In
mammals, the
melanopsin gene
(Opn4) encodes a sensory photopigment that underpins newly discovered inner retinal photoreceptors. Since its first discovery in
Xenopus laevis and subsequent description in humans and mice, melanopsin
genes have been described in all
vertebrate classes. Until now, all of these sequences have been considered representatives of a single orthologous gene (albeit with duplications in the teleost fish). Here, we describe the discovery and functional characterisation of a new melanopsin gene in fish, bird, and amphibian genomes, demonstrating that, in fact, the
vertebrates have evolved two quite separate
melanopsins. On the basis of sequence similarity, chromosomal localisation, and phylogeny, we identify our new melanopsins as the true orthologs of the melanopsin
gene previously described in mammals and term this grouping
Opn4m. By contrast, the previously published melanopsin genes in nonmammalian vertebrates represent a separate branch of the melanopsin family which we term
Opn4x. RT-PCR analysis in chicken, zebrafish, and
Xenopus identifies expression of both
Opn4m and
Opn4x genes in tissues known to be photosensitive (eye, brain, and skin). In the day-14 chicken eye,
Opn4m mRNA is found in a subset of cells in the outer nuclear, inner nuclear, and ganglion cell layers, the vast majority of which also express
Opn4x. Importantly, we show that a representative of the new melanopsins (chicken
Opn4m) encodes a photosensory pigment capable of activating G protein signalling cascades in a light- and retinaldehyde-dependent manner under heterologous expression in Neuro-2a cells. A comprehensive in silico analysis of vertebrate genomes indicates that while most vertebrate species have both
Opn4m and
Opn4x genes, the latter is absent from eutherian and, possibly, marsupial mammals, lost in the course of their evolution as a result of chromosomal reorganisation. Thus, our findings show for the first time that nonmammalian vertebrates retain two quite separate melanopsin genes, while mammals have just one. These data raise important questions regarding the functional differences between Opn4x and Opn4m pigments, the associated adaptive advantages for most vertebrate species in retaining both melanopsins, and the implications for mammalian biology of lacking Opn4x.
More abstracts about the Evolution of Melanopsin Photoreceptors: Discovery and Characterization of a New Melanopsin in Nonmam