Sequence analysis of mouse vomeronasal receptor gene clusters reveals common promoter motifs and a history of recent expansion

Clustering of vomeronasal receptor genes is required for transcriptional stability but not for choice

Rodents perceive pheromones via vomeronasal receptors encoded by highly evolutionarily dynamic Vr and Fpr gene superfamilies. We report here that high numbers of V1r pseudogenes are scattered in mammalian genomes, contrasting with the clustered organization of functional V1r and Fpr genes. We also found that V1r pseudogenes are more likely to be expressed when located in a functional V1r gene cluster than when isolated. To explore the potential regulatory role played by the association of functional vomeronasal receptor genes with their clusters, we dissociated the mouse Fpr-rs3 from its native cluster via transgenesis. Singular and specific transgenic Fpr-rs3 transcription was observed in young vomeronasal neurons but was only transient. Our study of natural and artificial dispersed gene duplications uncovers the existence of transcription-stabilizing elements not coupled to vomeronasal gene units but rather associated with vomeronasal gene clusters and thus explains the evolutionary conserved clustered organization of functional vomeronasal genes.

Coadaptation of the chemosensory system with voluntary exercise behavior in mice

Ethologically relevant chemical senses and behavioral habits are likely to coadapt in response to selection. As olfaction is involved in intrinsically motivated behaviors in mice, we hypothesized that selective breeding for a voluntary behavior would enable us to identify novel roles of the chemosensory system. Voluntary wheel running (VWR) is an intrinsically motivated and naturally rewarding behavior, and even wild mice run on a wheel placed in nature. We have established 4 independent, artificially evolved mouse lines by selectively breeding individuals showing high VWR activity (High Runners; HRs), together with 4 non-selected Control lines, over 88 generations. We found that several sensory receptors in specific receptor clusters were differentially expressed between the vomeronasal organ (VNO) of HRs and Controls. Moreover, one of those clusters contains multiple single-nucleotide polymorphism loci for which the allele frequencies were significantly divergent between the HR and Control lines, i.e., loci that were affected by the selective breeding protocol. These results indicate that the VNO has become genetically differentiated between HR and Control lines during the selective breeding process. Although the role of the vomeronasal chemosensory receptors in VWR activity remains to be determined, the current results suggest that these vomeronasal chemosensory receptors are important quantitative trait loci for voluntary exercise in mice. We propose that olfaction may play an important role in motivation for voluntary exercise in mammals.

Comparative Genomic Analysis of the Pheromone Receptor Class 1 Family (V1R) Reveals Extreme Complexity in Mouse Lemurs (Genus, Microcebus) and a Chromosomal Hotspot across Mammals

Sensory gene families are of special interest for both what they can tell us about molecular evolution and what they imply as mediators of social communication. The vomeronasal type-1 receptors (V1Rs) have often been hypothesized as playing a fundamental role in driving or maintaining species boundaries given their likely function as mediators of intraspecific mate choice, particularly in nocturnal mammals. Here, we employ a comparative genomic approach for revealing patterns of V1R evolution within primates, with a special focus on the small-bodied nocturnal mouse and dwarf lemurs of Madagascar (genera Microcebus and Cheirogaleus, respectively). By doubling the existing genomic resources for strepsirrhine primates (i.e. the lemurs and lorises), we find that the highly speciose and morphologically cryptic mouse lemurs have experienced an elaborate proliferation of V1Rs that we argue is functionally related to their capacity for rapid lineage diversification. Contrary to a previous study that found equivalent degrees of V1R diversity in diurnal and nocturnal lemurs, our study finds a strong correlation between nocturnality and V1R elaboration, with nocturnal lemurs showing elaborate V1R repertoires and diurnal lemurs showing less diverse repertoires. Recognized subfamilies among V1Rs show unique signatures of diversifying positive selection, as might be expected if they have each evolved to respond to specific stimuli. Furthermore, a detailed syntenic comparison of mouse lemurs with mouse (genus Mus) and other mammalian outgroups shows that orthologous mammalian subfamilies, predicted to be of ancient origin, tend to cluster in a densely populated region across syntenic chromosomes that we refer to as a V1R "hotspot."

Pronounced strain-specific chemosensory receptor gene expression in the mouse vomeronasal organ

Background

The chemosensory system plays an important role in orchestrating sexual behaviors in mammals. Pheromones trigger sexually dimorphic behaviors and different mouse strains exhibit differential responses to pheromone stimuli. It has been speculated that differential gene expression in the sensory organs that detect pheromones may underlie sexually-dimorphic and strain-specific responses to pheromone cues.

Results

We have performed transcriptome analyses of the mouse vomeronasal organ, a sensory organ recognizing pheromones and interspecies cues. We find little evidence of sexual dimorphism in gene expression except for Xist, an essential gene for X-linked gene inactivation. Variations in gene expression are found mainly among strains, with genes from immune response and chemosensory receptor classes dominating the list. Differentially expressed genes are concentrated in genomic hotspots enriched in these families of genes. Some chemosensory receptors show exclusive patterns of expression in different strains. We find high levels of single nucleotide polymorphism in chemosensory receptor pseudogenes, some of which lead to functionalized receptors. Moreover, we identify a number of differentially expressed long noncoding RNA species showing strong correlation or anti-correlation with chemoreceptor genes.

Conclusions

Our analyses provide little evidence supporting sexually dimorphic gene expression in the vomeronasal organ that may underlie dimorphic pheromone responses. In contrast, we find pronounced variations in the expression of immune response related genes, vomeronasal and G-protein coupled receptor genes among different mouse strains. These findings raised the possibility that diverse strains of mouse perceive pheromone cues differently and behavioral difference among strains in response to pheromone may first arise from differential detection of pheromones. On the other hand, sexually dimorphic responses to pheromones more likely originate from dimorphic neural circuits in the brain than from differential detection. Moreover, noncoding RNA may offer a potential regulatory mechanism controlling the differential expression patterns.

Electronic supplementary material

The online version of this article (10.1186/s12864-017-4364-4) contains supplementary material, which is available to authorized users.

