Meeting Abstract
Cypriniformes is a diverse group of freshwater fishes characterized by trophic novelties that include kinethmoid-mediated premaxillary protrusion and pharyngeal jaws formed solely from greatly hypertrophied 5th ceratobranchials. The hypertrophied 5th ceratobranchial has teeth ankylosed to it and since upper pharyngeal jaws are lost fish process food against the basioccipital. While investigating the skeletal anatomy of thyroid-disrupted zebrafish we identified profound anatomical changes to both the pharyngeal jaws and those structural elements associated with premaxillary protrusion. Thyroid hormone regulates tooth number; hypothyroid pharyngeal jaws had significantly more teeth, while hyperthyroid jaws had significantly fewer teeth. The anterior jaws, and the resulting kinethmoid-mediated premaxillary protrusion, were also strongly affected thyroid hormone. The kinethmoid is a sesamoid bone that sits ventral to the ascending process of the premaxilla and dorsal to the maxilla and has been shown to play a vital role in cypriniform jaw protrusion. In the hypothyroid zebrafish, the kinethmoid fails to ossify properly and lacks the characteristic lateral wings and dorsal process, as does the ascending process of the premaxilla. In the hyperthyroid zebrafish, the kinethmoid is hyperossified and improperly shaped as a result. Thyroid hormone therefore normally regulates the proper development of these feeding structures, and modulation of thyroid hormone likely gives rise to some of the anatomical differences observed both within Cypriniformes and outside of it. Understanding how the modulation of thyroid hormone changes these bones, and thus their ability to function, may lend insight into how they may have evolved.
