Positive selection on protein-length in the evolution of a primate sperm ion channel

Positive Darwinian selection on advantageous point substitutions has been demonstrated in many genes. We here provide empirical evidence, for the first time, that positive selection can also act on insertion/deletion (indel) substitutions in the evolution of a protein. CATSPER1 is a voltage-gated calcium channel found exclusively in the plasma membrane of the mammalian sperm tail and it is essential for sperm motility. We determined the DNA sequences of the first exon of the CATSPER1 gene from 15 primates, which encodes the intracellular N terminus region of approximately equal to 400 aa. These sequences exhibit an excessively high frequency of indels. However, all indels have lengths that are multiples of 3 nt (3n indels) and do not disrupt the ORF. The number of indel substitutions per site per year in CATSPER1 is five to eight times the corresponding rates calculated from two large-scale primate genomic comparisons, which represent the neutral rate of indel substitutions. Moreover, CATSPER1 indels are considerably longer than neutral indels. These observations strongly suggest that positive selection has been promoting the fixation of indel mutations in CATSPER1 exon 1. It has been shown in certain ion channels that the length of the N terminus region affects the rate of channel inactivation. This finding suggests that the selection detected may be related to the regulation of the CATSPER1 channel, which can affect sperm motility, an important determinant in sperm competition.