Sexual selection and pseudogenization in primate fertilization

The mouse sperm protein ZP3R interacts with proteins in the egg coat and mediates sperm-egg adhesion in a species-specific manner. Understanding the function and evolution of such genes has important implications for studies of speciation, reproductive success, and infertility. A recent analysis showed that (1) the human pseudogene C4BPAP1 is the ortholog of Zp3r, (2) ZP3R pseudogenization evolved independently in parallel among several primate lineages, and (3) functional ZP3R genes evolve under positive selection among other primate species. The causes of this pseudogenization of ZP3R are unknown. To explore one plausible cause (changes in sexual selection on males), we searched for ZP3R pseudogenes in recently published genomes, then compared sexually selected male traits among lineages with and without a functional ZP3R. We found that traits associated with more intense sexual selection on males (larger male body size, larger male canines, larger testes) were associated with functional ZP3R expression, and suggest that a relaxation of sexual selection may be associated with ZP3R pseudogenization. We propose the causal relationship implies an evolutionary cost to maintaining redundancy in the suite of primate fertilization genes.