A Candidate Subspecies Discrimination System Involving a Vomeronasal Receptor Gene with Different Alleles Fixed in M. m. domesticus and M. m. musculus

Laukaitis CM (2010) A Candidate Subspecies Discrimination System Involving a Vomeronasal Receptor Gene with Different Alleles Fixed in M. m. domesticus and M. m. musculus. PLoS ONE 5(9): e12638. doi:10.1371/journal.pone.0012638 A Candidate Subspecies Discrimination System Involving a Vomeronasal Receptor Gene with Different Alleles Fixed in M. m. domesticus and M. m. musculus Robert C. Karn 0 Janet M. Young 0 Christina M. Laukaitis 0 Sebastian D. Fugmann, National Institute on Aging, United States of America 0 1 Department of Medicine, College of Medicine, University of Arizona, Tucson, Arizona, United States of America, 2 Division of Human Biology, Fred Hutchinson Cancer Research Center , Seattle, Washington , United States of America Assortative mating, a potentially efficient prezygotic reproductive barrier, may prevent loss of genetic potential by avoiding the production of unfit hybrids (i.e., because of hybrid infertility or hybrid breakdown) that occur at regions of secondary contact between incipient species. In the case of the mouse hybrid zone, where two subspecies of Mus musculus (M. m. domesticus and M. m. musculus) meet and exchange genes to a limited extent, assortative mating requires a means of subspecies recognition. We based the work reported here on the hypothesis that, if there is a pheromone sufficiently diverged between M. m. domesticus and M. m. musculus to mediate subspecies recognition, then that process must also require a specific receptor(s), also sufficiently diverged between the subspecies, to receive the signal and elicit an assortative mating response. We studied the mouse V1R genes, which encode a large family of receptors in the vomeronasal organ (VNO), by screening Perlegen SNP data and identified one, Vmn1r67, with 24 fixed SNP differences most of which (15/24) are nonsynonymous nucleotide substitutions between M. m. domesticus and M. m. musculus. We observed substantial linkage disequilibrium (LD) between Vmn1r67 and Abpa27, a mouse salivary androgen-binding protein gene that encodes a proteinaceous pheromone (ABP) capable of mediating assortative mating, perhaps in conjunction with its bound small lipophilic ligand. The LD we observed is likely a case of association rather than residual physical linkage from a very recent selective sweep, because an intervening gene, Vmn1r71, shows significant intra(sub)specific polymorphism but no inter(sub)specific divergence in its nucleotide sequence. We discuss alternative explanations of these observations, for example that Abpa27 and Vmn1r67 are coevolving as signal and receptor to reinforce subspecies hybridization barriers or that the unusually divergent Vmn1r67 allele was not a product of fast positive selection, but was derived from an introgressed allele, possibly from Mus spretus. - Funding: JMY was supported by National Institutes of Health grant DC004209 to Barbara Trask. CML was supported by career development funding from a National Cancer Institute Specialized Program of Research Excellence in Gastrointestinal Cancer (P50 CA95060). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. Pheromones, specific substances secreted to the exterior of an organism, communicate information about sex, species and related states to other members of the same species, whereupon the pheromones elicit specific reactions such as behavior and/or endocrine changes [1]. Many pheromones in terrestrial animals are volatile airborne molecules, however, large non-volatile molecules such as peptides and proteins may also be utilized for communication (reviewed in [2,3]). Olfactory cues represent the primary means of communication in nocturnal animals such as the house mouse [4,5] and two families of receptors in the vomeronasal organ (VNO), the V1Rs and the V2Rs, are thought to detect pheromonal signals [6]. Studies of putative mouse pheromones have proliferated over the past several decades, however, it is one thing to propose a pheromonal function but quite another to elucidate a mechanism, including identification of the receptor by which the pheromone is recognized. In the case of putative mouse pheromones, receptor identification has generally relied on cell biology experiments which have indicated that the VNO and the accessory olfactory bulb comprise the system receiving and processing the information [7,8], but in those experiments identification of the specific VNO receptors has been elusive. In this report, we demonstrate a purely genetic and evolutionary approach with which to identify specific receptors for a particular pheromonal function, subspecies recognition [911], thought to mediate assortative mating where two different subspecies make secondary contact, e.g. the European house mouse hybrid zone described below. The house mouse, Mus musculus, comprises at least three relatively distinct parapatric gene pools given subspecies status by some and full species status by others (for reviews see [12,13]). Two subspecies, Mus musculus domesticus and M. m. musculus, occupy distinct geographic ranges in western and eastern Europe, respectively. Where these make contact across southern Danish Jutland and through Central Europe from the Baltic Sea to the Black Sea coast, they form a narrow hybrid zone, which is a region of limited gene exchange [1214]. Two lines of indirect evidence suggest that selection is acting against hybrids: (1) hybrid male sterility and partial female sterility have been described in different crosses of laboratory or wild populations [1523]; and (2) limited introgression of sex chromosome markers as compared to autosomes has been shown across four studied hybrid zone transects [2431]. The reduced fitness of hybrid animals within the zone has been proposed to create a genetic sink, where genes entering the zone are eliminated by selection [32]. Assortative mating is a potentially efficient prezygotic reproductive barrier, which may prevent loss of genetic potential into unfit hybrids [3340]. When partial postzygotic isolation acts in the presence of divergent specific mate recognition systems, selection for increased mating specificity, the phenomenon of reinforcement, may lead to complete speciation [34,4143]. This idea predicts that if hybrids are less fit, reinforcement should then amplify homo(sub)specific preference most close to a contact zone, a phenomenon called reproductive character displacement. Reinforcement is best studied in closely related or recently divergent taxa, such as the subspecies of house mice, where limited hybridization still occurs and speciation may be incipient. Here, selection may act to reinforce prezygotic isolation in regions of secondary contact, e.g. the European mouse hybrid zone, leading to avoidance of disadvantageous hetero(sub)specific mating. A divergent subspecific m (...truncated)