The molecular basis of colour vision in colourful fish : four long wave-sensitive (LWS) opsins in guppies (Poecilia reticulata) are defined by amino acid substitutions at key functional sites.




Ward, Matthew Norman

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Comparisons of functionally important changes at the molecular level in model systems have identified key adaptations driving isolation and speciation. For example, recent studies suggest mutations in opsins, the genes that encode retinal photoreceptors, influence colour-based female mate choice and have, thereby, contributed to the radiation of African cichlids. To test the hypothesis that the evolution of elaborate colouration in guppies (Poecilia reticulata) is also associated with opsin gene diversity, long wavelength-sensitive (LWS) opsin genes were sequenced in six species of the family Poeciliidae. Sequences of four LWS genes were amplified from the guppy genome and from mRNA isolated from adult eyes. Variation in expression was quantified using qPCR. Three of the four paralogs encode opsins that appear to be most sensitive to different wavelengths of light, as they vary at key amino acid positions. This family of LWS opsin genes was produced by a diversity of duplication events. One gene appears to be the product of retrotransposition. Between-gene PCR and DNA sequencing show that two of the guppy LWS opsins are linked in an inverted orientation. The fourth locus is a hybrid, with the first five exons most similar to one of the tandem duplicates and the last exon most similar to the other. The single-exon LWS opsin was produced prior to the divergence of families Fundulidae and Poeciliidae. The inverted tandem duplication event occurred near the base of the poecilid tree, in the common ancestor of Poecilia and Xiphophorus, and the hybrid locus is found only in the genus Poecilia. Enhanced wavelength discrimination, a likely consequence of opsin gene duplication and divergence, might have been an evolutionary prerequisite for colour based sexual selection and have led to the extraordinary colouration now observed in male guppies and in many other poeciliids.



Guppies, Color vision