Genomic Organization of Duplicated Short Wave-Sensitive and Long Wave-Sensitive Opsin Genes in the Green Swordtail, Xiphophorus helleri

Background: Long wave-sensitive (LWS) opsin genes have undergone multiple lineage-specific duplication eventsthroughout the evolution of teleost fishes. LWS repertoire expansions in live-bearing fishes (family Poeciliidae) haveequipped multiple species in this family with up to four LWS genes. Given that color vision, especially attraction toorange male coloration, is important to mate choice within poeciliids, LWS opsins have been proposed ascandidate genes driving sexual selection in this family. To date the genomic organization of these genes has notbeen described in the family Poeciliidae, and little is known about the mechanisms regulating the expression ofLWS opsins in any teleost.Results: Two BAC clones containing the complete genomic repertoire of LWS opsin genes in the green swordtailfish, Xiphophorus helleri, were identified and sequenced. Three of the four LWS loci identified here were linked in atandem array downstream of two tightly linked short wave-sensitive 2 (SWS2) opsin genes. The fourth LWS opsingene, containing only a single intron, was not linked to the other three and is the product of a retrotranspositionevent. Genomic and phylogenetic results demonstrate that the LWS genes described here share a commonevolutionary origin with those previously characterized in other poeciliids. Using qualitative RT-PCR and MSP weshowed that each of the LWS and SWS2 opsins, as well as three other cone opsin genes and a single rod opsingene, were expressed in the eyes of adult female and male X. helleri, contributing to six separate classes of adultretinal cone and rod cells with average lmax values of 365 nm, 405 nm, 459 nm, 499 nm, 534 nm and 568 nm.Comparative genomic analysis identified two candidate teleost opsin regulatory regions containing putative CRXbinding sites and hormone response elements in upstream sequences of LWS gene regions of seven teleostspecies, including X. helleri.Conclusions: We report the first complete genomic description of LWS and SWS2 genes in poeciliids. These datawill serve as a reference for future work seeking to understand the relationship between LWS opsin genomicorganization, gene expression, gene family evolution, sexual selection and speciation in this fish family.