A new study by Taboada et al (2020) describes how dozens of frog lineages have independently evolved green coloration, but not through the typical route using skin cells containing pigments (chromatophores). Instead, these frogs have convergently co-opted serpin proteins to capture biliverdin, a cyan pigment that is a common by-product of the breakdown of hemoglobin in red blood cells. Using mass-spectrometry and spectrophotometry, Taboada and colleagues determine that green coloration of many frogs is conferred by three characters: 1) relatively clear skin, which allows for the visibility of 2) yellow pigments (hyloins and carotenoids) and cyan-colored serpin-bound biliverdin in subcutaneous lymph fluid. These pigments selectively absorb green and red light that is reflected by 3) a reflective basal tissue layer containing guanine crystals. The spectral reflectance of frogs that use biliverdin-based coloration, such as many glass frogs and hylid treefrogs, likely allows them to effectively camouflage against a green-leaf background. Independent origins of biliverdin-based coloration in frogs may have resulted from independent recruitment of different serpin genes. Future studies of serpin evolution in these frogs require more genetic resources to fully understand.