Identification of a uniquely expanded V1R (ORA) gene family in the Japanese grenadier anchovy (Coilia nasus)

A number of studies have suggested that olfaction plays an important role in fish migration. Fish use several distinct families of olfactory receptors to detect environmental odorants, including MORs (main olfactory receptors), V1Rs (vomeronasal type-1 receptors), V2Rs (vomeronasal type-2 receptors), TAARs (trace amine-associated receptors), and FPRs (formyl peptide receptors). The V1Rs have been reported to detect pheromones, and a pheromone hypothesis for the spawning migration of anadromous fish has been proposed. Examining whether Coilia nasus relies on V1R-mediated olfaction for spawning migration is important for understanding the molecular basis of spawning migration behavior. Here, we explored the V1R gene family in anadromous C. nasus. Six V1R genes previously reported in other teleost fish were successfully identified. Interestingly, we detected the largest V1R repertoire in teleost fish from C. nasus and identified a species-specific expansion event of V1R3 gene that has previously been detected as single-copy genes in other teleost fish. The V1R loci were found to be populated with repetitive sequences, especially in the expanded V1R3 genes. Additionally, the divergence of V1R3 genetic structures in different populations of C. nasus indicates the copy number variation (CNV) in V1R3 gene among individuals of C. nasus. Most of the putative C. nasus V1R genes were expressed primarily in the olfactory epithelium, consistent with the role of the gene products as functional olfactory receptors. Significant differences in the expression levels of V1R genes were detected between the anadromous and non-anadromous C. nasus. This study represents a first step in the elucidation of the olfactory communication system of C. nasus at the molecular level. Our results indicate that some V1R genes may be involved in the spawning migration of C. nasus, and the study provides new insights into the spawning migration and genome evolution of C. nasus.

Chemosensory receptor specificity and regulation

The senses provide a means by which data on the physical and chemical properties of the environment may be collected and meaningfully interpreted. Sensation begins at the periphery, where a multitude of different sensory cell types are activated by environmental stimuli as different as photons and odorant molecules. Stimulus sensitivity is due to expression of different cell surface sensory receptors, and therefore the receptive field of each sense is defined by the aggregate of expressed receptors in each sensory tissue. Here, we review current understanding on patterns of expression and modes of regulation of sensory receptors.

The molecular evolutionary dynamics of the vomeronasal receptor (class 1) genes in primates: a gene family on the verge of a functional breakdown

Olfaction plays a critical role in both survival of the individual and in the propagation of species. Studies from across the mammalian clade have found a remarkable correlation between organismal lifestyle and molecular evolutionary properties of receptor genes in both the main olfactory system (MOS) and the vomeronasal system (VNS). When a large proportion of intact (and putatively functional) copies is observed, the inference is made that a particular mode of chemoreception is critical for an organism’s fit to its environment and is thus under strong positive selection. Conversely, when the receptors in question show a disproportionately large number of pseudogene copies, this contraction is interpreted as evidence of relaxed selection potentially leading to gene family extinction. Notably, it appears that a risk factor for gene family extinction is a high rate of nonsynonymous substitution. A survey of intact vs. pseudogene copies among primate vomeronasal receptor Class one genes (V1Rs) appears to substantiate this hypothesis. Molecular evolutionary complexities in the V1R gene family combine rapid rates of gene duplication, gene conversion, lineage-specific expansions, deletions, and/or pseudogenization. An intricate mix of phylogenetic footprints and current adaptive landscapes have left their mark on primate V1Rs suggesting that the primate clade offers an ideal model system for exploring the molecular evolutionary and functional properties of the VNS of mammals. Primate V1Rs tell a story of ancestral function and divergent selection as species have moved into ever diversifying adaptive regimes. The sensitivity to functional collapse in these genes, consequent to their precariously high rates of nonsynonymous substitution, confer a remarkable capacity to reveal the lifestyles of the genomes that they presently occupy as well as those of their ancestors.

Molecular evolutionary characterization of a V1R subfamily unique to strepsirrhine primates

Vomeronasal receptor genes have frequently been invoked as integral to the establishment and maintenance of species boundaries among mammals due to the elaborate one-to-one correspondence between semiochemical signals and neuronal sensory inputs. Here, we report the most extensive sample of vomeronasal receptor class 1 (V1R) sequences ever generated for a diverse yet phylogenetically coherent group of mammals, the tooth-combed primates (suborder Strepsirrhini). Phylogenetic analysis confirms our intensive sampling from a single V1R subfamily, apparently unique to the strepsirrhine primates. We designate this subfamily as V1Rstrep. The subfamily retains extensive repertoires of gene copies that descend from an ancestral gene duplication that appears to have occurred prior to the diversification of all lemuriform primates excluding the basal genus Daubentonia (the aye-aye). We refer to the descendent clades as V1Rstrep-α and V1Rstrep-β. Comparison of the two clades reveals different amino acid compositions corresponding to the predicted ligand-binding site and thus potentially to altered functional profiles between the two. In agreement with previous studies of the mouse lemur (genus, Microcebus), the majority of V1Rstrep gene copies appear to be intact and under strong positive selection, particularly within transmembrane regions. Finally, despite the surprisingly high number of gene copies identified in this study, it is nonetheless probable that V1R diversity remains underestimated in these nonmodel primates and that complete characterization will be limited until high-coverage assembled genomes are available.

Positive Darwinian selection in the singularly large taste receptor gene family of an 'ancient' fish, Latimeria chalumnae

Background

Chemical senses are one of the foremost means by which organisms make sense of their environment, among them the olfactory and gustatory sense of vertebrates and arthropods. Both senses use large repertoires of receptors to achieve perception of complex chemosensory stimuli. High evolutionary dynamics of some olfactory and gustatory receptor gene families result in considerable variance of chemosensory perception between species. Interestingly, both ora/v1r genes and the closely related t2r genes constitute small and rather conserved families in teleost fish, but show rapid evolution and large species differences in tetrapods. To understand this transition, chemosensory gene repertoires of earlier diverging members of the tetrapod lineage, i.e. lobe-finned fish such as Latimeria would be of high interest.

Results

We report here the complete T2R repertoire of Latimeria chalumnae, using thorough data mining and extensive phylogenetic analysis. Eighty t2r genes were identified, by far the largest family reported for any species so far. The genomic neighborhood of t2r genes is enriched in repeat elements, which may have facilitated the extensive gene duplication events resulting in such a large family. Examination of non-synonymous vs. synonymous substitution rates (dN/dS) suggests pronounced positive Darwinian selection in Latimeria T2Rs, conceivably ensuring efficient neo-functionalization of newly born t2r genes. Notably, both traits, positive selection and enrichment of repeat elements in the genomic neighborhood, are absent in the twenty v1r genes of Latimeria. Sequence divergence in Latimeria T2Rs and V1Rs is high, reminescent of the corresponding teleost families. Some conserved sequence motifs of Latimeria T2Rs and V1Rs are shared with the respective teleost but not tetrapod genes, consistent with a potential role of such motifs in detection of aquatic chemosensory stimuli.

