PHYLOGENY AND PHYSIOLOGY OF DROSOPHILA OPSINS

Phylogenetic and physiological methods were used to study the evolutio n of the opsin gene family in Drosophila. A phylogeny based on DNA seq uences from 13 opsin genes including representatives from the two majo r subgenera of Drosophila shows six major, well-supported clades: The ''blue opsin'' clade includes all of the Rh1 and Rh2 genes and is sepa rated into two distinct subclades of Rh1 sequences and Rh2 sequences; the ultraviolet opsin clade includes all Rh3 and Rh4 genes and bifurca tes into separate Rh3 and Rh4 clades. The duplications that generated this gene family most likely took place before the evolution of the su bgenera Drosophila and Sophophora and their component species groups. Numerous changes have occurred in these genes since the duplications, including the loss and/or gain of introns in the different genes and e ven within the Rh1 and Rh4 clades. Despite these changes, the spectral sensitivity of each of the opsins has remained remarkably fixed in a sample of four species representing two species groups in each of the two subgenera. All of the strains that were investigated had R1-6 (Rh1 ) spectral sensitivity curves that peaked at or near 480 nm, R7 (Rh3 a nd Rh4) peaks in the ultraviolet range, and ocellar (Rh2) peaks near 4 20 nm. Each of the four gene clades on the phylogeny exhibits very con servative patterns of amino acid replacement in domains of the protein thought to influence spectral sensitivity, reflecting strong constrai nts on the spectrum of light visible to Drosophila.