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 Hawaii and Guam have strong species-specificity of fertilization. Crosses between the two species found in Hawaii, E. mathaei and E. oblonga, were compared in order to determine which steps of gamete interaction are responsible for fertilization barriers. The acrosome reaction, attachment of sperm to eggs, and fusion of sperm and egg membranes were measured in crosses between species and compared to within-species controls. In all crosses, eggs induced the acrosome reaction in 50-100% of sperm within 20 s. However, eggs bound about 3-5 times fewer heterospecific than conspecific sperm. In addition, electrical continuity between heterospecific gametes was achieved rarely under conditions that allowed conspecific gametes to achieve it readily. Only two sperm-egg fusion events were recorded in more than 80 min of heterospecific sperm interaction on 22 eggs. Accordingly, species-specific fertilization in these urchins results firstly from reduced attachment of the heterospecific 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, incompatibilities are reciprocal. Thus a barrier to gene flow is mediated by molecular interactions during a specific part of the fertilization process, as the sperm acrosomal surface and the egg vitelline layer contact each other. Recognition molecules mediating these steps of fertilization may be capable of relatively rapid change, leading to species-specificity of fertilization.