Abstract
BackgroundCysteine-RIch Secretory Proteins (CRISP) are expressed in the reproductive tract of mammalian males and are involved in fertilization and related processes. Due to their important role in sperm performance and sperm-egg interaction, these genes are likely to be exposed to strong selective pressures, including postcopulatory sexual selection and/or male-female coevolution. We here perform a comparative evolutionary analysis of Crisp genes in mammals. Currently, the nomenclature of CRISP genes is confusing, as a consequence of discrepancies between assignments of orthologs, particularly due to numbering of CRISP genes. This may generate problems when performing comparative evolutionary analyses of mammalian clades and species. To avoid such problems, we first carried out a study of possible orthologous relationships and putative origins of the known CRISP gene sequences. Furthermore, and with the aim to facilitate analyses, we here propose a different nomenclature for CRISP genes (EVAC1–4, “EVolutionarily-analyzed CRISP”) to be used in an evolutionary context.ResultsWe found differing selective pressures among Crisp genes. CRISP1/4 (EVAC1) and CRISP2 (EVAC2) orthologs are found across eutherian mammals and seem to be conserved in general, but show signs of positive selection in primate CRISP1/4 (EVAC1). Rodent Crisp1 (Evac3a) seems to evolve under a comparatively more relaxed constraint with positive selection on codon sites. Finally, murine Crisp3 (Evac4), which appears to be specific to the genus Mus, shows signs of possible positive selection. We further provide evidence for sexual selection on the sequence of one of these genes (Crisp1/4) that, unlike others, is thought to be exclusively expressed in male reproductive tissues.ConclusionsWe found differing selective pressures among CRISP genes and sexual selection as a contributing factor in CRISP1/4 gene sequence evolution. Our evolutionary analysis of this unique set of genes contributes to a better understanding of Crisp function in particular and the influence of sexual selection on reproductive mechanisms in general.
Highlights
Cysteine-RIch Secretory Proteins (CRISP) are expressed in the reproductive tract of mammalian males and are involved in fertilization and related processes
In this study we present an overview of selective pressures and tendencies of sexual selection-driven evolution of CRISP gene sequences
Even though EVAC1 and EVAC2 both seem to be conserved in general, which might be explained by their potential additional roles in other processes [17], positive selection can still be a factor in the evolution of these two genes
Summary
Cysteine-RIch Secretory Proteins (CRISP) are expressed in the reproductive tract of mammalian males and are involved in fertilization and related processes Due to their important role in sperm performance and sperm-egg interaction, these genes are likely to be exposed to strong selective pressures, including postcopulatory sexual selection and/or male-female coevolution. The nomenclature of CRISP genes is confusing, as a consequence of discrepancies between assignments of orthologs, due to numbering of CRISP genes This may generate problems when performing comparative evolutionary analyses of mammalian clades and species. Proteins of the reproductive system that affect male and female traits are thought to be targets of accelerated gene sequence evolution [1, 2] Whereas this is generally true, evolutionary rates of reproductive proteins vary depending on their involvement in different reproductive. Proteins found on the sperm surface [16] and those with roles in sperm motility and sperm-egg interaction [5] have been found to have high evolutionary rates
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