Frogs have layers: A comparative histological study of color polymorphism in Oophaga poison frogs
Abstract
Background
Color polymorphism in aposematic species is often associated with ecological divergence and predator selection. In amphibians, coloration 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 coloration. 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 coloration 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 color measurements. While all species have a chromatophore unit of xanthophores, iridophores, and melanophores, we observed interspecific and intraspecific variation in the organization, thickness, and density of these layers.
Results
In O. pumilio and O. vicentei, red morphs consistently exhibited greater xanthophore and iridophore coverage and higher xanthophore to melanophore ratios. In contrast, O. granulifera displayed minimal differences between morphs and a more disorganized chromatophore unit, however melanosome size and melanophore density was higher for green morphs. Spectral reflectance measurements correlated significantly with chromatophore traits, supporting a link between skin histology and color phenotype. These findings suggest that coloration in Oophaga arises from less conserved chromatophore unit structure, reflecting diverse evolutionary routes to equivalent coloration.