Characterization of novel GPCR gene coding locus in amphioxus genome: Gene structure, expression, and phylogenetic analysis with implications for its involvement in chemoreception

Chemosensation is the primary sensory modality in almost all metazoans. The vertebrate olfactory receptor genes exist as tandem clusters in the genome, so that identifying their evolutionary origin would be useful for understanding the expansion of the sensory world in relation to a large‐scale genomic duplication event in a lineage leading to the vertebrates. In this study, I characterized a novel GPCR (G‐protein‐coupled receptor) gene‐coding locus from the amphioxus genome. The genomic DNA contains an intronless ORF whose deduced amino acid sequence encodes a seven‐transmembrane protein with some amino acid residues characteristic of vertebrate olfactory receptors (ORs). Surveying counterparts in the Ciona intestinalis (Asidiacea, Urochordata) genome by querying BLAST programs against the Ciona genomic DNA sequence database resulted in the identification of a remotely related gene. In situ hybridization analysis labeled primary sensory neurons in the rostral epithelium of amphioxus adults. Based on these findings, together with comparison of the developmental gene expression between amphioxus and vertebrates, I postulate that chemoreceptive primary sensory neurons in the rostrum are an ancient cell population traceable at least as far back in phylogeny as the common ancestor of amphioxus and vertebrates. genesis 41:47–57, 2005. © 2005 Wiley‐Liss, Inc.