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

Abstract

SUMMARY Rod absorbance spectra, characterized by the wavelength of peak absorbance (λ 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 λ max differed between populations in a manner correlated with differences in the spectral light transmission of the respective water bodies [λ max : (A)≈503 nm; (E and S)≈505–506 nm; (B)≈508 nm]. A distinguishing feature of B was the wide within-population variation of λ 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 (λ max ≈507 nm) and the relatively red-shifted Baltic P. microps (λ max ≈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 withλ max at 505.6–507.5 nm were identical to S, but those with λ 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 λ max .