Skin Ultrastructural Analysis Reveals a Lack of Iridophores in Adult Individuals of Differently-Colored Species of Galápagos Iguanas

Abstract

Critically Endangered Galápagos pink land iguanas (Conolophus marthae) are one of the rarest species of iguana, found exclusively on Isabela Island, in the Galápagos. A previous histological study provided an explanation of the unique pink color of adult C. marthae. Still, it also raised a question related to the recent discovery that C. marthae hatchlings are not pink upon emergence. Like hatchlings of C. subcristatus, another Galápagos iguana species, they are maculated, but contrary to C. subcristatus, C. marthae hatchlings exhibit a green color pattern. Generally, greenish, and bluish colors in reptiles are primarily due to an interaction between different types of chromatophores in different skin layers. Iridophores are particularly important among chromophores, as they contain guanine nanocrystals capable of reflecting and diffracting light. Green color emerges when the mostly diffracted blue wavelength interferes with carotenoids in xanthophores. In a multiapproach analysis, we investigated the histological, ultrastructural and immunohistochemical features of differently-colored skin areas in three species of Galápagos iguanas. We tested for the presence of different types of chromatophores and described characteristics of stratus corneum, stratus germinativum and stratum laxum in the different species. Results demonstrated the absence of iridophores in the skin of adults of the three species, regardless of color, and the presence of stratified epithelium in the stratum germinativum of C. marthae non-black-colored skin. This research furthers interest in the evolutionary significance of pink coloration in C. marthae. The study also calls for additional research to demonstrate the presence of iridophores in pink iguana hatchlings and document possible ontogenetic changes that may lead to the lack of iridophores in adults. An investigation that could take advantage of genome mapping and gene expression along a developmental gradient would be greatly beneficial for advancing the understanding of the genomic basis of pink iguanas and, in general, reptile coloration.