Hashiguchi Yasuyuki, Nishida Mutsumi
Division of Molecular Marine Biology, Ocean Research Institute, University of Tokyo, Tokyo, Japan.
BMC Evol Biol. 2006 Oct 3;6:76. doi: 10.1186/1471-2148-6-76.
In teleost fishes that lack a vomeronasal organ, both main odorant receptors (ORs) and vomeronasal receptors family 2 (V2Rs) are expressed in the olfactory epithelium, and used for perception of water-soluble chemicals. In zebrafish, it is known that both ORs and V2Rs formed multigene families of about a hundred copies. Whereas the contribution of V2Rs in zebrafish to olfaction has been found to be substantially large, the composition and structure of the V2R gene family in other fishes are poorly known, compared with the OR gene family.
To understand the evolutionary dynamics of V2R genes in fishes, V2R sequences in zebrafish, medaka, fugu, and spotted green pufferfish were identified from their draft genome sequences. There were remarkable differences in the number of intact V2R genes in different species. Most V2R genes in these fishes were tightly clustered in one or two specific chromosomal regions. Phylogenetic analysis revealed that the fish V2R family could be subdivided into 16 subfamilies that had diverged before the separation of the four fishes. Genes in two subfamilies in zebrafish and another subfamily in medaka increased in their number independently, suggesting species-specific evolution in olfaction. Interestingly, the arrangements of V2R genes in the gene clusters were highly conserved among species in the subfamily level. A genomic region of tetrapods corresponding to the region in fishes that contains the V2R cluster was found to have no V2R gene in any species.
Our results have indicated that the evolutionary dynamics of fish V2Rs are characterized by rapid gene turnover and lineage-specific phylogenetic clustering. In addition, the present phylogenetic and comparative genome analyses have shown that the fish V2Rs have expanded after the divergence between teleost and tetrapod lineages. The present identification of the entire V2R repertoire in fishes would provide useful foundation to the future functional and evolutionary studies of fish V2R gene family.
在缺乏犁鼻器的硬骨鱼类中，主要嗅觉受体（ORs）和犁鼻器受体家族2（V2Rs）均在嗅觉上皮中表达，并用于感知水溶性化学物质。在斑马鱼中，已知ORs和V2Rs都形成了约有一百个拷贝的多基因家族。虽然已发现斑马鱼中V2Rs对嗅觉的贡献相当大，但与OR基因家族相比，其他鱼类中V2R基因家族的组成和结构却鲜为人知。
为了解鱼类中V2R基因的进化动态，从斑马鱼、青鳉、河豚和斑绿河豚的基因组草图序列中鉴定出V2R序列。不同物种中完整V2R基因的数量存在显著差异。这些鱼类中的大多数V2R基因紧密聚集在一两个特定的染色体区域。系统发育分析表明，鱼类V2R家族可细分为16个亚家族，这些亚家族在这四种鱼类分化之前就已经分化。斑马鱼两个亚家族和青鳉另一个亚家族中的基因数量独立增加，表明嗅觉存在物种特异性进化。有趣的是，亚家族水平上，基因簇中V2R基因的排列在物种间高度保守。发现四足动物中与鱼类中包含V2R簇的区域相对应的基因组区域在任何物种中都没有V2R基因。
我们的结果表明，鱼类V2Rs的进化动态特征是基因快速更替和谱系特异性系统发育聚类。此外，目前的系统发育和比较基因组分析表明，硬骨鱼和四足动物谱系分化后鱼类V2Rs发生了扩张。目前对鱼类中整个V2R库的鉴定将为未来鱼类V2R基因家族的功能和进化研究提供有用的基础。
