Iguanas are a diverse group of reptiles belonging to the family Iguanidae, exhibiting a complex pattern of colors and behaviors that reflect their adaptation to various habitats and different communication strategies. The different coloration patterns in the skin of iguanas and reptiles, in general, suggest that the integumentary system is particularly crucial in adapting to diverse habitats, acting as a dynamic organ that responds to environmental cues. The skin, an interface between an organism and its environment, plays a pivotal role in maintaining homeostasis, providing protection, and facilitating various physiological functions. In this scenario, the coloration of iguanas is often intricately linked to their habitat, representing evolutionary adaptations for thermoregulation, camouflage, and communication (1).
Iguanas are among the most spectacular representative species of the Galápagos Islands (2). Four named species of Galápagos iguanas, all endemic, are currently recognized: Amblyrhynchus cristatus (marine iguanas), Conolophus subcristatus (common Galápagos land iguanas, hereafter yellow iguanas), C. pallidus (Barrington iguanas) and C. marthae (Galápagos pink land iguanas, or simply pink iguanas) (3, 4). Whereas marine iguanas are widely distributed along coastlines throughout the archipelago (5), C. subcristatus occurs only on a few islands, and C. pallidus and C. marthae are limited to Santa Fe and Isabela islands, respectively (6). Currently, many factors contribute to their threatened status at various levels of risk (IUCN Red List, https://www.iucnredlist.org/). Critically Endangered pink iguanas (7) were described in 2009 (8). Pink iguanas, which derive their common name from their color, have become a symbol of the incredible biodiversity and evolutionary processes that have unfolded in the isolated ecosystems of the Galápagos archipelago (9).
While studies have explored external coloration and its ecological implications, the ultrastructural aspects of iguana skin remain less known, and no such information is currently available for any of the Galápagos iguanas. In a recent paper (10), gross and microscopic analyses of skin structure provided histological details on the nature of coloration of three of the four named Galápagos iguana species, A. cristatus, C. subcristatus and C. marthae. Uniquely for C. marthae, the authors found that "pink" dermal areas are devoid of melanophores, and a rich network of confluent capillaries exists. A poor interaction between xanthopores and iridophores was also speculated to explain the peculiar pattern of coloration, as the study could not confirm or falsify the presence of iridophores. While this study provided solid evidence to explain the pink color of adult C. marthae, it also raised a question related to the recent discovery that C. marthae hatchlings are not pink upon emergence (11). Like C. subcristatus (Fig. 1A), they are maculated, but contrary to C. subcristatus, C. marthae hatchlings show a green color pattern (Fig. 1B).
Generally, greenish, and bluish colors in reptiles are primarily due to an interaction between different types of chromatophores in different skin layers. Chromatophores is a general term that includes melanophores, iridophores (granulophores or guanophores), xanthophores, and erythrophores. Iridophores are particularly important, as they contain guanine nanocrystals capable of reflecting and diffracting light. Iridophores may be located in the superficial and deeper dermal layers. Whereas in the deeper layers, crystals protect against excessive UV radiation, crystals in the more superficial iridophores can diffract light, generating different colors, depending on the wavelength diffracted, which may depend on the density and organization of crystals. Green color emerges when the mostly diffracted blue wavelength interferes with carotenoids in xanthophores (1, 12). Chromatophores may be well characterized by their ultrastructure, and melanophores, iridophores, and xanthophores may be distinguished by Transmission Electron Microscopy (TEM) (13).
Starting from these considerations and given the predicted presence of iridophores in C. marthae hatchlings, the present study investigated the histological, ultrastructural and immunohistochemical features to comprehensively characterize the skin in adults of three species of Galápagos iguanas (A. cristatus, C. subcristatus and C. marthae), and test for the presence of different types of chromatophores in their skin.