Thyroid hormone coordinates developmental trajectories but does not underlie developmental truncation in Danionins
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 PointsO_LIThyroid hormone regulates shifts in relative growth trajectories in different zebrafish tissues C_LIO_LIThyroid hormone inhibits head growth in juvenile zebrafish, and regulates juvenile growth patterns C_LIO_LIThyroid hormone stimulates formation of neuromast canals in the head and stitches on the trunk of zebrafish C_LIO_LIDanionella, the miniaturized sister group to Danio morphologically resemble larval zebrafish and do not form neuromast canals C_LIO_LIHypothyroidism is not the cause of paedomorphosis in Danionella C_LI Grant SponsorsNIH R00GM105874 NIH R03HD091634 Burroughs Wellcome Collaborative Research Travel Grant 1017439