Conclusions

The singularly large T2R repertoire of Latimeria may have been generated by facilitating local gene duplication via increased density of repeat elements, and efficient neofunctionalization via positive Darwinian selection.

The high evolutionary dynamics of tetrapod t2r gene families precedes the emergence of tetrapods, i.e. the water-to-land transition, and thus constitutes a basal feature of the lobe-finned lineage of vertebrates.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-650) contains supplementary material, which is available to authorized users.

Common promoter elements in odorant and vomeronasal receptor genes

In mammals, odorants and pheromones are detected by hundreds of odorant receptors (ORs) and vomeronasal receptors (V1Rs and V2Rs) expressed by sensory neurons that are respectively located in the main olfactory epithelium and in the vomeronasal organ. Even though these two olfactory systems are functionally and anatomically separate, their sensory neurons show a common mechanism of receptor gene regulation: each neuron expresses a single receptor gene from a single allele. The mechanisms underlying OR and VR gene expression remain unclear. Here we investigated if OR and V1R genes share common sequences in their promoter regions.

We conducted a comparative analysis of promoter regions of 39 mouse V1R genes and found motifs that are common to a large number of promoters. We then searched mouse OR promoter regions for motifs that resemble the ones found in the V1R promoters. We identified motifs that are present in both the V1R and OR promoter regions. Some of these motifs correspond to the known O/E like binding sites while others resemble binding sites for transcriptional repressors. We show that one of these motifs specifically interacts with proteins extracted from both nuclei from olfactory and vomeronasal neurons. Our study is the first to identify motifs that resemble binding sites for repressors in the promoters of OR and V1R genes. Analysis of these motifs and of the proteins that bind to these motifs should reveal important aspects of the mechanisms of OR/V1R gene regulation.

The vomeronasal organ mediates interspecies defensive behaviors through detection of protein pheromone homologs

Potential predators emit uncharacterized chemosignals that warn receiving species of danger. Neurons that sense these stimuli remain unknown. Here we show that detection and processing of fear-evoking odors emitted from cat, rat, and snake require the function of sensory neurons in the vomeronasal organ. To investigate the molecular nature of the sensory cues emitted by predators, we isolated the salient ligands from two species using a combination of innate behavioral assays in naive receiving animals, calcium imaging, and c-Fos induction. Surprisingly, the defensive behavior-promoting activity released by other animals is encoded by species-specific ligands belonging to the major urinary protein (Mup) family, homologs of aggression-promoting mouse pheromones. We show that recombinant Mup proteins are sufficient to activate sensory neurons and initiate defensive behavior similarly to native odors. This co-option of existing sensory mechanisms provides a molecular solution to the difficult problem of evolving a variety of species-specific molecular detectors.

Extreme variability among mammalian V1R gene families

We report an evolutionary analysis of the V1R gene family across 37 mammalian genomes. V1Rs comprise one of three chemosensory receptor families expressed in the vomeronasal organ, and contribute to pheromone detection. We first demonstrate that Trace Archive data can be used effectively to determine V1R family sizes and to obtain sequences of most V1R family members. Analyses of V1R sequences from trace data and genome assemblies show that species-specific expansions previously observed in only eight species were prevalent throughout mammalian evolution, resulting in "semi-private" V1R repertoires for most mammals. The largest families are found in mouse and platypus, whose V1R repertoires have been published previously, followed by mouse lemur and rabbit (approximately 215 and approximately 160 intact V1Rs, respectively). In contrast, two bat species and dolphin possess no functional V1Rs, only pseudogenes, and suffered inactivating mutations in the vomeronasal signal transduction gene Trpc2. We show that primate V1R decline happened prior to acquisition of trichromatic vision, earlier during evolution than was previously thought. We also show that it is extremely unlikely that decline of the dog V1R repertoire occurred in response to selective pressures imposed by humans during domestication. Functional repertoire sizes in each species correlate roughly with anatomical observations of vomeronasal organ size and quality; however, no single ecological correlate explains the very diverse fates of this gene family in different mammalian genomes. V1Rs provide one of the most extreme examples observed to date of massive gene duplication in some genomes, with loss of all functional genes in other species.

Mammalian olfactory receptors

Perception of chemical stimuli from the environment is essential to most animals; accordingly, they are equipped with a complex olfactory system capable of receiving a nearly unlimited number of odorous substances and pheromones. This enormous task is accomplished by olfactory sensory neurons (OSNs) arranged in several chemosensory compartments in the nose. The sensitive and selective responsiveness of OSNs to odorous molecules and pheromones is based on distinct receptors in their chemosensory membrane; consequently, olfactory receptors play a key role for a reliable recognition and an accurate processing of chemosensory information. They are therefore considered as key elements for an understanding of the principles and mechanisms underlying the sense of smell. The repertoire of olfactory receptors in mammals encompasses hundreds of different receptor types which are highly diverse and expressed in distinct subcompartments of the nose. Accordingly, they are categorized into several receptor families, including odorant receptors (ORs), vomeronasal receptors (V1Rs and V2Rs), trace amine-associated receptors (TAARs), formyl peptide receptors (FPRs), and the membrane guanylyl cyclase GC-D. This large and complex receptor repertoire is the basis for the enormous chemosensory capacity of the olfactory system.

Exceptional LINE density at V1R loci: the Lyon repeat hypothesis revisited on autosomes

The mammalian olfactory system utilizes three large receptor families: the olfactory receptors (ORs) of the main nose and the vomeronasal type-1 and type-2 receptor genes (V1Rs and V2Rs) of the vomeronasal organ. We find that these loci are among the most long interspersed nuclear element (LINE)-dense regions of mammalian genomes. We investigate two evolutionary models to account for this cohabitation. First, we investigate an adaptive selection model, in which LINEs have contributed to expansions of mouse V1R repertoires. We find that even evolutionarily stable V1R loci are exceptionally LINE-rich compared to other genome loci, including loci containing other large gene clusters. Also, a more detailed analysis of specific V1R duplications does not reveal LINE patterns predicted by common LINE-mediated duplication mechanisms. Next, we investigate neutral models, in which LINEs were tolerated by, but not advantageous for, surrounding V1R genes. We find that V1R loci are exceptionally LINE-rich compared to other regions of similar AT base composition, and that duplicated V1R gene blocks are generally depleted of LINE elements, suggesting that these loci did not become densely populated with LINEs simply as a consequence of targeted integration or passive multiplication along with the genes. Finally, we show that individual LINE repeats of a given age at V1R, V2R, and OR loci exhibit a significantly longer average length than at other autosomal loci, suggesting a reduced tendency for these LINEs to be disrupted. We speculate that LINEs at V1R, V2R, and OR loci might be selectively retained because they contribute to allelic regulation of these three gene families.

