Fertilization is fundamental for sexual reproduction, yet its molecular 
mechanisms are poorly understood. We found that an oocyte-expressed Ly6/uPAR 
protein, which we call Bouncer, is a crucial fertilization factor in zebrafish. 
Membrane-bound Bouncer mediates sperm-egg binding and is thus essential for 
sperm entry into the egg. Remarkably, Bouncer not only is required for sperm-egg 
interaction but is also sufficient to allow cross-species fertilization between 
zebrafish and medaka, two fish species that diverged more than 200 million years 
ago. Our study thus identifies Bouncer as a key determinant of species-specific 
fertilization in fish. Bouncer's closest homolog in tetrapods, SPACA4, is 
restricted to the male germline in internally fertilizing vertebrates, which 
suggests that our findings in fish have relevance to human biology.