Primary gamete binding: Quantitative determination of its specificity in echinoid fertilization

The occurrence of primary gamete binding in homologous vs heterologous crosses of three species of echinoids was assayed quantitatively. The three sympatric echinoids, Lytechinus, Tripneustes, and Echinometra were chosen for this investigation because, under normal conditions, crossfertilization between these species is extremely low. In each of the three homologous crosses, large numbers of sperm become bound to the egg and are released, following the cortical reaction. A similar binding pattern does not occur in any of the six heterologous crosses, although we have previously shown that in these crosses sperm penetrate and undergo acrosomal reaction within the heterologous egg jelly 13]. The results of the present investigation demonstrate that primary gamete binding is the chief mechanism of reproductive isolation (species-exclusive event) of echinoid fertilization. Failure of reacted, heterologous sperm to undergo primary binding precludes subsequent interaction with the ovum and thus minimizes cross-fertilization.