Frogs have layers: a comparative histological study of colour polymorphism in Oophaga poison frogs.

BACKGROUND: Colour polymorphism in aposematic species is often associated with ecological divergence and predator selection. In amphibians, colouration is achieved through the chromatophore unit, a layered structure of pigment-containing cells (xanthophores, melanophores) and structural cells (iridophores) that interact to produce the final skin colouration. In dendrobatid frogs of the genus Oophaga, striking red and green morphs coexist within and across species, yet the cellular mechanisms underlying this variation remain poorly understood. METHODS: We investigated the histological basis of colouration in three Oophaga species (O. granulifera, O. pumilio, and O. vicentei) by comparing chromatophore layer structure in red and green morphs and correlating these traits with spectrometric colour measurements. While all species have a chromatophore unit of xanthophores, iridophores, and melanophores, we observed interspecific and intraspecific variation in the organization, thickness, and relative area of these layers. RESULTS: In O. pumilio and O. vicentei, red morphs consistently exhibited greater xanthophore and iridophore coverage and more xanthophore contribution. In contrast, O. granulifera displayed minimal differences between morphs and a more disorganized chromatophore unit, however melanosome size was larger for green morphs. Spectral reflectance measurements correlated significantly with chromatophore traits, supporting a link between skin histology and colour phenotype. These findings suggest that colouration in Oophaga arises from less conserved chromatophore unit structure, reflecting diverse evolutionary routes to equivalent colouration.