Many members of the Peruvian poison frog genus Ranitomeya show a wide variety of color patterns, and two or more color morphs can exist within a single species. R. sirensis has yellow stripes over most of its range, but has a deep red color in the Sira mountains near the Brazilian border. Twomey et al. (2020) investigate this divergent coloration to determine the nature of the pigments in the skin of these frogs and the patterns of gene expression underlying pigment processing. First, with chromatography, they identified the carotenoid pigments in the skin and liver. As in previous studies, they found carotenoids associated with yellow coloration in both morphs (with a substantially higher amount in the red morph), as well as a number of ketocarotenoids in the red morph. These are modified carotenoids associated with red coloration. They then found in genetic tests that the red morph expresses a dysfunctional form of beta-carotene oxygenase 2 (BCO2: a carotenoid cleavage enzyme), which normally degrades carotenoids. This allows carotenoids to build to higher concentrations in the red morph, consistent with the results of their chromatographic analyses. They also found significantly higher expression of a carotenoid ketolase (CYP3A80: a cytochrome P450 enzyme) in the livers of red morph frogs. This appears to be the key enzyme converting carotenoids to ketocarotenoids, hence giving the red morph its distinctive appearance. By combining chemical analyses of pigmentation with genomic analyses of gene expression, this study bridges the gap between genotype and phenotype, a key goal of evolutionary analysis.
