Thyroid hormone signaling in the evolution of carotenoid coloration in Neotropical cichlids with reversed sexual dichromatism

Neotropical cichlids are a group of fish among which there are species with reversed sexual dichromatism, where females have carotenoid-based coloration that males lack. Carotenoid coloration can vary depending on the environment and the distribution of other types of chromatophores in the pigment pattern. The role of thyroid hormones (THs), one of the main signaling molecules regulating fish ontogeny, in the diversity of carotenoid coloration in Neotropical cichlid females remained poorly understood. In this work, using the species Amatitlania nigrofasciata , the effect of elevated TH level on the development of reversed sexual dichromatism was shown. Increases in TH signaling in experimental fish led to heterochrony in the development of the pigment pattern and phenotypic variability with the appearance of variants of carotenoid pigmentation similar to other Neotropical cichlid species. The extent of carotenoid-based ventral coloration of females depended on TH-induced changes in the melanistic pattern. The results obtained testify in favor of the important role of the TH signaling pathway in the diversification of the carotenoid coloration of Neotropical cichlids and provide new perspectives for studying the role of plasticity in the evolutionary adaptation of cichlid fishes.