Pheromone sensing in mice

Beginning with the neuroepithelium of the vomeronasal organ, the accessory olfactory system in rodents runs parallel to the main olfactory system and is specialized in the detection of pheromones. Only a small number of vomeronasal agonists carrying pheromonal information have been identified this far. These structurally diverse classes of chemicals include peptides secreted by exocrine glands and range from small volatile molecules to proteins and fragments thereof present in urine. Most pheromones activate both vomeronasal and main olfactory sensory neurons, making the identification of functionally relevant populations of sensory neurons difficult. Analyses of gene-targeted mice selectively affecting either vomeronasal or main olfactory signaling have attempted to elucidate the functional contribution of the different chemosensory epithelia to pheromone sensing in mice. These mouse models suggest that both the main and the accessory olfactory systems can converge and synergize to express the complex array of stereotyped behaviors and hormonal changes triggered by pheromones.

Odorant and pheromone receptor gene regulation in vertebrates

The largest mammalian gene family codes for odorant receptors and is exclusively devoted to the perception of the outside world. Its expression is very peculiar, since olfactory sensory neurons are only allowed to express a single of its numerous members, from a single parental allele. How this is achieved is unknown, but recent work points to multiple regulatory mechanisms, possibly shared by pheromone receptor genes, acting at (a) a general level, via the expression of the chemoreceptor itself and (b) a more restricted level, defined by activator elements.

Co-regulation of a large and rapidly evolving repertoire of odorant receptor genes

The olfactory system meets niche- and species-specific demands by an accelerated evolution of its odorant receptor repertoires. In this review, we describe evolutionary processes that have shaped olfactory and vomeronasal receptor gene families in vertebrate genomes. We emphasize three important periods in the evolution of the olfactory system evident by comparative genomics: the adaptation to land in amphibian ancestors, the decline of olfaction in primates, and the delineation of putative pheromone receptors concurrent with rodent speciation. The rapid evolution of odorant receptor genes, the sheer size of the repertoire, as well as their wide distribution in the genome, presents a developmental challenge: how are these ever-changing odorant receptor repertoires coordinated within the olfactory system? A central organizing principle in olfaction is the specialization of sensory neurons resulting from each sensory neuron expressing only ~one odorant receptor allele. In this review, we also discuss this mutually exclusive expression of odorant receptor genes. We have considered several models to account for co-regulation of odorant receptor repertoires, as well as discussed a new hypothesis that invokes important epigenetic properties of the system.

Sensory genes and mate choice: Evidence that duplications, mutations, and adaptive evolution alter variation in mating cue genes and their receptors

Fascinating new data, revealed through gene sequencing, comparative genomics, and genetic engineering, precisely establish which genes are involved in mate choice and mating activity--behaviors that are surprisingly understudied from a genetic perspective. Discussed here are some of the recently identified visual and chemosensory genes that are involved in mate choice and mating behavior. These genes' products are involved in the production, transmission, and receipt of crucial sensory mate-choice cues that affect fitness. This review exposes newfound evidence that alternative splicing, gene-expression pattern changes, and molecular genetic variation in sensory genes are crucial for both intra- and interspecific mate choice and mating success. Many sensory genes have arisen through gene duplications, and data amassed from studies conducted at scales ranging from individual genes to genomic comparisons show that strong, positive Darwinian selection acts on several mating-related genes and that these genes evolve rapidly.

V1R promoters are well conserved and exhibit common putative regulatory motifs

Background

The mouse vomeronasal organ (VNO) processes chemosensory information, including pheromone signals that influence reproductive behaviors. The sensory neurons of the VNO express two types of chemosensory receptors, V1R and V2R. There are ~165 V1R genes in the mouse genome that have been classified into ~12 divergent subfamilies. Each sensory neuron of the apical compartment of the VNO transcribes only one of the repertoire of V1R genes. A model for mutually exclusive V1R transcription in these cells has been proposed in which each V1R gene might compete stochastically for a single transcriptional complex. This model predicts that the large repertoire of divergent V1R genes in the mouse genome contains common regulatory elements. In this study, we have characterized V1R promoter regions by comparative genomics and by mapping transcription start sites.

Results

We find that transcription is initiated from ~1 kb promoter regions that are well conserved within V1R subfamilies. While cross-subfamily homology is not evident by traditional methods, we developed a heuristic motif-searching tool, LogoAlign, and applied this tool to identify motifs shared within the promoters of all V1R genes. Our motif-searching tool exhibits rapid convergence to a relatively small number of non-redundant solutions (97% convergence). We also find that the best motifs contain significantly more information than those identified in controls, and that these motifs are more likely to be found in the immediate vicinity of transcription start sites than elsewhere in gene blocks. The best motifs occur near transcription start sites of ~90% of all V1R genes and across all of the divergent subfamilies. Therefore, these motifs are candidate binding sites for transcription factors involved in V1R co-regulation.

Conclusion

Our analyses show that V1R subfamilies have broad and well conserved promoter regions from which transcription is initiated. Results from a new motif-finding algorithm, LogoAlign, designed for this context and more generally for searching large, hierarchical datasets, suggest the existence of common information-rich regulatory motifs that are shared across otherwise divergent V1R subfamilies.

Gene cluster lock after pheromone receptor gene choice

In mammals, perception of pheromones is based on the expression in each vomeronasal sensory neuron of a limited set of receptor genes, chosen among a large repertoire. Here, we report an extremely tight control of the monogenic and monoallelic transcription of the V1rb2 receptor gene. Combining genetic and electrophysiological approaches, we show that the transcription of a non-functional V1r allele leads to the coexpression of another, functional V1r gene. The choice of this coexpressed gene surprisingly includes genes located on the cluster homologous to the one from which the mutant allele is transcribed. However, V1r genes located in cis relative to the transcribed mutant allele are excluded from the coexpression choice. Our observations strongly suggest a monogenic regulatory mechanism acting (a) at a general level, via the expression of the V1r receptor itself, and (b) at a more local level, defined by the V1r gene cluster.

