Ancient colour vision: multiple opsin genes in the ancestral vertebrates

Molecular investigation of the origin of colour vision has discovered five visual pigment (opsin) genes, all of which are expressed in an agnathan (jawless) fish, the lamprey Geotria australis. Lampreys are extant representatives of an ancient group of vertebrates whose origins are thought to date back to at least the early Cambrian, approximately 540 million years ago [1]. Phylogenetic analysis has identified the visual pigment opsin genes of G. australis as orthologues of the major classes of vertebrate opsin genes. Therefore, multiple opsin genes must have originated very early in vertebrate evolution, prior to the separation of the jawed and jawless vertebrate lineages, and thereby provided the genetic basis for colour vision in all vertebrate species. The southern hemisphere lamprey Geotria australis (Figure 1A,B) possesses a predominantly cone-based visual system designed for photopic (bright light) vision [2,3]. Previous work identified multiple cone types suggesting that the potential for colour vision may have been present in the earliest members of this group. In order to trace the molecular evolution and origins of vertebrate colour vision, we have examined the genetic complement of visual pigment opsins in G. australis. We used RT-PCR with a set of degenerate primers to amplify and clone partial cDNA sequences from each of the G. australis opsin genes. Overlapping cDNA sequences were isolated by 5′-and 3′-RACE-PCR and the complete coding sequence of each gene was reconstructed. The sequence of the G. australis opsin genes was confirmed by amplification of each cDNA as a single fragment using a proof reading DNA polymerase. The sequences of the PCR primers and sequence alignments of the five G. australis opsin genes are provided as supplementary material (Genbank accession numbers: AY366491–AY366495). The five G. australis opsin cDNA sequences were compared with the major groups of opsin genes from a broad range of representative vertebrate species. A codon-matched nucleotide sequence alignment provided the basis for estimating the evolutionary distances between the gnathostome (jawed vertebrate) and G. australis opsins and the neighbour-joining method [4] was used to infer phylogenetic relationships. The resulting phylogenetic tree (Figure 1C) shows that the G. australis LWS, SWS1 and SWS2 opsin genes are orthologous to the LWS, SWS1 and SWS2 opsin genes of the jawed vertebrates, with the branching order within each gene group largely following the accepted order of appearance of the vertebrate species. The two remaining G. australis opsin genes, RhA and RhB, are most closely related …