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Session Schedule & Abstracts

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Thursday 30th June, 2016

Symposium : The vertebrate pharynx: crossroads in evolution and development 2

Room: Salon H   11:30 am–1:00 pm

Moderator(s): A. Huysseune & A. Tucker
PHA2-1  11:30 am  Endoderm out of the mouth: pre-oral gut in non-teleost fishes reveals an ancient mode of foregut development. Cerny R.*, Department of Zoology, Charles University in Prague, Czech Republic; Metscher B.D., Department of Theoretical Biology, University of Vienna, Austria; Arias Rodriguez L., Laboratorio de acuicultura tropical, Universidad Juárez Autónoma de Tabasco, Villahermosa, Mexico; Gela D., Research Institute of Fish Culture and Hydrobiology, University of South Bohemia in Ceske Budejovice, Czech Republic; Minarik M., Department of Zoology, Charles University in Prague, Czech Republic
Abstract: In all vertebrates, oro-pharyngeal development is considered rather uniform. From anterior to posterior, it comprises progressive formation of mouth and pharyngeal arches, where ectoderm generally outlines the outer, whereas endoderm the inner surfaces and structures. Here we present evidence that in all non-teleost fishes, development of the mouth is preceded by considerable foregut evagination that forms a distinct "pre-oral gut" along the roof of the prospective mouth. MicroCT imaging of complete embryonic series of African bichirs, American gars, and European sturgeons detailed an early pouching of their anteriormost archenteron with subsequent formation of prominent diverticula in the premandibular domain. Further in vivo lineage tracing mapped a contribution of this pre-oral endoderm to orofacial epidermis, including lips, sensory barbels, attachment organs, and teeth. This presents the first direct evidence of external surfaces and structures of the vertebrate head to be thoroughly derived from the endoderm. Embryonic formation of the pre-oral gut is prominent in all three basal (non-teleost) fish lineages and thus seems arguably ancestral for ray-finned fishes (Actinopterygii). In teleosts, such a foregut morphogenesis has been suppressed probably due to radical transformation of their early embryonic development and foregut compression. On the other hand, pre-oral gut formation seems to be at least rudimentarily present in many other vertebrates and the early foregut expansion that forms diverticula with a central lumen continuous with the buccal cavity appears in many Deuterostomes. Hatschek's diverticulum of embryonic Amphioxus (Cephalochordates), oral (buccal) glands of appendicularian tunicates, or stomochord (buccal diverticulum) of hemichordate acorn worms are all examples of this kind of foregut morphogenesis. The above-described peculiar formation of the prominent pre-oral gut in non-teleost fishes thus reveals an ancient blueprint of foregut morphogenesis rather than a clade-specific curiosity.

PHA2-2  12:00 pm  The first pouch in formation and evolution of the amniote middle ear. Tucker AS*, King's College London
Abstract: The endodermally derived first pharyngeal pouch classically forms the middle ear cavity in tetrapods with a tympanic ear, creating an air filled space that is continuous with the pharynx. In mice, however, the endoderm of the first pouch breaks during embryonic development creating a middle ear that is lined by both endoderm and neural crest derived mesenchyme. A similar break does not appear to occur in birds and reptiles but has been observed in a variety of mammals. This has consequences for the development of the tympanic membrane, with the Pars flaccida of mammals appearing to have no contribution from the first pouch. This new consideration of the role of the first pouch has consequences for our understanding of evolution of the tympanic ear and for our understanding of why the mammalian middle ear appears particularly susceptible to disorders.

PHA2-3  12:30 pm  Evolutionary and developmental relationships between pharyngeal pouches and teeth . Huysseune A*, Ghent University, Biology Department; Witten PE, Ghent University, Biology Department
Abstract: During vertebrate development, the pharyngeal endoderm produces a series of pouches separating the pharyngeal arches. The pouches may extend towards, and eventually make contact with, the ectodermal cover (skin). In primary aquatic osteichthyans, gill slits develop at these contact points. Based on paleontological and neontological evidence, we have previously proposed that the places of contact allow the interaction between ectoderm and endoderm and that this interaction is required for pharyngeal tooth formation (Huysseune et al., 2009). Likewise, the need for a close correlation between pouch formation (more specifically, ectodermal/endodermal contacts) and pharyngeal tooth development could explain the evolutionary loss of pharyngeal teeth in tetrapods. We have tested this hypothesis using mutant and transgenic zebrafish (Danio rerio), a teleost fish species possessing pharyngeal teeth only. For example, zebrafish van gogh mutants, defective in the transcription factor tbx1, display impaired pouch formation and defects in their pharyngeal dentition. Dentition defects coincide with the level of impairment of ectodermal-endodermal contacts. We have further tested the association between pouches and teeth by blocking endogenous retinoic acid synthesis, which also leads to defective tooth formation. We propose that impaired tooth formation is not a direct result from blocking endogenous retinoic acid (as suggested by Gibert et al., 2012), but is a byproduct of defective pouch formation. Together, our results lend support to the association between pouch and (pharyngeal) tooth formation. The authors acknowledge support from the Special Research Fund (BOF, 2-4 yr project) of Ghent University. Huysseune et al., 2009. J. Anat. 214: 465-476. Gibert et al., 2010. FASEB J. 24: 3298-3309.

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