Differential regulation of opsin gene expression in response to internal and external stimuli

Determining how internal and external stimuli interact to determine developmental trajectories of traits is a challenge that requires the integration of different subfields of biology. Internal stimuli, such as hormones, control developmental patterns of phenotypic changes, which might be modified by external environmental cues (e.g., plasticity). Thyroid hormone (TH) modulates the timing of opsin gene expression in developing Midas cichlid fish (Amphilophus citrinellus). Moreover, fish reared in red light accelerate this developmental timing compared to fish reared in white light. Hence, we hypothesized that plasticity caused by variation in light conditions has co-opted the TH signaling pathway to induce changes in opsin gene expression. We treated Midas cichlids with TH and crossed this treatment with two light conditions, white and red. We observed that the pattern of opsin expression responded similarly to TH and red light, but also that at high TH levels the response to light conditions was overridden. Transcriptomic analysis of the retina showed that genes in the TH pathway were affected by TH, but not by the light treatments. Co-expression network analyses further suggested that the response to light was independent and potentially mutually exclusive with responses to TH manipulations. Taken together, our results suggest independent mechanisms mediating development and plasticity during development of opsin gene expression, and that responses to environmental stimuli could be overridden by internal stimuli. This conditional developmental response to external factors depending on internal ones (e.g., hormones) might play a fundamental role in the patterns of phenotypic divergence observed in Midas cichlids, and potentially other organisms.