Practice question

Fertilization requires species-specific molecular lock-and-key recognition to prevent cross-species hybrids in mixed spawning assemblages. In sea urchins this specificity resides in interaction between sperm protein bindin on acrosomal process and its complementary glycoprotein receptor EBR1 embedded in vitelline envelope. Carbohydrate moieties and protein loops vary rapidly between species, creating selective adhesion. Mixing experiments show bindin from Strongylocentrotus binds strongly only to conspecific EBR1; antibodies to EBR1 block fertilization species-specifically. Flagellar proteins, actin filaments, or ATPases provide motility and exocytosis machinery but lack discriminatory binding capacity, making bindin-receptor coevolution primary determinant of reproductive isolation.