Avian Dopamine Receptors DRD1a and DRD1b Arise from an Ancient Vertebrate Whole Genome Duplication

Dopamine is an important neurotransmitter that functions through two main classes of dopamine receptors to influence neural activity, behavior, and gene expression. Here, we identify and characterize Class 1 (D1a, D1b, D1c/d, D1e) and Class 2 (D2, D3, D4) dopamine receptor gene families in 43 bird genomes. Each gene was validated by inferring gene similarity and genomic region synteny with previously characterized receptors in other species. Phylogenetic trees were reconstructed (RAxML) for the superfamily of genes using aligned DNA and protein sequences (MUSCLE). Overall tree topography was as previously described for the dopamine superfamily in other vertebrate species. Further investigation showed, using collinear gene order of homologous genes, that chromosome regions containing the DRD1a and DRD1b receptors are syntenic between and within bird species. In addition, the synonymous mutation rates of these regions are common between a number of other syntenic regions, indicative that many of these regions originated simultaneously, possibly from a from a whole genome duplication event (WGD). We mapped these two genes to orthologous regions in the human genome previously shown as likely derived from an ancient WGD event in the vertebrate lineage. Taken together, these lines of evidence show that gene families DRD1a and DRD1b arose through an ancient WGD event in the early vertebrate lineage. In conclusion our findings about this key neurotransmitter family not only provide insights into the evolutionary development of an important component of the central nervous system in vertebrates, but also will help unravel the functional relationships between homologous dopamine receptors.