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Abstract
BackgroundA common feature of chemosensory systems is the involvement of G protein-coupled receptors (GPCRs) in the detection of environmental stimuli. Several lineages of GPCRs are involved in vertebrate olfaction, including trace amine-associated receptors, type 1 and 2 vomeronasal receptors and odorant receptors (ORs). Gene duplication and gene loss in different vertebrate lineages have lead to an enormous amount of variation in OR gene repertoire among species; some fish have fewer than 100 OR genes, while some mammals possess more than 1000. Fascinating features of the vertebrate olfactory system include allelic exclusion, where each olfactory neuron expresses only a single OR gene, and axonal guidance where neurons expressing the same receptor project axons to common glomerulae. By identifying homologous ORs in vertebrate and in non-vertebrate chordates, we hope to expose ancestral features of the chordate olfactory system that will help us to better understand the evolution of the receptors themselves and of the cellular components of the olfactory system.ResultsWe have identified 50 full-length and 11 partial ORs in Branchiostoma floridae. No ORs were identified in Ciona intestinalis. Phylogenetic analysis places the B. floridae OR genes in a monophyletic clade with the vertebrate ORs. The majority of OR genes in amphioxus are intronless and many are also tandemly arrayed in the genome. By exposing conserved amino acid motifs and testing the ability of those motifs to discriminate between ORs and non-OR GPCRs, we identified three OR-specific amino acid motifs common in cephalochordate, fish and mammalian and ORs.ConclusionHere, we show that amphioxus has orthologs of vertebrate ORs. This conclusion demonstrates that the receptors, and perhaps other components of vertebrate olfaction, evolved at least 550 million years ago. We have also identified highly conserved amino acid motifs that may be important for maintaining receptor conformation or regulating receptor activity. We anticipate that the identification of vertebrate OR orthologs in amphioxus will lead to an improved understanding of OR gene family evolution, OR gene function, and the mechanisms that control cell-specific expression, axonal guidance, signal transduction and signal integration.
Highlights
A common feature of chemosensory systems is the involvement of G proteincoupled receptors (GPCRs) in the detection of environmental stimuli
This similarity-based approach, in which query sequences are used to identify orthologs and paralogs, is a staple of both molecular and bioinformatics research. These and subsequent studies have uncovered over a thousand rat and mouse odorant receptors [2,3,4,5] and have led to the identification of other GPCR families involved in vertebrate olfaction such as the trace amine-associated receptors (TAARs) [6], the type 1 [7] and type 2 vomeronasal receptors [8,9,10] and the formyl peptide receptor-like proteins [11]
Hidden Markov Model (HMM) and Blastp When we searched the B. floridae protein predictions using the HMM model derived from fish odorant receptors with an e-value cut off of E-10, three B. floridae proteins were identified
Summary
A common feature of chemosensory systems is the involvement of G proteincoupled receptors (GPCRs) in the detection of environmental stimuli. This similarity-based approach, in which query sequences are used to identify orthologs and paralogs, is a staple of both molecular and bioinformatics research These and subsequent studies have uncovered over a thousand rat and mouse odorant receptors [2,3,4,5] and have led to the identification of other GPCR families involved in vertebrate olfaction such as the trace amine-associated receptors (TAARs) [6], the type 1 [7] and type 2 vomeronasal receptors [8,9,10] and the formyl peptide receptor-like proteins [11]. We report the results of our search for orthologs of vertebrate ORs in the tunicate, Ciona intestinalis (subphylum Urochordata), and in amphioxus, Branchiostoma floridae (subphylum Cephalochordata)
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