Individual variation in rod absorbance spectra correlated with opsin gene polymorphism in sand goby (Pomatoschistus minutus).

Rod absorbance spectra, characterized by the wavelength of peak absorbance (lambda(max)) were related to the rod opsin sequences of individual sand gobies (Pomatoschistus minutus) from four allopatric populations [Adriatic Sea (A), English Channel (E), Swedish West Coast (S) and Baltic Sea (B)]. Rod lambda(max) differed between populations in a manner correlated with differences in the spectral light transmission of the respective water bodies [lambda(max): (A) approximately 503 nm; (E and S) approximately 505-506 nm; (B) approximately 508 nm]. A distinguishing feature of B was the wide within-population variation of lambda(max) (505.6-511.3 nm). The rod opsin gene was sequenced in marked individuals whose rod absorbance spectra had been accurately measured. Substitutions were identified using EMBL/GenBank X62405 English sand goby sequence as reference and interpreted using two related rod pigments, the spectrally similar one of the Adriatic P. marmoratus (lambda(max) approximately 507 nm) and the relatively red-shifted Baltic P. microps (lambda(max) approximately 515 nm) as outgroups. The opsin sequence of all E individuals was identical to that of the reference, whereas the S and B fish all had the substitution N151N/T or N151T. The B fish showed systematic within-population polymorphism, the sequence of individuals with lambda(max) at 505.6-507.5 nm were identical to S, but those with lambda(max) at 509-511.3 nm additionally had F261F/Y. The substitution F261Y is known to red-shift the rod pigment and was found in all P. microps. We propose that ambiguous selection pressures in the Baltic Sea and/or gene flow from the North Sea preserves polymorphism and is phenotypically evident as a wide variation in lambda(max).