Churcher Allison M, Taylor John S
Department of Biology, University of Victoria, Victoria BC, Canada.
BMC Evol Biol. 2009 Oct 5;9:242. doi: 10.1186/1471-2148-9-242.
A 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.
We 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.
Here, 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.
化学感应系统的一个共同特征是G蛋白偶联受体（GPCRs）参与环境刺激的检测。几种GPCRs谱系参与脊椎动物嗅觉，包括痕量胺相关受体、1型和2型犁鼻器受体以及气味受体（ORs）。不同脊椎动物谱系中的基因复制和基因丢失导致物种间OR基因库存在大量差异；一些鱼类的OR基因少于100个，而一些哺乳动物拥有超过1000个。脊椎动物嗅觉系统的迷人特征包括等位基因排斥，即每个嗅觉神经元仅表达单个OR基因，以及轴突导向，即表达相同受体的神经元将轴突投射到共同的嗅小球。通过鉴定脊椎动物和非脊椎动物脊索动物中的同源ORs，我们希望揭示脊索动物嗅觉系统的祖先特征，这将有助于我们更好地理解受体本身以及嗅觉系统细胞成分的进化。
我们在佛罗里达文昌鱼中鉴定出50个全长ORs和11个部分ORs。在肠鳃纲动物中未鉴定出ORs。系统发育分析将佛罗里达文昌鱼的OR基因置于与脊椎动物ORs的一个单系分支中。文昌鱼中的大多数OR基因无内含子，许多在基因组中也是串联排列的。通过揭示保守的氨基酸基序并测试这些基序区分ORs和非OR GPCRs的能力，我们鉴定出了在头索动物、鱼类和哺乳动物以及ORs中常见的三个OR特异性氨基酸基序。
在此，我们表明文昌鱼具有脊椎动物ORs的直系同源物。这一结论表明这些受体以及脊椎动物嗅觉的其他成分至少在5.5亿年前就已进化。我们还鉴定出了高度保守的氨基酸基序，这些基序可能对维持受体构象或调节受体活性很重要。我们预计，在文昌鱼中鉴定脊椎动物OR直系同源物将有助于更好地理解OR基因家族的进化、OR基因功能以及控制细胞特异性表达、轴突导向、信号转导和信号整合的机制。
