Composition and evolution of the V2r vomeronasal receptor gene repertoire in mice and rats

Abstract

Pheromones are chemicals produced and detected by conspecifics to elicit social/sexual physiological and behavioral responses, and they are perceived primarily by the vomeronasal organ (VNO) in terrestrial vertebrates. Two large superfamilies of G protein-coupled receptors, V1rs and V2rs, have been identified as pheromone receptors in vomeronasal sensory neurons. Based on a computational analysis of the mouse and rat genome sequences, we report the first global draft of the V2r gene repertoire, composed of ∼200 genes and pseudogenes. Rodent V2rs are subject to rapid gene births/deaths and accelerated amino acid substitutions, likely reflecting the species-specific nature of pheromones. Vertebrate V2rs appear to have originated twice prior to the emergence of the VNO in ancestral tetrapods, explaining seemingly inconsistent observations among different V2rs. The identification of the entire V2r repertoire opens the door to genomic-level studies of the structure, function, and evolution of this diverse group of sensory receptors.

Introduction

Pheromones are chemicals emitted and detected by individuals of the same species. They stimulate sexual and social changes in physiology and behavior, such as inducing estrus, terminating pregnancy, initiating copulatory behavior, and controlling intermale aggression [1]. Although some pheromones are detectable by the main olfactory system, most pheromones are probably sensed by the vomeronasal organ (VNO) in mammals [1]. VNO is encased in a bony capsule on the anterior nasal septum, and is anatomically and physiologically separated from the main olfactory system that detects thousands of odorants [1]. Two distinct superfamilies of seven-transmembrane G protein-coupled receptors, V1rs and V2rs, have been identified as vomeronasal pheromone receptors [2], [3], [4], [5]. V1r genes have intronless coding regions. They are coexpressed with the G protein subunit Gα_i2 in sensory neurons whose cell bodies are located in the apical part of the vomeronasal epithelium [1], [6]. In contrast, V2rs are characterized by the presence of a long, highly variable N-terminal domain. They are encoded by multiexon genes expressed in Gα_O-positive neurons whose cell bodies are located basally in the vomeronasal epithelium [1], [6]. Neurons expressing V1r and V2r receptors project to the anterior and posterior accessory olfactory bulb, respectively, where they form multiple glomeruli in spatially conserved domains [1]. In addition to the distinct molecular structures and expression locations, V1r and V2r receptors also differ in a number of ways that suggest their different physiological functions. First, current experimental evidence shows that V1rs bind small volatile molecules, whereas V2rs bind peptides [7], [8], [9], [10], [11]. Second, gene knockout experiments suggest that V1rs are involved in gender discrimination, whereas V2rs control pheromone-induced male–male aggression [1], [12], [13], [14]. Third, V2rs, but not V1rs, are coexpressed with M10 and M1 families of major histocompatibility complex (MHC) class Ib molecules in a selective fashion [14], [15]. M10 molecules appear to function as escort molecules in the transport of V2rs to the cell membrane of vomeronasal sensory neurons [14].

With these differences, both V1rs and V2rs should be studied to gain a full understanding of the molecular mechanisms responsible for pheromone-based chemical communications. V1r genes are relatively easy to identify from genome sequences by computational methods because of their simple gene structures. To date, the complete V1r gene repertoire has been described in the human, chimpanzee, mouse, rat, dog, cow, and opossum [16], [17], [18], [19], [20], [21], [22], with the number of intact genes varying from a few in the human, chimpanzee, and dog to over 150 in the mouse. The evolution of the mammalian V1r repertoire is characterized by rapid gene turnover, lineage-specific phylogenetic clustering, accelerated nonsynonymous substitutions, and dramatic among-species variations of the repertoire size [10], [18], [19], [20], [21], [22], [23], [24]. However, little is known about V2rs. Even in the model organisms of mouse and rat, only a few V2r genes have been described, although the total number of V2r genes is believed to be on the order of 100 [3], [4], [5]. This scarcity of knowledge is mainly due to the complex structure of V2r genes that makes their identification from genome sequences difficult. Here we combine several computational methods in an attempt to identify all V2r genes from the mouse and rat genome sequences. Subsequent analyses reveal both common features and unexpected differences between V1rs and V2rs in repertoire organization and evolution.