Role of thyroid hormones in color diversity of male guppies: experimental data on Endler’s guppy ( Poecilia wingei )

Male guppies display an outstanding diversity of color patterns which is formed as a result of a complex interplay between sexual selection, predation, and other environmental factors. The heterogeneity of the environment affects the variability of ornamental traits in male guppies through genotype–environment interaction. Thyroid hormones (THs) are important regulators of the ontogeny of fish and serve as a link between environmental changes and phenotypic development. However, the role of THs in the formation of a variety of color patterns in male guppies remained poorly understood. In this work, an experimental assessment of the effect of THs on the variability of ornamental traits in Poecilia wingei males was carried out. The fish were reared from birth to the initial stages of the formation of melanistic elements in males and until the final formation of the color pattern; they were subjected to different hormonal regimes: euthyroidism (natural TH status), hyperthyroidism (high TH level, at a triiodo-L-thyronine concentration of 0.15 μg/mL), and hypothyroidism (TH-deficiency, at a thiourea concentration of 0.025%). Alterations in the TH status caused changes in the timing and rate of the development of coloration and affected the transformation of various elements of the color pattern in males. These changes led to an increase in phenotypic variability and the appearance of ornamental traits in the male color patterns that were characteristic of closely related species of Poecilia . Thus, the data obtained indicate a potentially important role of thyroid hormones in the diversification of guppy color patterns and open up new prospects for studying the role of endocrine regulatory mechanisms in the adaptive evolution of poeciliid fish.