FERTILIZATION BETWEEN CLOSELY-RELATED SEA-URCHINS IS BLOCKED BY INCOMPATIBILITIES DURING SPERM-EGG ATTACHMENT AND EARLY STAGES OF FUSION

Closely related sea urchin species in the genus Echinometra from Hawai i and Guam have strong species-specificity of fertilization. Crosses b etween the two species found in Hawaii, E. mathaei and E. oblonga, wer e compared in order to determine which steps of gamete interaction are responsible for fertilization barriers. The acrosome reaction, attach ment of sperm to eggs, and fusion of sperm and egg membranes were meas ured in crosses between species and compared to within-species control s. In all crosses, eggs induced the acrosome reaction in 50-100% of sp erm within 20 s. However, eggs bound about 3-5 times fewer heterospeci fic than conspecific sperm. In addition, electrical continuity between heterospecific gametes was achieved rarely under conditions that allo wed conspecific gametes to achieve it readily. Only two sperm-egg fusi on events were recorded in more than 80 min of heterospecific sperm in teraction on 22 eggs. Accordingly, species-specific fertilization in t hese urchins results firstly from reduced attachment of the heterospec ific sperm acrosomal process to the egg vitelline layer, and secondly from inability of attached heterospecific sperm to develop continuity with the egg plasma membrane. At both of these steps, incompatibilitie s are reciprocal. Thus a barrier to gene now is mediated by molecular interactions during a specific part of the fertilization process, as t he sperm acrosomal surface and the egg vitelline layer contact each ot her. Recognition molecules mediating these steps of fertilization may be capable of relatively rapid change, leading to species-specificity of fertilization.