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Background  G protein-coupled receptors (GPCRs) constitute a large family of integral transmembrane receptor proteins that play a central role in signal transduction in eukaryotes. The genome of the protochordate  Ciona intestinalis  has a compact size with an ancestral complement of many diversified gene families of vertebrates and is a good model system for studying protochordate to vertebrate diversification. An analysis of the  Ciona  repertoire of GPCRs from a comparative genomic perspective provides insight into the evolutionary origins of the GPCR signalling system in vertebrates.    Results  We have identified 169 gene products in the  Ciona  genome that code for putative GPCRs. Phylogenetic analyses reveal that  Ciona  GPCRs have homologous representatives from the five major GRAFS ( Glutamate, Rhodopsin, Adhesion, Frizzled  and  Secretin ) families concomitant with other vertebrate GPCR repertoires. Nearly 39% of  Ciona  GPCRs have unambiguous orthologs of vertebrate GPCR families, as defined for the human, mouse, puffer fish and chicken genomes. The  Rhodopsin  family accounts for ~68% of the  Ciona  GPCR repertoire wherein the LGR-like subfamily exhibits a lineage specific gene expansion of a group of receptors that possess a novel domain organisation hitherto unobserved in metazoan genomes.    Conclusion  Comparison of GPCRs in  Ciona  to that in human reveals a high level of orthology of a protochordate repertoire with that of vertebrate GPCRs. Our studies suggest that the ascidians contain the basic ancestral complement of vertebrate GPCR genes. This is evident at the subfamily level comparisons since  Ciona  GPCR sequences are significantly analogous to vertebrate GPCR subfamilies even while exhibiting  Ciona  specific genes. Our analysis provides a framework to perform future experimental and comparative studies to understand the roles of the ancestral chordate versions of GPCRs that predated the divergence of the urochordates and the vertebrates.

The repertoire of G protein-coupled receptors in the sea squirt Ciona intestinalis