MECHANISMS OF WAVELENGTH TUNING IN THE ROD OPSINS OF DEEP-SEA FISHES

The main object of this study was to investigate the molecular basis f or changes in the spectral sensitivity of the visual pigments of deep- sea fishes. The four teleost species studied? Hoplostethus mediterrane us, Cataetyx laticeps, Gonostoma elongatum and Histiobranchus bathybiu s, are phylogenetically distant from each other and live at depths ran ging from 500 to almost 5000 m. A single fragment of the intronless ro d opsin gene was PCR-amplified from each fish and sequenced. The wavel ength of peak sensitivity for the rod visual pigments of the four deep -sea species varies from 483 nm in H. mediterraneus and G. elongatum t o 468 nm in C. laticeps. Six amino acids at sites on the inner face of the chromophore-binding pocket formed by the seven transmembrane a-he lices are identified as candidates for spectral tuning. Substitutions at these sites involve either a change of charge, or a gain or loss of a hydroxyl group. Two of these, at positions 83 and 292, are consiste ntly substituted in the visual pigments of all four species and are li kely to be responsible for the shortwave sensitivity of the pigments. Shifts to wavelengths shorter than 480 nm may involve substitution at one or more of the remaining four sites. None of the modifications fou nd in the derived sequences of these opsins suggest functional adaptat ions, such as increased content of hydroxyl-bearing or proline residue s, to resist denaturation by the elevated hydrostatic pressures of the deep sea. Phylogenetic evidence for the duplication of the rod opsin gene in the Anguilliform lineage is presented.