The jelly role: high-impact carbs in sea urchin fertilization

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.

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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 Salish Sea comprising 742 green sea urchin individuals were genotyped at six polymorphic microsatellite loci. The genetic diversity was high with heterozygosities of 0.57, 0.7, and 0.9 at the three loci analyzed thus far. One locus deviated substantially from Hardy-Weinberg-equilibrium. Two other loci each showed significant population subdivision: one of them strongly divided populations on either side of Boundary Passage along the US/Canadian border, and the other differentiated between populations around San Juan Island and between those and more distant samples. Populations from east of Cypress Island and from the west side of Whidbey Island are among the least isolated. Intertidal populations do not appear to differ from nearby subtidal samples. The gene flow results will be compared to oceanographic data such as those from drift card studies.