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Session Schedule & Abstracts

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Friday 1st July, 2016

Symposium: The many faces of the skeleton: a tribute to the achievements of Brian K. Hall 1

Room: Salon A   9:30 am–11:00 am

Moderator(s): P. E. Witten & M. Vickaryous
HAL1-1  9:30 am  Facing plasticity in the skeleton of teleosts. Witten PE*, Ghent University, Department of Biology; Huysseune A, Ghent University, Department of Biology
Abstract: Black and white thinking or digitalising nature by assigning characters to zero or one, is not the way that Brian K. Hall understands evolution and development. A prominent example is how Brian Hall explains us the many gradients that exist between homology and convergence (1). Darwin took a similar approach when he addressed the species problem (2): "Species are merely artificial combinations made for convenience. This may not be a cheering prospect; but we shall at least be freed from the vain search for the undiscovered and undiscoverable essence of the term species". Perhaps one can say, in a very Darwinian way Brian teaches us to look at the nature of skeletal tissues. We assign skeletal tissues and cells to distinctive categories, but nature is full of intermediate skeletal tissues. Grading of tissue types and the transition of skeletal cells into other cell types are common processes, not exceptions. They are required for development, adaptation and regeneration. Teleost skeletal and dental development provides us with numerous examples of plasticity, in terms of how skeletal structures are patterned, as well as of how they achieve their morphological end product. We will provide examples taken from our studies on dental patterning and on development of dermal and endoskeletal elements in teleosts. Clearly, our research would not be possible without the Brian Hall's understanding of the skeleton (3). (1) Hall BK (2003) Descent with modification: the unity underlying homology and homoplasy as seen through an analysis of development and evolution. Biol. Rev 78:409 -433 (2) Darwin C (1859) The origin of species. 552 pages (3) Hall BK (2015) Bone and Cartilage. Evolutionary Developmental Skeletal Biology. Academic Press. 892pages

HAL1-2  10:00 am  Dermal skeletal plasticity and evolution of the chondrichthyan dentition . Meredith Smith Moya*, Kings Colleg London; Johanson Z, Natural History Museum London; Underwood , Birckbeck College London   
Abstract: Adaptive structure of the dermal skeleton during its evolution depends on plasticity of developmental mechanisms for skeletogenesis to form bone, separately and coupled with dentine. Dentine forms within a morphogenetic unit, the odontode, and requires neural crest cells to interact with epithelium; in skin and oropharyngeal mucosa odontodes are serial homologues by shared regulatory genes. Major divergences occur through developmental plasticity for adaptive function as odontodes transform through evolution. This diversity of odontodes in different morphogenetic fields occurs within as yet uncharacterised developmental boundaries. Dentitions are uniquely patterned in an oral morphogenetic field that may arise in evolution by co-opting modules from serially ordered, enlarged dermal denticles, as in axially aligned tail scales, or axial and paraxial dorsal rows. One such pre-patterned field, exemplified by extended rostra equipped with "saw-teeth", is present in the sawsharks, the sawfish and fossil Sclerorhynchoidea. We examined both adults, and embryos to determine the developmental plasticity of chondrichthyan dermal denticles and whether or not, they could have been co-opted to provide a module for the dentition. In the chondrichthyan dentition, toothed fields are restricted to a continuous dental lamina, where from the jaw symphysis teeth form as left right mirror images. This oral field regulates the pattern (in time and space) of tooth addition along the jaw (distal to proximal) and in each developmental set of lingual successor teeth. These comprise a sequence addition model of two adjacent replacement sets formed in alternate time and space, proposed to explain alternate tooth pattern in many chondrichthyan jaws. Alternatively, in the single file tooth pattern, each is iterative, as disto-proximal tooth files spaced out along the jaw, where all file teeth are central cusp aligned, an arrangement that may be ancestral for crown gnathostomes.

HAL1-3  10:30 am  A neural crest origin of trunk dermal denticles in the little skate, Leucoraja erinacea. Gillis JA*, University of Cambridge; Alsema EC
Abstract: Although many stem-gnathostomes possessed an extensive dentinous trunk exoskeleton, this feature is present only in relatively few extant taxa (e.g. in the form of dermal denticles in cartilaginous fishes, and in the scales of Polypterus and Latimeria). The embryonic origin of the dentine producing cells (odontoblasts) of the vertebrate trunk exoskeleton is a longstanding unresolved question in vertebrate evolutionary-developmental biology. The odontoblasts of vertebrate teeth derive exclusively from cranial neural crest cells. However, trunk neural crest cells are generally regarded as non-skeletogenic/odontogenic, leading to suggestions that trunk odontoblasts may derive from cranial neural crest cells that undergo an exceptionally long caudal migration, or from mesodermally derived progenitors. We have experimentally tested the odontogenic fate of trunk neural crest cells in embryos of a cartilaginous fish, the little skate (Leucoraja erinacea). Using histology and mRNA in situ hybridisation, we have characterised the emigration of trunk neural crest cells in the early skate embryo, and by labelling the trunk neural tube with CM-DiI prior to neural crest cell emigration, we demonstrate that the odontoblasts of skate trunk dermal denticles are, in fact, neural crest-derived. Our findings highlight the odontogenic potential of trunk neural crest cells in cartilaginous fishes, and point to the neural crest as the primitive source of dentinous tissues in the vertebrate exoskeleton.

HAL1-4  10:45 am  Evolution and development of scleral ossicles. Franz-Odendaal T.A.*, Mount Saint Vincent University
Abstract: My research into the evolution and development of scleral ossicles was begun in Dr BK Hall's laboratory while I was a post-doctoral fellow. Over the last decade, scleral ossicles have remained a key focus of my ongoing research program. Scleral ossicles are flat bony plates present in the sclera of most vertebrate eyes. They have a long evolutionary history but remain poorly understood. Through gross morphological and experimental developmental biology studies, we have shown that while ossicles vary in shape and size within different reptilian lineages, they are highly constrained during development. In contrast, the morphology of the scleral ossicles is highly conserved amongst teleosts. From a developmental perspective, scleral ossicles develop via different modes of ossification in reptiles and teleosts, and therefore they are not homologous. Using a variety of approaches that include analysing vasculature, surgically over expressing inhibitors for major signaling pathways in development (e.g. the Hedgehog and TGF ß families), gene expression, and cell tracking, we have gained significant insight into how the sclerotic ring develops, how it is constrained in development, and how variation arises. This multi-faceted approach has led to major advances in our understanding of the evo-devo of this intriguing skeletal element. This research was funded by the Natural Sciences and Engineering Research Council of Canada.

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