Abstract
BackgroundCompetition between spermatozoa from rival males for success in fertilization (i.e., sperm competition) is an important selective force driving the evolution of male reproductive traits and promoting positive selection in genes related to reproductive function. Positive selection has been identified in reproductive proteins showing rapid divergence at nucleotide level. Other mutations, such as insertions and deletions (indels), also occur in protein-coding sequences. These structural changes, which exist in reproductive genes and result in length variation in coded proteins, could also be subjected to positive selection and be under the influence of sperm competition. Catsper1 is one such reproductive gene coding for a germ-line specific voltage-gated calcium channel essential for sperm motility and fertilization. Positive selection appears to promote fixation of indels in the N-terminal region of CatSper1 in mammalian species. However, it is not known which selective forces underlie these changes and their implications for sperm function.ResultsWe tested if length variation in the N-terminal region of CatSper1 is influenced by sperm competition intensity in a group of closely related rodent species of the subfamily Murinae. Our results revealed a negative correlation between sequence length of CatSper1 and relative testes mass, a very good proxy of sperm competition levels. Since CatSper1 is important for sperm flagellar motility, we examined if length variation in the N-terminus of CatSper1 is linked to changes in sperm swimming velocity. We found a negative correlation between CatSper1 length and several sperm velocity parameters.ConclusionsAltogether, our results suggest that sperm competition selects for a shortening of the intracellular region of CatSper1 which, in turn, enhances sperm swimming velocity, an essential and adaptive trait for fertilization success.
Highlights
Competition between spermatozoa from rival males for success in fertilization is an important selective force driving the evolution of male reproductive traits and promoting positive selection in genes related to reproductive function
Catsper1 phylogenies built by Neighbor-Joining and Maximum Likelihood approaches (Additional file 3) showed almost identical topology, but they showed some differences when compared to the species tree (Figure 2), which suggests that CatSper1 may be subjected to selective forces that would alter the evolutionary pattern expected from the phylogenetic relationships among the species
Because CatSper1 is a possible candidate for direct involvement in sperm competition, we assessed whether the fixation of indel substitutions in the N-terminal region of CatSper1 is associated to this selective force
Summary
Competition between spermatozoa from rival males for success in fertilization (i.e., sperm competition) is an important selective force driving the evolution of male reproductive traits and promoting positive selection in genes related to reproductive function. Positive selection has been identified in reproductive proteins showing rapid divergence at nucleotide level Other mutations, such as insertions and deletions (indels), occur in protein-coding sequences. These structural changes, which exist in reproductive genes and result in length variation in coded proteins, could be subjected to positive selection and be under the influence of sperm competition. Positive selection appears to promote fixation of indels in the N-terminal region of CatSper in mammalian species It is not known which selective forces underlie these changes and their implications for sperm function. An increase in the size of sperm components is generally associated to increases in sperm swimming velocity [14,15,16,17]
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