AbstractBackgroundLong wave-sensitive (LWS) opsin genes have undergone multiple lineage-specific duplication events throughout the evolution of teleost fishes.LWSrepertoire expansions in live-bearing fishes (family Poeciliidae) have equipped multiple species in this family with up to fourLWSgenes. Given that color vision, especially attraction to orange male coloration, is important to mate choice within poeciliids,LWSopsins have been proposed as candidate genes driving sexual selection in this family. To date the genomic organization of these genes has not been described in the family Poeciliidae, and little is known about the mechanisms regulating the expression ofLWSopsins in any teleost.ResultsTwo BAC clones containing the complete genomic repertoire ofLWSopsin genes in the green swordtail fish,Xiphophorus helleri, were identified and sequenced. Three of the fourLWSloci identified here were linked in a tandem array downstream of two tightly linked short wave-sensitive 2 (SWS2) opsin genes. The fourthLWSopsin gene, containing only a single intron, was not linked to the other three and is the product of a retrotransposition event. Genomic and phylogenetic results demonstrate that theLWSgenes described here share a common evolutionary origin with those previously characterized in other poeciliids. Using qualitative RT-PCR and MSP we showed that each of theLWSandSWS2opsins, as well as three other cone opsin genes and a single rod opsin gene, were expressed in the eyes of adult female and maleX. helleri, contributing to six separate classes of adult retinal cone and rod cells with average λmaxvalues 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 CRX binding sites and hormone response elements in upstream sequences ofLWSgene regions of seven teleost species, includingX. helleri.ConclusionsWe report the first complete genomic description ofLWSandSWS2genes in poeciliids. These data will serve as a reference for future work seeking to understand the relationship betweenLWSopsin genomic organization, gene expression, gene family evolution, sexual selection and speciation in this fish family.