone in dense spawning aggregations, chemically mediated navigation may instead promote conspecific gamete interactions and prevent sperm from pointlessly tracking heterospecific eggs. Species-specificity may thus evolve when finding the right target confers a selective advantage on sperm by preventing gamete wastage, even though reproductive isolation fundamentally resides at the level of membrane recognition proteins.