Figure 10. An evolutionary hypothesis regarding the origin of dopamine receptor genes in vertebrates.

The vertebrate ancestor possessed two dopamine receptors, one of each class. However, after the two rounds of whole genome duplications (WGD) that occurred in the ancestor of the group each ancestral gene (DRD_1anc and DRD_2anc) gave rise to four genes. In the case of the DRD₁ class of receptors three out of the four genes were retained in the genome of the vertebrate ancestor. In the gnathostome ancestor, the DRD_1C∕E gene underwent a duplication event that gave rise to the actual DRD_1C and DRD_1E genes. Thus, the gnathostome ancestor had a repertoire of four DRD₁ genes: DRD₁, DRD₅, DRD_1C and DRD_1E. In the case of the DRD₂ group of receptors, the vertebrate WGDs originated four genes, three of which were maintained in the genome of extant species (DRD_2∕2l, DRD₃ and DRD_4∕4rs). In the ancestor of gnathostomes, the DRD_2∕2l gene underwent a duplication event that gave rise to an extra copy of the gene. Similarly, the DRD_4∕4rs gene also underwent a duplication event that gave rise to an extra copy of the gene. Thus, the ancestor of gnathostome vertebrates possessed a repertoire of five DRD₂ genes: DRD₂, DRD_2l, DRD₃, DRD₄ and DRD_4rs.