Thyroid hormone modulation during zebrafish development recapitulates evolved diversity in danionin jaw protrusion mechanics

One of three vertebrates belongs to a fish lineage for which protrusile jaws are a synapomorphy. Identifying the developmental determinants of protrusion ability will improve our understanding of an important area of evolutionary diversification. The high water viscosities experienced by tiny fish larvae inhibit the viability of protrusile jaws. In the zebrafish protrusion does not arise until after metamorphosis. Fish metamorphosis typically includes significant changes in trophic morphology, accompanies a shift in feeding niche and coincides with increased thyroid hormone production. We tested whether thyroid hormone affects the development of zebrafish feeding mechanics. We found that it affected all developmental stages examined, but that these effects were most pronounced after metamorphosis. Thyroid hormone levels affected the development of jaw morphology, feeding mechanics, shape variation and cranial ossification. Adult zebrafish utilize protrusile jaws, but an absence of thyroid hormone eliminated postmetamorphic remodeling of the premaxilla and the premaxillary structure that permits protrusion never formed. The premaxillae of late juvenile and adult zebrafish are similar to those found in the adults of other Danio species. Premaxillae from early juvenile zebrafish and hypothyroid adult zebrafish resemble those from adults in the genera Danionella, Devario and Microdevario that show little to no jaw protrusion.