Divergent evolution among teleost V1r receptor genes

The survival of vertebrate species is dependent on the ability of individuals to adequately interact with each other, a function often mediated by the olfactory system. Diverse olfactory receptor repertoires are used by this system to recognize chemicals. Among these receptors, the V1rs, encoded by a very large gene family in most mammals, are able to detect pheromones. Teleosts, which also express V1r receptors, possess a very limited V1r repertoire. Here, taking advantage of the possibility to unequivocally identify V1r orthologs in teleosts, we analyzed the olfactory expression and evolutionary constraints of a pair of clustered fish V1r receptor genes, V1r1 and V1r2. Orthologs of the two genes were found in zebrafish, medaka, and threespine stickleback, but a single representative was observed in tetraodontidae species. Analysis of V1r1 and V1r2 sequences from 12 different euteleost species indicate different evolutionary rates between the two paralogous genes, leading to a highly conserved V1r2 gene and a V1r1 gene under more relaxed selective constraint. Moreover, positively-selected sites were detected in specific branches of the V1r1 clade. Our results suggest a conserved agonist specificity of the V1R2 receptor between euteleost species, its loss in the tetraodontidae lineage, and the acquisition of different chemosensory characteristics for the V1R1 receptor.

Comparative genomic analysis identifies an evolutionary shift of vomeronasal receptor gene repertoires in the vertebrate transition from water to land

Two evolutionarily unrelated superfamilies of G-protein coupled receptors, V1Rs and V2Rs, bind pheromones and "ordinary" odorants to initiate vomeronasal chemical senses in vertebrates, which play important roles in many aspects of an organism's daily life such as mating, territoriality, and foraging. To study the macroevolution of vomeronasal sensitivity, we identified all V1R and V2R genes from the genome sequences of 11 vertebrates. Our analysis suggests the presence of multiple V1R and V2R genes in the common ancestor of teleost fish and tetrapods and reveals an exceptionally large among-species variation in the sizes of these gene repertoires. Interestingly, the ratio of the number of intact V1R genes to that of V2R genes increased by approximately 50-fold as land vertebrates evolved from aquatic vertebrates. A similar increase was found for the ratio of the number of class II odorant receptor (OR) genes to that of class I genes, but not in other vertebrate gene families. Because V1Rs and class II ORs have been suggested to bind to small airborne chemicals, whereas V2Rs and class I ORs recognize water-soluble molecules, these increases reflect a rare case of adaptation to terrestrial life at the gene family level. Several gene families known to function in concert with V2Rs in the mouse are absent outside rodents, indicating rapid changes of interactions between vomeronasal receptors and their molecular partners. Taken together, our results demonstrate the exceptional evolutionary fluidity of vomeronasal receptors, making them excellent targets for studying the molecular basis of physiological and behavioral diversity and adaptation.

Origin and evolution of the vertebrate vomeronasal system viewed through system-specific genes

Tetrapods have two distinct nasal chemosensory systems, the main olfactory system and the vomeronasal system (VNS). Defined by certain morphological components, the main olfactory system is present in all groups of vertebrates, while the VNS is found only in tetrapods. Previous attempts to identify a VNS precursor in teleost fish were limited by functional and morphological characters that could not clearly distinguish between homologous and analogous systems. In the past decade, several genes that specifically function in the VNS have been discovered. Here we first describe recent evolutionary studies of mammalian VNS-specific genes. We then review evidence showing the presence and tissue-specific expression of the VNS-specific genes in teleosts, as well as co-expression patterns of these genes in specific regions of the teleost olfactory epithelium. We propose that a VNS precursor exists in teleosts and that its evolutionary origin predated the separation between teleosts and tetrapods.

Comprehensive comparisons of the current human, mouse, and rat RefSeq, Ensembl, EST, and FANTOM3 datasets: Identification of new human genes with specific tissue expression profile

Our understanding of functional genetic elements in the genomes is continuously growing and new entries are entered in various databases on a regular basis. We have here merged the genetic elements in RefSeq, Ensembl, FANTOM3, HINV, and NCBI:s ESTdb using the genome assemblies in order to achieve a comprehensive picture of the current status of the identity and gene number in human, mouse, and rat. The number of human protein coding genes has not increased (25,043) while the increased sequencing of mouse transcripts has provided the considerably higher number of protein coding genes (31,578) in mouse. The results indicate large discrepancies between the datasets, as considerable numbers of unique transcripts can be found in each dataset. Despite the high number of ncRNA (38,129 in mouse) there are also almost 20,000 EST clusters in both mouse and humans with more than one EST that do not overlap any transcript suggesting that several new genetic elements are still to be found. We also demonstrated presence of new genes by identifying new human ones that have specific tissue profiles, using RT-PCR on rat tissues.

Adaptive Diversification of Vomeronasal Receptor 1 Genes in Rodents

The vomeronasal receptor 1 (V1R) are believed to be pheromone receptors in rodents. Here we used computational methods to identify 95 and 62 new putative V1R genes from the draft rat and mouse genome sequence, respectively. The rat V1R repertoire consists of 11 subfamilies, 10 of which are shared with the mouse, while rat appears to lack the H and I subfamilies found in mouse and possesses one unique subfamily (M). The estimations of the relative divergence times suggest that many subfamilies originated after the split of rodents and primates. The analysis also reveals that these clusters underwent an expansion very close to the split of mouse and rat. In addition, maximum likelihood analysis showed that the nonsynonymous and synonymous rate ratio for most of these clusters was much higher than one, suggesting the role of positive selection in the diversification of these duplicated V1R genes. Because V1R are thought to mediate the process of signal transduction in response to pheromone detection, we speculate that the V1R genes have evolved under positive Darwinian selection to maintain the ability to discriminate between large and complex pheromonal mixtures.

