SEQUENCE DIVERGENCE WITHIN THE SPERM-SPECIFIC POLYPEPTIDE TCTE1 IS CORRELATED WITH SPECIES-SPECIFIC DIFFERENCES IN SPERM BINDING TO ZONA-INTACT EGGS

The T-complex-associated testes-expressed (TCTE1) gene encodes a novel sperm cell-specific polypeptide (TCTE1) that is conserved across vert ebrate species. TCTE1 is absolutely required for fertilization and is expressed in earlier stages of spermatogenesis. When the amino acid se quence of the TCTE1 gene product is compared among various mammalian s pecies, a large, highly conserved domain is observed, along with a div ergent domain encoding the 56-58 residues at the N terminus. In this s tudy, the N-terminal regions of the TCTE1 polypeptide from three roden t species-mouse, gerbil, and rat-were compared. The results show that while the gerbil and mouse species are most distant in evolutionary te rms, their TCTE1 homologs have not undergone significant divergence. I n contrast, the N-terminal region of the rat TCTE1 homolog has evolved rapidly, a finding that indicates positive Darwinian selection. We ha ve tested the correlation between TCTE1 divergence and heterospecific sperm-egg binding ability in the three species under study. Gerbil spe rm bind to mouse eggs, while no significant binding is observed betwee n rat sperm and mouse eggs. The results obtained support the hypothesi s that the sperm-specific polypeptide TCTE1 may facilitate species-spe cific divergence of sperm function.