Comparative Analysis of Testis Protein Evolution in Rodents
Turner, Leslie M., Chuong, Edward B., Hoekstra, Hopi E., Genetics
Genes expressed in testes are critical to male reproductive success, affecting spermatogenesis, sperm competition, and sperm-egg interaction. Comparing the evolution of testis proteins at different taxonomic levels can reveal which genes and functional classes are targets of natural and sexual selection and whether the same genes are targets among taxa. Here we examine the evolution of testis-expressed proteins at different levels of divergence among three rodents, mouse (Mus musculus), rat (Rattus norvegicus), and deer mouse (Peromyscus maniculatus), to identify rapidly evolving genes. Comparison of expressed sequence tags (ESTs) from testes suggests that proteins with testis-specific expression evolve more rapidly on average than proteins with maximal expression in other tissues. Genes with the highest rates of evolution have a variety of functional roles including signal transduction, DNA binding, and egg-sperm interaction. Most of these rapidly evolving genes have not been identified previously as targets of selection in comparisons among more divergent mammals. To determine if these genes are evolving rapidly among closely related species, we sequenced 11 of these genes in six Peromyscus species and found evidence for positive selection in five of them. Together, these results demonstrate rapid evolution of functionally diverse testis-expressed proteins in rodents, including the identification of amino acids under lineage-specific selection in Peromyscus. Evidence for positive selection among closely related species suggests that changes in these proteins may have consequences for reproductive isolation.
ONE of the most striking patterns in molecular evolution is that reproductive proteins evolve faster than other protein classes, a pattern consistent across diverse taxa (Singh and Kulathinal 2000; Swanson and Vacquier 2002; Clark et al. 2006). These rapidly evolving proteins serve diverse functions in both males and females and act at various stages of the fertilization process ranging from navigation of sperm through the female reproductive tract through egg?sperm fusion (Clark et al. 2006). Many questions, however, remain unresolved: (1) Do proteins involved in particular biological functions or participating in specific steps of fertilization evolve more rapidly than others?, (2) Are the same proteins and amino acid sites targets of selection in different taxa?, and (3) Does divergence in reproductive proteins contribute to reproductive isolation between closely related taxa?
In mammals, research on reproductive protein evolution has focused primarily on sequence analysis of candidate genes chosen because of their role in fertilization. This approach has identified positive selection (mainly on the basis of relative rates of nonsynonymous vs. synonymous change) acting on genes involved in sperm motility, semen coagulation, sperm?egg binding, and sperm?egg fusion (Clark et al. 2006). The functions of numerous proteins involved in fertilization, however, are unknown ( Jansen et al. 2001; Tanphaichitr et al. 2007); therefore, candidate gene approaches are likely to miss important targets of selection. In contrast, a genomewide analysis of reproductive proteins can characterize general patterns of evolution as well as identify rapidly evolving genes. Such genomic approaches have been particularly useful in identifying rapidly evolving male accessory gland proteins (Acps) in Drosophila (Swanson et al. 2001a) and crickets (Andres et al. 2006; Braswell et al. 2006), female reproductive tract proteins in Drosophila (Swanson et al. 2001b), and seminal proteins in primates (Clark and Swanson 2005).
We use a genomic approach to characterize reproductive protein evolution in three rodents, mouse (Mus musculus), rat (Rattus norvegicus), and deer mouse (Peromyscus maniculatus). Rodents are an excellent system for investigating mammalian reproductive protein evolution. Fertilization is better characterized in Mus than any other mammal, due to its importance as a model in human reproductive health research (Nixon et al. …