The repertoire of G-protein-coupled receptors in fully sequenced genomes

The superfamily of G-protein-coupled receptors (GPCRs) is one of the largest and most studied families of proteins. We created Hidden Markov Models derived from sorted groups of GPCRs from our previous detailed phylogenetic classification of human GPCRs and added several other models derived from receptors not found in mammals. We used these models to search entire Genscan data sets from 13 species whose genomes are nearly completely sequenced. We found more than 5000 unique GPCRs that were divided into 15 main groups, and the largest one, the Rhodopsin family, was subdivided into 13 subclasses. The results show that the main families in the human genome, Glutamate, Rhodopsin, Adhesion, Frizzled, and Secretin, arose before the split of nematodes from the chordate lineage. Moreover, several of the subgroups of the Rhodopsin family arose before the split of the linage leading to vertebrates. We also searched expressed sequence tag (EST) databases and identified more than 20,000 sequences that match GPCRs. Although the GPCRs represent typically 1 to 2% of the Genscan predictions, the ESTs that match GPCRs are typically only 0.01 to 0.001%, indicating that GPCRs in most of the groups are expressed at low levels. We also provide searchable data sets that may be used for annotation and further detailed analysis of the GPCR family. This study provides an extensive overview of the expansion of the gene repertoire for families and subgroups of GPCRs.

Dramatic variation of the vomeronasal pheromone receptor gene repertoire among five orders of placental and marsupial mammals

Pheromones are chemicals emitted and sensed by conspecifics to elicit social and sexual responses and are perceived in terrestrial vertebrates primarily by the vomeronasal organ (VNO). Pheromone receptors in the mammalian VNO are encoded by the V1R and V2R gene superfamilies. The V1R superfamily contains 187 and 102 putatively functional genes in the mouse and rat, respectively. To investigate whether this large repertoire size is typical among mammals with functional VNOs, we here describe the V1R repertoires of dog, cow, and opossum based on their draft genome sequences. The dog and cow have only 8 and 32 intact V1R genes, respectively. Thus, the intact V1R repertoire size varies by at least 23-fold among placental mammals with functional VNOs. To our knowledge, this size ratio represents the greatest among-species variation in gene family size of all mammalian gene families. Phylogenetic analysis of placental V1R genes suggests multiple losses of ancestral genes in carnivores and artiodactyls and gains of many new genes by gene duplication in rodents, manifesting massive gene births and deaths. We also identify 49 intact opossum V1R genes and discover independent expansions of the repertoire in placentals and marsupials. We further show a concordance between the V1R repertoire size and the complexity of VNO morphology, suggesting that the latter could indicate the sophistication of pheromone communications within species. In sum, our results demonstrate tremendous diversity and rapid evolution of mammalian V1R gene inventories and caution the generalization of VNO biology from rodents to all mammals.

Olfactory expression of a single and highly variable V1r pheromone receptor-like gene in fish species

Sensory neurons expressing members of the seven-transmembrane V1r receptor superfamily allow mice to perceive pheromones. These receptors, which exhibit no sequence homology to any known protein except a weak similarity to taste receptors, have only been found in mammals. In the mouse, the V1r repertoire contains >150 members, which are expressed by neurons of the vomeronasal organ, a structure present exclusively in some tetrapod species. Here, we report the existence of a single V1r gene in multiple species of a non-terrestrial, vomeronasal organ-lacking taxon, the teleosts. In zebrafish, this V1r gene is expressed in chemosensory neurons of the olfactory rosette with a punctate distribution, strongly suggesting a role in chemodetection. This unique receptor gene exhibits a remarkably high degree of sequence variability between fish species. It likely corresponds to the original V1r present in the common ancestor of vertebrates, which led to the large and very diverse expansion of vertebrate pheromone receptor repertoires, and suggests the presence of V1rs in multiple nonmammalian phyla.

Nine new human Rhodopsin family G-protein coupled receptors: identification, sequence characterisation and evolutionary relationship

We report nine new members of the Rhodopsin family of human G protein-coupled receptors (GPCRs) found by searches in the genome databases. BLAST searches and phylogenetic analyses showed that only four of the receptors are closely related to previously characterised GPCRs, GPR150 and GPR154 to oxytocin/vasopressin receptors, GPR152 to CRTH2/FPRs and GPR165 to GPR72/NPYR. Four of the receptors, GPR139, GPR146, GPR153 and GPR162, have one other orphan GPCRs as close relative while GPR148 lacks close relatives. We have identified in total 37 orthologues for the new receptors, primarily from rat, mouse, chicken, fugu and zebrafish. GPR162 and GPR139 are remarkably well conserved while GPR148 seems to be evolving rapidly. Analyses using expressed sequence tags (ESTs) indicate that all the new receptors except GPR153 have the CNS as a major site of expression.

Rhox: a new homeobox gene cluster

Homeobox genes encode transcription factors notable for their ability to regulate embryogenesis. Here, we report the discovery of a cluster of 12 related homeobox genes on the X chromosome expressed in male and female reproductive tissues in adult mice. These reproductive homeobox on the X chromosome (Rhox) genes are expressed in a cell type-specific manner; several are hormonally regulated, and their expression pattern during postnatal testis development corresponds to their chromosomal position. Most of the Rhox genes are expressed in Sertoli cells, the nurse cells in direct contact with developing male germ cells, suggesting that they regulate the expression of somatic-cell gene products critical for germ cell development. In support of this, targeted disruption of Rhox5 increased male germ cell apoptosis and reduced sperm production, sperm motility, and fertility. Identification of this family of homeobox genes provides an opportunity to study colinear gene regulation and the transcriptional control of reproduction.

Comparative genomic analysis of the clade B serpin cluster at human chromosome 18q21: amplification within the mouse squamous cell carcinoma antigen gene locus

The human clade B serpins neutralize serine or cysteine proteinases and reside predominantly within the intracellular compartment. Genomic analysis shows that the 13 human clade B serpins map to either 6p25 (n = 3) or 18q21 (n = 10). Similarly, the mouse clade B serpins map to syntenic loci at 13A3.2 and 1D, respectively. The mouse clade B cluster at 13A3.2 shows a marked expansion in the number of serpin genes (n = 15). The purpose of this study was to determine whether a similar expansion occurred at 1D. Using STS-content mapping, comparative genomic DNA sequence analysis, and cDNA cloning, we found that the mouse clade B cluster at 1D showed nearly complete conservation of gene number, order, and orientation relative to those of 18q21. The only exception was the squamous cell carcinoma antigen (SCCA) locus. The human SCCA locus contains two genes, SERPINB3 (SCCA1) and SERPINB4 (SCCA2), whereas the mouse locus contains four serpins and three pseudogenes. Based on phylogenetic analysis and predicted amino acid sequences, amplification of the mouse SCCA locus occurred after rodents and primates diverged and was associated with some diversification of proteinase inhibitory activity relative to that of humans.

