Morphological Novelties and the Role of Thyroid Hormone in Cypriniform Skeletogenesis
Open AccessMorphological novelties are new forms or functions that are shaped during ontogeny but are not present in the ancestral state, and thus tracing the development of morphological novelties is key to understanding their evolutionary history. Cypriniformes are characterized by several trophic morphological novelties: the pharyngeal jaws, kinethmoid, and palatal organ. However, unlike the pharyngeal jaws and kinethmoid, the palatal organ (PO) has been poorly studied. This muscular structure at the roof of the buccal cavity aids goldfish and carp trap and sort food, but the development of the PO has not been well investigated. Using zebrafish, which are cypriniforms as well as a biomedical tool, we found that the PO grows between 60 and 96 hours post fertilization (hpf) in three stages: the emergence, anterior growth and integration, and thickening phase. Based on the development of the PO prior to the onset of feeding behaviors in zebrafish and based on its function in goldfish and carp, the PO may serve as a muscular hydrostat in zebrafish as well. To better understand the forces that shape morphology, we also investigated the consequences of thyroid hormone (TH) disruption on morphology, as TH plays a key role in coordinating skeletogenesis. We first performed an anatomical assessment of adult hypo- and hyperthyroid zebrafish and found that the morphology of the pectoral girdle, craniofacial elements, and Weberian apparatus is significantly impacted. Given that the morphology of the lower jaw, hyomandibula, pharyngeal jaws, and tripus were disrupted in the adults, the development of these bones was described anatomically post metamorphosis and analyzed via geometric morphometrics. The morphology and shape of the tripus, lower jaw, and hyomandibula were particularly affected, and the changes in morphology observed have clinical, functional, and evolutionary implications that should be further investigated.
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