Rapid changes in a protein's amino acid sequence over evolutionary time have often been attributed to the effects of positive selection, characterized at the molecular level by an excess of nonsynonymous nucleotide substitutions compared with synonymous ones (Nei and Gojobori 1986
; Hughes and Nei 1988
; Yang and Bielawski 2000
; Nielsen 2005
; Jensen et al. 2007
). Repeated selection for amino acid substitutions is thought to be a form of adaptive evolution, resulting in an increase in the fitness of the organism. Proteins involved in reproductive fitness have been shown to have evolved unusually rapidly across diverse groups of organisms (Swanson and Vacquier 2002
; Emes et al. 2003
; Bustamante et al. 2005
; Clark et al. 2006
). In the case of reproductive proteins, coevolutionary cycles involving adaptation and counter adaptation are expected to apply continuous selective pressure, resulting in rapid changes at amino acid sites involved in the function of the protein. These proteins often have roles in sperm competition, host immunity to pathogens, and manipulation of female reproductive physiology and behavior; however, in many other cases, the function of the rapidly evolving protein is unknown.
We sought to determine the extent of positive selection in rodent seminal fluid proteins and to identify new proteins subject to positive selection. Evolutionary screens, that is, “scanning” for elevated dN/dS between genes in a designated group, or in the whole genome, are a powerful way to identify, in a single experiment, many candidate genes undergoing positive selection. Because there is no a priori requirement to know the function of the protein before determining that it has experienced positive selection, those with either unknown or poorly understood functions can be identified for further study and, as a consequence, our knowledge about proteins with important effects on reproductive fitness should grow more rapidly.
We performed an evolutionary screen of rodent proteins and compared the results with similar studies in diverse taxonomic groups including Drosophila
and primates (Clark and Swanson 2005
; Dean et al. 2007
; Ramm et al. 2007
). Although most studies focus on one taxonomic group, we are interested in comparing evolutionary pressures on reproductive proteins across a wide taxonomic spectrum. For example, we find that, although several rodent and primate prostate proteins evolved rapidly, their complement of highly expressed genes are largely different. The availability of the mouse (Mus musculus
) and rat (Rattus norvegicus
) genomes (Waterston et al. 2002
; Gibbs et al. 2004
) and the mouse prostate-expressed sequence tag (EST) database (Nelson et al. 2002
) presented the opportunity to conduct an evolutionary screen of murid rodent taxa. The advantage of this screen is that it provides candidate proteins for studies of rapid evolution in rodent seminal fluid proteins.
In the process of our evolutionary screen of rodents, we identified and characterized a group of candidate seminal vesicle secretion (Svs) proteins under selection and showed that one of them, Svs7, is evolving very rapidly in rodents. Consequently, we also studied Svs7 evolution in primates and found a similar high rate of evolution. We note that the regions of protein structure changing most rapidly are quite similar between rodent and primate Svs7 proteins. Svs7 has been identified as mouse caltrin (for reviews, see Lardy 1985
), a protein involved in sperm capacitation, the process responsible for the timing of changes in sperm activity and behavior, following ejaculation. This is interesting in regard to the strong selection on Svs7 that we report here because Svs7 is only one member of a heterogeneous group of proteins identified by classical biochemical characterizations as having caltrin activity in various mammals (Lardy 1985
). Nonetheless, our data suggest that Svs7 is involved in a competitive aspect of reproductive fitness in rodents and primates, and we present arguments that sperm competition must be considered as a likely competitive mechanism driving the rapid evolution seen in this protein.