Practice question

While initial sperm attraction by egg jelly provides coarse guidance, lasting species-specific recognition at binding step depends critically on precise molecular complementarity between sperm protein bindin and its large glycoprotein receptor EBR1 on vitelline envelope. Bindin sequence shows hypervariable regions among sea urchin species, and receptor binding pocket compatibility determines adhesion strength and specificity. Heterologous bindin experimentally fails to agglutinate eggs despite vigorous motility and intact flagellar proteins. Flagellar components, actin filaments or ATPase enzymes do not confer selectivity. Competition with recombinant bindin isoforms blocks fertilization species-specifically, demonstrating bindin-receptor complex as primary determinant enforcing prezygotic isolation during broadcast spawning.