Practice question

Species-specific gamete recognition in sea urchins involves Bindin protein exposed on acrosomal process after calcium-triggered acrosome reaction. Bindin interacts with Egg Bindin Receptor EBR1, a heavily glycosylated transmembrane protein on vitelline layer of egg containing species-specific sulfated carbohydrate motifs and EGF repeats. EBR1 displays high polymorphism among species and selects conspecific Bindin variants, preventing hybridization and ensuring reproductive isolation. This Bindin-EBR1 lock-key interaction mediates tight adhesion and triggers fusion, analogous to ZP3 in mammals but unique to echinoids. Fertilin and integrins mediate mammalian fusion, not primary initial adhesion here.