Positive selection for indel substitutions in the rodent sperm protein Catsper1

Catsperl is a voltage-gated calcium channel located in the plasma membrane of the sperm tail and is necessary for sperm motility and fertility in mice. We here examine the evolutionary pattern of Catsperl from nine species of the rodent subfamily Murinae of family Muridae. We show that the rate of insertion/deletion (indel) substitutions in exon I of the gene is 4-15 times that in introns or neutral genomic regions, suggesting the presence of strong positive selection that promotes fixations of indel mutations in exon 1. The number of indel polymorphisms within species appears higher than expected from interspecific comparisons, although there are too little data to provide a statistically significant conclusion. These results, together with an earlier report in primates, indicate that positive selection promoting length variation in Catsperl may be widespread in mammals. A structural model of Catsperl suggested the importance of the exon I-encoded region in regulating channel inactivation, which may affect sperm mobility and sperm competition. Our findings provide a necessary foundation for future experimental investigations of Catsperl's function in sperm physiology and role in sperm competition using rodent models.