Visual pigment chromophore usage in Nicaraguan Midas cichlids: phenotypic plasticity and genetic assimilation of cyp27c1 expression

The wide-ranging photic conditions found across aquatic habitats may act as selective pressures that potentially drive the rapid evolution and diversity of the visual system in teleost fish. Teleost fish fine-tune their visual sensitivities by regulating the two components of visual pigments, the opsin protein and the chromophore. Compared with opsin protein variation, chromophore usage across photic habitats has received little attention. The Nicaraguan Midas cichlid species complex, Amphilophus cf citrinellus [Günther 1864], has independently colonized seven isolated crater lakes with different photic conditions, resulting in several recent adaptive radiations. Here, we investigate variation in cyp27c1, the main enzyme modulating chromophore exchange. We measured cyp27c1 expression in photic environments in the wild, its genetic component in laboratory-reared fish, and its response to different light conditions during development. We found that photic environments significantly predict variation in cyp27c1 expression in wild populations and that this variation seems to be genetically assimilated in two populations. Furthermore, light-induced cyp27c1 expression exhibited genotype-by-environment interactions in our manipulative experiments. Overall, within-lake variation in cyp27c1 expression was higher and inversely related to variation in opsin gene expression along the photic gradient, emphasizing the key role of cyp27c1 in the visual ecology of cichlid fish.