Background Differences in postembryonic developmental trajectories can profoundly alter adult phenotypes and life histories. Thyroid hormone (TH) regulates metamorphosis in many vertebrate taxa with multiphasic ecologies, and alterations to TH metabolism underlie notable cases of paedomorphosis in amphibians. We tested the requirement for TH in multiple postembryonic developmental processes in zebrafish, which has a monophasic ecology, and asked if TH production was compromised in paedomorphic Danionella. Results We showed that TH regulates allometric growth in juvenile zebrafish, and inhibits relative head growth. The lateral line system showed differential requirements for TH: the hormone promotes canal neuromast formation and inhibits neuromast proliferation in the head, but causes expansion of the neuromast population in the trunk. While Danionella morphology resembled that of larval zebrafish, the two Danionella species analyzed were not similar to hypothyroid zebrafish in their shape or neuromast distribution, and both possessed functional thyroid follicles. Conclusions Although zebrafish do not undergo a discrete ecological transformation, we found that multiple tissues undergo transitions in developmental trajectories that are dependent on TH, suggesting the TH axis and its downstream pathways as likely targets for adaptation. Nonetheless, we found no evidence that evolutionary paedomorphosis in Danionella is the result of compromised TH production.

BackgroundChanges in post-embryonic developmental trajectories can profoundly alter adult phenotypes and life history transitions. In vertebrate clades with complex, biphasic life cycles, thyroid hormone (TH) regulates metamorphosis, and alterations to TH metabolism underlie famous cases of paedomorphosis. We tested the requirement for TH in multiple post-embryonic developmental processes in the zebrafish, and asked if TH loss was associated with paedomorphosis in miniaturized Danionellas.ResultsWe found that TH regulates the relative growth of different body parts in zebrafish, inhibiting head growth at juvenile stages. The lateral line also showed sensitivity to the hormone: head neuromasts were inhibited by TH, while trunk neuromasts required TH for their proliferation. While several aspects of Danionella morphology resembled that of larval zebrafish, these species did not resemble hypothyroid zebrafish in their shape or lateral line, and these fish showed functional thyroid follicles.ConclusionsAlthough zebrafish do not undergo a discrete ecological transformation, we found that multiple tissues go through a protracted metamorphosis, and that TH ushers in growth patterns and processes typical of juveniles. We found no evidence that compromised TH metabolism is responsible for paedomorphosis in Danionellas. Nonetheless, modulations to TH-sensitive pathways affect evolutionarily-relevant traits, and are likely important targets for adaptation.Bullet PointsThyroid hormone regulates shifts in relative growth trajectories in different zebrafish tissuesThyroid hormone inhibits head growth in juvenile zebrafish, and regulates juvenile growth patternsThyroid hormone stimulates formation of neuromast canals in the head and ‘stitches’ on the trunk of zebrafishDanionella, the miniaturized sister group to Danio morphologically resemble larval zebrafish and do not form neuromast canalsHypothyroidism is not the cause of paedomorphosis in DanionellaGrant SponsorsNIH R00GM105874NIH R03HD091634Burroughs Wellcome Collaborative Research Travel Grant 1017439