Divergent V1R repertoires in five species: Amplification in rodents, decimation in primates, and a surprisingly small repertoire in dogs

The V1R gene family comprises one of two types of putative pheromone receptors expressed in the mammalian vomeronasal organ (VNO). We searched the most recent mouse, rat, dog, chimpanzee, and human genome sequence assemblies to compile a near-complete repertoire of V1R genes for each species. Dog, human, and chimpanzee have very few intact V1Rs (8, 2, and 0, respectively) compared to more than a hundred intact V1Rs in each of the rat (106) and mouse (165) genomes. We also provide the first description of the diversity of V1R pseudogenes in these species. We identify at least 165 pseudogenes in mouse, 110 in rat, 102 in chimpanzee, 115 in human, and 54 in dog. Primate and dog pseudogenes are distributed among almost all V1R subfamilies seen in rodents, indicating that the common ancestor of these species had a diverse V1R repertoire. We find that V1R genes were subject to strikingly different fates in different species and in different subfamilies. In rodents, some subfamilies remained relatively stable or underwent roughly equivalent expansion in mouse and rat; other subfamilies expanded in one species but not the other. The small number of intact V1Rs in the dog genome is unexpected given the presumption that dogs, like rodents, have a functional VNO, and a complex system of pheromone-based behaviors. We identify an intact transient receptor potential channel 2beta in the dog genome, consistent with a functional VNO in dogs. The diminished V1R repertoire in dogs raises questions about the relative contributions of V1Rs versus other candidate pheromone receptor genes in the establishment of complex pheromone systems in mammals.

Pheromone reception in mammals

Pheromonal communication is the most convenient way to transfer information regarding gender and social status in animals of the same species with the holistic goal of sustaining reproduction. This type of information exchange is based on pheromones, molecules often chemically unrelated, that are contained in body fluids like urine, sweat, specialized exocrine glands, and mucous secretions of genitals. So profound is the relevance of pheromones over the evolutionary process that a specific peripheral organ devoted to their recognition, namely the vomeronasal organ of Jacobson, and a related central pathway arose in most vertebrate species. Although the vomeronasal system is well developed in reptiles and amphibians, most mammals strongly rely on pheromonal communication. Humans use pheromones too; evidence on the existence of a specialized organ for their detection, however, is very elusive indeed. In the present review, we will focus our attention on the behavioral, physiological, and molecular aspects of pheromone detection in mammals. We will discuss the responses to pheromonal stimulation in different animal species, emphasizing the complicacy of this type of communication. In the light of the most recent results, we will also discuss the complex organization of the transduction molecules that underlie pheromone detection and signal transmission from vomeronasal neurons to the higher centers of the brain. Communication is a primary feature of living organisms, allowing the coordination of different behavioral paradigms among individuals. Communication has evolved through a variety of different strategies, and each species refined its own preferred communication medium. From a phylogenetic point of view, the most widespread and ancient way of communication is through chemical signals named pheromones: it occurs in all taxa, from prokaryotes to eukaryotes. The release of specific pheromones into the environment is a sensitive and definite way to send messages to other members of the same species. Therefore, the action of an organism can alter the behavior of another organism, thereby increasing the fitness of either or both. Albeit slow in transmission and not easily modulated, pheromones can travel around objects in the dark and over long distances. In addition, they are emitted when necessary and their biosynthesis is usually economic. In essence, they represent the most efficient tool to refine the pattern of social behaviors and reproductive strategies.

Rapid turnover and species-specificity of vomeronasal pheromone receptor genes in mice and rats

Pheromones are used by individuals of the same species to elicit behavioral or physiological changes, and they are perceived primarily by the vomeronasal organ (VNO) in terrestrial vertebrates. VNO pheromone receptors are encoded by the V1r and V2r gene superfamilies in mammals. A comparison of the V1r and V2r repertoires between closely related species can provide significant insights into the evolutionary genetic mechanisms responsible for species-specific pheromone communications. A total of 137 putatively functional V1r genes of 12 families were previously identified from the mouse genome. We report the identification of 95 putatively functional V1r genes from the draft rat genome sequence. These genes map primarily to four blocks in two chromosomes. The rat V1r genes can be phylogenetically grouped into 10 families, which are shared with mouse, and 2 new families, which are rat-specific. Even in many shared families, gene numbers differ between the two species, apparently due to frequent gene duplication and pseudogenization after the separation of the two species. Molecular dating suggests that most of the rat V1r families emerged before or during the radiation of mammalian orders, but many duplications within families occurred as recently as in the past 10 million years (MY). Our results show that the evolution of the V1r repertoire is characterized by exceptionally fast gene turnover via gains and losses of individual genes, suggesting rapid and substantial changes in pheromone communication between species.

Pheromone receptors in mammals

In most mammals, pheromone perception mediates intraspecies interactions related to reproduction, such as mate recognition, intermale aggressive behaviors, or exchanges between females and their offspring. Recent molecular findings, particularly the identification of two large pheromone receptor gene superfamilies, provide today invaluable tools to better understand the way mammals make sense of pheromonal information.

Species specificity in rodent pheromone receptor repertoires

The mouse V1R putative pheromone receptor gene family consists of at least 137 intact genes clustered at multiple chromosomal locations in the genome. Species-specific pheromone receptor repertoires may partly explain species-specific social behavior. We conducted a genomic analysis of an orthologous pair of mouse and rat V1R gene clusters to test for species specificity in rodent pheromone systems. Mouse and rat have lineage-specific V1R repertoires in each of three major subfamilies at these loci as a result of postspeciation duplications, gene loss, and gene conversions. The onset of this diversification roughly coincides with a wave of Line1 (L1) retrotranspositions into the two loci. We propose that L1 activity has facilitated postspeciation V1R duplications and gene conversions. In addition, we find extensive homology among putative V1R promoter regions in both species. We propose a regulatory model in which promoter homogenization could ensure that V1R genes are equally competitive for a limiting transcriptional structure to account for mutually exclusive V1R expression in vomeronasal neurons.

