Sex-Linked Mammalian Sperm Proteins Evolve Faster Than Autosomal Ones

X-linked genes can evolve slower or faster depending on whether most recessive, or at least partially recessive alleles are deleterious or beneficial due to their hemizygous expression in males. Molecular studies of X chromosome divergence have provided conflicting evidence for both a higher and lower rate of nucleotide substitution at both synonymous and nonsynonymous sites, depending on the nucleotide sites sampled. Using human and mouse orthologous genes, we tested the hypothesis that genes encoding male-specific sperm proteins are evolving faster on the X chromosome compared with autosomes. X-linked sperm proteins have an average nonsynonymous mutation rate almost twice as high as sperm genes found on autosomes, unlike other tissue-specific genes, where no significant difference in the nonsynonymous mutation rate between the X chromosome and autosomes was found. However, no difference was found in the average synonymous mutation rate of X-linked versus autosomal sperm proteins, which along with corresponding higher values of Ka/Ks in X-linked sperm proteins suggest that differences in selective forces and not mutation rates are the underlying cause of higher X-linked mammalian sperm protein divergence.