The Molecular Basis of Color Vision in Colorful Fish: Four Long Wave-Sensitive (LWS) Opsins in Guppies (Poecilia reticulata) Are Defined by Amino Acid Substitutions at Key Functional Sites

Background: Comparisons of functionally important changes at the molecular level in modelsystems have identified key adaptations driving isolation and speciation. In cichlids, for example,long wavelength-sensitive (LWS) opsins appear to play a role in mate choice and male colorvariation within and among species. To test the hypothesis that the evolution of elaboratecoloration in male guppies (Poecilia reticulata) is also associated with opsin gene diversity, wesequenced long wavelength-sensitive (LWS) opsin genes in six species of the family Poeciliidae.Results: Sequences of four LWS opsin genes were amplified from the guppy genome and frommRNA isolated from adult guppy eyes. Variation in expression was quantified using qPCR. Threeof the four genes encode opsins predicted to be most sensitive to different wavelengths of lightbecause they vary at key amino acid positions. This family of LWS opsin genes was produced by adiversity of duplication events. One, an intronless gene, was produced prior to the divergence offamilies Fundulidae and Poeciliidae. Between-gene PCR and DNA sequencing show that two of theguppy LWS opsins are linked in an inverted orientation. This inverted tandem duplication eventoccurred near the base of the poeciliid tree in the common ancestor of Poecilia and Xiphophorus.The fourth sequence has been uncovered only in the genus Poecilia. In the guppies surveyed here,this sequence is a hybrid, with the 5' end most similar to one of the tandem duplicates and the 3'end identical to the other.Conclusion: Enhanced wavelength discrimination, a possible consequence of opsin geneduplication and divergence, might have been an evolutionary prerequisite for color-based sexualselection and have led to the extraordinary coloration now observed in male guppies and in manyother poeciliids.