Accelerated evolution and loss of a domain of the sperm-egg-binding protein SED1 in ancestral primates

Proteins involved in sperm-egg binding have been shown to evolve rapidly in several groups of invertebrates and vertebrates. Mammalian SED1 (secreted protein containing N-terminal Notch-like type II epidermal growth factor (EGF) repeats and C-terminal discoidin/F5/8 C domains) is a recently identified sperm surface protein that binds the egg zona pellucida and facilitates sperm-egg adhesion. SED1-null male mice are subfertile. Here we examine the SED1 gene from 11 mammalian species and provide evidence that it underwent accelerated evolution in ancestral primates, most likely driven by positive selection. Specifically, the intensity of the positive selection across various protein domains of SED1 was heterogeneous. Although one of the 2 Notch-like EGF domains, which mediate protein-protein binding, was lost in primate SED1, the second EGF domain evolved under strong positive selection favoring polar to nonpolar amino acid replacements. By contrast, the 2 discoidin/F5/8 type C domains, which are involved in protein-cell membrane binding, do not show definite signs of positive selection. The structural modification and occurrence of directional selection in ancestral primates but not any other lineage suggest that the function of SED1 may have changed during primate evolution. These results reveal a different evolutionary pattern of SED1 from that of many other sperm-egg-binding proteins, which often show diversifying selection occurring in multiple lineages.