CONSPECIFIC SPERM PRECEDENCE IN STRONGYLOCENTROTUS
SEA URCHINS
(Christiane H. Biermann, Tom A. White, and Stephen R. Palumbi)
When given no choice, eggs of the green sea urchin Strongylocentrotus droebachiensis can be fertilized with sperm from the related, sympatric species S. pallidus and S. purpuratus. When green urchin sperm are added along with heterospecific sperm, conspecific sperm father over 90% of eggs - as assessed by genotyping three-day old embryos. The conspecific sperm precedence remains strong even if there is an excess of heterospecific sperm. However, if heterospecific sperm are given a head start, hybrid fertilizations increase. This increase is linear with delay time; conspecific fertilization reaches zero when heterospecific sperms had a one minute head start. In the reciprocal cross with eggs from S. pallidus, no-choice hybridization with S. droebachiensis sperm is lower with a mean of 40%, and conspecific sperm precedence is 100%. Strongylocentrotus purpuratus eggs block hybrid fertilization efficiently, even if given no choice. Strongylocentotus purpuratus sperm are very successful at fertilizing heterospecific eggs, and bypass the other species’ conspecific precedence to a small degree. These results suggest that hybrid fertilization studies should include sperm competition experiments since no-choice crosses are not representative even in simple broadcast-spawning systems.
THE JELLY ROLE: HIGH-IMPACT CARBS IN SEA URCHIN FERTILIZATION
The discovery that reproductive proteins are the most rapidly diverging proteins among mammals was stimulated by finding quickly evolving gamete recognition proteins in snails and sea urchins. Here, we extend the phenomenon of fast-evolving sperm-egg recognition molecules from proteins to carbohydrates. To examine sea urchin reproductive isolating mechanisms we studied sperm activation in three closely related, sympatric Strongylocentrotus species. Sperm undergo acrosomal exocytosis upon contact with sulfated polysaccharides in the egg-jelly coat. This acrosome reaction exposes the protein bindin, and is therefore a precondition for sperm binding to the egg. We found that sulfated carbohydrates from egg jelly induce the acrosome reaction species-specifically between S. droebachiensis and S. pallidus. There appear to be no other significant barriers to interspecific fertilization between these two species: the egg-jelly polysaccharides may have been correlated with speciation or reinforcement of reproductive barriers. Other species pairs in the same genus acrosome-react nonspecifically to egg jelly, but exhibit species-specific sperm binding. We thus show that different cell-cell communication systems mediate mate recognition among very closely related species. By comparing sperm reactions to egg-jelly compounds from different species and genera we identify the major structural feature of the polysaccharides required for the specific recognition by sperm: the position of the glycosidic bond of the sulfated a-L-fucans. This is one of the few examples of highly specific, pure-carbohydrate signal transduction.
(see Publications (CV) for references)
POPULATION GENETIC STRUCTURE OF GREEN
SEA URCHINS IN WASHINGTON’S NORTHWEST STRAITS
Green sea urchins (Strongylocentrotus
droebachiensis) are an important component of the
Northwest Straits marine ecosystem, and are being harvested commercially. Their
planktotrophic larvae get transported by currents; we
can infer realized dispersal by assessing the genetic subdivision among adult
populations. Sixteen populations from the