Exploration of human trichromatic origin by studying visual pigment genes of nocturnal primates and zebrafish

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Vertebrate visual pigments can be classified into five classes : RH1, SWS1, SWS2, RH2, and LWS. Only primates and bony fish are known to have sub-categories of pigments under the classes, presumably reflecting their photo-environmental adaptation. However, the information regarding genomic organization, spectral properties and expressional regulation of the visual pigments of primates and fish is limited.We found that owl monkey, the only nocturnal monkey among higher primates, has LWS pigment gene(s) on Y chromosome as well as on X chromosome, while all the other mammals have been believed to have it on X chromosome. We also found that SWS1 genes of galagos and loris, nocturnal prosimians, have multiple insertions and deletions in the coding region and are non-functionalized. Absorption spectrum of galago's LWS pigment was shown to be blue-shifted compared to those of diurnal animals. These results indicate that visual pigments of nocturnal primates have evolved in their own way to adapt to nocturnal light environment.We showed that zebrafish has two red (LWS-1 and LWS-2), four green (RH2-1, RH2-2, RH2-3 and RH2-4) and single blue (SWS2) and ultraviolet (SWS1) opsin genes in the genome, among which LWS-2, RH2-2 and RH2-3 are novel. SWS2, LWS-1 and LWS-2 are located in tandem and RH2-1, RH2-2, RH2-3 and RH2-4 form another tandem gene cluster. The peak absorption spectra of the reconstituted photopigments from the opsin cDNAs differed markedly among them. The quantitative RT-PCR revealed a considerable difference among the opsin genes in the expression level in the retina. We also found that 1 kb upstream of RH1 can lead to rod cell-specific expression of GFP in the spatio-temporal pattern indistinguishable from that of endogenous RH1 gene.