Seven evolutionarily conserved human rhodopsin G protein-coupled receptors lacking close relatives

We report seven new members of the superfamily of human G protein-coupled receptors (GPCRs) found by searches in the human genome databases, termed GPR100, GPR119, GPR120, GPR135, GPR136, GPR141, and GPR142. We also report 16 orthologues of these receptors in mouse, rat, fugu (pufferfish) and zebrafish. Phylogenetic analysis shows that these are additional members of the family of rhodopsin-type GPCRs. GPR100 shows similarity with the orphan receptor SALPR. Remarkably, the other receptors do not have any close relative among other known human rhodopsin-like GPCRs. Most of these orphan receptors are highly conserved through several vertebrate species and are present in single copies. Analysis of expressed sequence tag (EST) sequences indicated individual expression patterns, such as for GPR135, which was found in a wide variety of tissues including eye, brain, cervix, stomach and testis. Several ESTs for GPR141 were found in marrow and cancer cells, while the other receptors seem to have more restricted expression patterns.

Atypical expansion in mice of the sensory neuron-specific Mrg G protein-coupled receptor family

The Mas-related genes (Mrgs) comprise a family of >50 G protein-coupled receptors (GPCRs), many of which are expressed in specific subsets of nociceptive sensory neurons in mice. In contrast, humans contain a related but nonorthologous family of genes, called MrgXs or sensory neuron-specific receptors, of which many fewer appear to be expressed in sensory neurons. To determine whether the diversity of murine Mrgs is generic to rodents or is an atypical feature of mice, we characterized MrgA, MrgB, MrgC, and MrgD subfamilies in rat and gerbil. Surprisingly, although mice have approximately 22 MrgA and approximately 14 MrgC genes, rats and gerbils have just a single MrgA and MrgC gene. This murine-specific expansion likely reflects recent retrotransposon-mediated unequal crossover events. The expression of Mrgs in rat sensory ganglia suggests that the extensive cellular diversity in mice can be simplified to a core subset of approximately four different genes (MrgA, MrgB, MrgC, and MrgD), defining a similar number of neuronal subpopulations. Our results suggest more generally that mouse-human genomic comparisons may sometimes reveal differences atypical of rodents.

The G-protein-coupled receptors in the human genome form five main families. Phylogenetic analysis, paralogon groups, and fingerprints

The superfamily of G-protein-coupled receptors (GPCRs) is very diverse in structure and function and its members are among the most pursued targets for drug development. We identified more than 800 human GPCR sequences and simultaneously analyzed 342 unique functional nonolfactory human GPCR sequences with phylogenetic analyses. Our results show, with high bootstrap support, five main families, named glutamate, rhodopsin, adhesion, frizzled/taste2, and secretin, forming the GRAFS classification system. The rhodopsin family is the largest and forms four main groups with 13 sub-branches. Positions of the GPCRs in chromosomal paralogons regions indicate the importance of tetraploidizations or local gene duplication events for their creation. We also searched for "fingerprint" motifs using Hidden Markov Models delineating the putative inter-relationship of the GRAFS families. We show several common structural features indicating that the human GPCRs in the GRAFS families share a common ancestor. This study represents the first overall map of the GPCRs in a single mammalian genome. Our novel approach of analyzing such large and diverse sequence sets may be useful for studies on GPCRs in other genomes and divergent protein families.

Relaxed selective pressure on an essential component of pheromone transduction in primate evolution

The vomeronasal organ (VNO) detects pheromones in many vertebrate species but is likely to be vestigial in humans. TRPC2(TRP2), a gene that is essential for VNO function in the mouse, is a pseudogene in humans. Because TRPC2 is expressed only in the VNO, the loss of selective pressure on this gene can serve as a molecular marker for the time at which the VNO became vestigial. By analyzing sequence data from the TRPC2 gene of 15 extant primate species, we provide evidence that the VNO was most likely functional in the common ancestor of New World monkeys and Old World monkeys and apes, but then became vestigial in the common ancestor of Old World monkeys and apes. We propose that, at this point in evolution, other modalities, notably the development of color vision, may have largely replaced signaling by pheromones.

There exist at least 30 human G-protein-coupled receptors with long Ser/Thr-rich N-termini

We report six novel members of the superfamily of human G-protein coupled receptors (GPCRs) found by searches in the human genome databases, termed GPR123, GPR124, GPR125, GPR126, GPR127, and GPR128. Phylogenetic analysis demonstrates that these are additional members of the family of GPCRs with long N-termini, previously termed EGF-7TM, LNB-7TM, B2 or LN-7TM, showing that there exist at least 30 such GPCRs in the human genome. Three of these receptors form their own phylogenetic cluster, while two other places in a cluster with the previously reported HE6 and GPR56 (TM7XN1) and one with EMR1-3. All the novel receptors have a GPS domain in their N-terminus, except GPR123, as well as long Ser/Thr rich regions forming mucin-like stalks. GPR124 and GPR125 have a leucine rich repeat (LRR), an immunoglobulin (Ig) domain, and a hormone-binding domain (HBD). The Ig domain shows similarities to motilin and titin, while the LRR domain shows similarities to LRIG1 and SLIT1-2. GPR127 has one EGF domain while GPR126 and GPR128 do not contain domains that are readily recognized in other proteins beyond the GPS domain. We found several human EST sequences for most of the receptors showing differential expression patterns, which may indicate that some of these receptors participate in central functions while others are more likely to have a role in the immune or reproductive systems.

Deficient pheromone responses in mice lacking a cluster of vomeronasal receptor genes

The mammalian vomeronasal organ (VNO), a part of the olfactory system, detects pheromones--chemical signals that modulate social and reproductive behaviours. But the molecular receptors in the VNO that detect these chemosensory stimuli remain undefined. Candidate pheromone receptors are encoded by two distinct and complex superfamilies of genes, V1r and V2r (refs 3 and 4), which code for receptors with seven transmembrane domains. These genes are selectively expressed in sensory neurons of the VNO. However, there is at present no functional evidence for a role of these genes in pheromone responses. Here, using chromosome engineering technology, we delete in the germ line of mice an approximately 600-kilobase genomic region that contains a cluster of 16 intact V1r genes. These genes comprise two of the 12 described V1r gene families, and represent approximately 12% of the V1r repertoire. The mutant mice display deficits in a subset of VNO-dependent behaviours: the expression of male sexual behaviour and maternal aggression is substantially altered. Electrophysiologically, the epithelium of the VNO of such mice does not respond detectably to specific pheromonal ligands. The behavioural impairment and chemosensory deficit support a role of V1r receptors as pheromone receptors.