Sexual Selection and the Adaptive Evolution of Mammalian Ejaculate Proteins

Sexual Selection and the Adaptive Evolution of Mammalian Ejaculate Proteins Steven A. Ramm,*1 Peter L. Oliver, 1 Chris P. Ponting, Paula Stockley,* and Richard D. Emesà2 *Mammalian Behaviour and Evolution Group, Faculty of Veterinary Science, University of Liverpool, Liverpool, United Kingdom; MRC Functional Genetics Unit, Department of Physiology, Anatomy and Genetics, University of Oxford, Oxford, United Kingdom; and àDepartment of Biology, University College London, London, United Kingdom Introduction Reproduction-related genes are known to have evolved unusually rapidly across diverse animal groups (Swanson and Vacquier 2002; Emes et al. 2003; Bustamante et al. 2005; Richards et al. 2005; Clark et al. 2006). Of several factors suggested as major influences on the evolutionary rate of reproductive genes (Swanson and Vacquier 2002), those hypothesized to contribute most involve continual selective pressure deriving from coevolutionary cycles of adaptation and counteradaptation. Such coevolutionary cycles could occur either between or within genomes. Thus, rapid evolution could be driven by host–pathogen coevolution because many of the genes expressed in reproductive tissues possess important immune-related functions (Good and Nachman 2005). Alternatively or additionally, the rapid evolution of reproductive genes may result from episodes of sexual selection and coevolution between the sexes (Swanson and Vacquier 2002). In support of the latter idea, many rapidly evolving genes are sex limited or sex biased in their expression (e.g., Pröschel et al. 2006); for example, male accessory gland-specific proteins found in seminal fluid evolve rapidly for both vertebrate (Clark and Swanson 2005) and invertebrate (Swanson, Clark, et al. 2001; Andrés et al. 2006) taxa. Despite the potentially pervasive influence of sexual selection in driving adaptation at the molecular level, very few studies to date have reported evidence directly linking 1 These authors contributed equally to this work. Present address: Institute for Science and Technology in Medicine, School of Medicine, Keele University, Staffordshire, United Kingdom. Key words: adaptive evolution, positive selection, primates, rodents, sexual selection, sperm competition. 2 E-mail: . Mol. Biol. Evol. 25(1):207–219. 2008 doi:10.1093/molbev/msm242 Advance Access publication November 20, 2007 Ó The Author 2007. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution. All rights reserved. For permissions, please e-mail: episodes of positive selection to changes in relevant physiological or behavioral traits. So far, these studies have concentrated on examining whether species’ differences in evolutionary rates correlate with diversity in mating systems or with other proxy measures of sexual selection intensity (Dorus et al. 2004; Wagstaff and Begun 2005; Turner and Hoekstra 2006; Herlyn and Zischler 2007; Hurle et al. 2007; Nadeau et al. 2007; Wagstaff and Begun 2007). One such study concluded that differences in female promiscuity among primate species are responsible for variation in the rate of evolution of SEMG2, which encodes the major protein present in primate seminal fluid (Dorus et al. 2004). However, a subsequent reanalysis of SEMG2 and related genes based on a larger data set failed to establish support for this conclusion (Hurle et al. 2007). Nevertheless, others have now shown that the evolutionary rate of the primate sperm ligand zonadhesin (ZAN) correlates negatively with sexual size dimorphism and that an avian pigmentation gene (MC1R) evolves fastest in lineages with greatest sexual dichromatism (Herlyn and Zischler 2007; Nadeau et al. 2007). More broadly, Wagstaff and Begun (2005, 2007) have noted the generally higher rates of Drosophila accessory gland protein (Acp) gene divergence in repleta group versus melanogaster subgroup flies, potentially driven by the higher rates of female remating in repleta group females. Although these studies clearly highlight sexual selection’s potential explanatory power, whether or not variation in sexual selection intensity can generally account for interspecific diversity in evolutionary rates for single genes clearly warrants further investigation. Here, we address this question by investigating the molecular evolution of several genes expressed in the male reproductive tract of muroid rodents. We seek evidence of adaptive evolution specifically for those lineages that are associated with large relative testis size (RTS). This is An elevated rate of substitution characterizes the molecular evolution of reproductive proteins from a wide range of taxa. Although the selective pressures explaining this rapid evolution are yet to be resolved, recent evidence implicates sexual selection as a potentially important explanatory factor. To investigate this hypothesis, we sought evidence of a high rate of adaptive gene evolution linked to postcopulatory sexual selection in muroid rodents, a model vertebrate group displaying a broad range of mating systems. Specifically, we sequenced 7 genes from diverse rodents that are expressed in the testes, prostate, or seminal vesicles, products of which have the potential to act in sperm competition. We inferred positive Darwinian selection in these genes by estimation of the ratio of nonsynonymous (dN, amino acid changing) to synonymous (dS, amino acid retaining) substitution rates (x 5 dN/dS). Next, we tested whether variation in this ratio among lineages could be attributed to interspecific variation in mating systems, as inferred from the variation in these rodents’ relative testis sizes (RTS). Four of the 7 genes examined (Prm1, Sva, Acrv1, and Svs2, but not Svp2, Msmb, or Spink3) exhibit unambiguous evidence of positive selection. One of these, the seminal vesicle–derived protein Svs2, also shows some evidence for a concentration of positive selection in those lineages in which sperm competition is common. However, this was not a general trend among all the rodent genes we examined. Using the same methods, we then reanalyzed previously published data on 2 primate genes, SEMG1 and SEMG2. Although SEMG2 also shows evidence of positive selection concentrated in lineages subject to high levels of sperm competition, no such trend was found for SEMG1. Overall, despite a high rate of positive selection being a feature of many ejaculate proteins, these results indicate that the action of sexual selection potentially responsible for elevated evolutionary rates may be difficult to detect on a gene-by-gene basis. Although the extreme diversity of reproductive phenotypes exhibited in nature attests to the power of sexual selection, the extent to which this force predominates in driving the rapid molecular evolution of reproductive genes therefore remains to be determined. 208 Ramm et al. olysis when deposited in the female reproductive tract (Xuan et al. 1999). The testis-specific acrosomal pro (...truncated)