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Session Schedule & Abstracts

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Saturday 2nd July, 2016

Symposium: Mechanisms of whole dentition patterning in extant and extinct amniotes 1

Room: Salon B   9:30 am–11:00 am

Moderator(s): J. Richman, L. Hlusko, & T. Grieco
DEN1-1  9:30 am  Asymmetry and developmental integration in the replacing leopard gecko dentition (Squamata: Eublepharis macularius) provide evidence for in ovo jaw patterning maintained throughout life. Grieco TM*, University of British Columbia; Richman JM, University of British Columbia
Abstract: Jaw-wide control over tooth shedding may be necessary in many animals to preserve the integrity of the functional tooth row. Reptilian teeth are initiated de novo throughout life, but it is unclear whether tooth replacement patterns are dependent on conditions established in ovo. We used the leopard gecko as a model to study the patterns and synchrony of dental replacement over space and time. We first determined the characteristics of the dentition present at hatching, prior to function. Micro-CT scans revealed a high degree of symmetry in erupted and unerupted teeth between the right and left jaw halves. To test the subsequent degree of independence between tooth positions, we studied the ontogeny of shedding asymmetry during two periods in 10 post-hatching geckos: an early period where the dentition was entirely comprised of teeth initiated in ovo, and a much later time where teeth were initiated after hatching. We analyzed data from wax bite impressions by tooth position, scoring the teeth that were shed either unilaterally (asymmetric) or bilaterally (symmetric). Two distinct motifs of asymmetric shedding were also observed. The majority of animals displayed a time lag of 3-4 days between right and left tooth shedding. The other group of animals had a position difference such that teeth at consecutive positions on right and left sides were lost. These motifs were strongly propagated in the early and late periods, and the percentage asymmetry remained constant between time periods for all animals. Large regions of the posterior maxilla are coordinated in the right-left polarity of their asymmetry, refuting the null hypothesis of shedding event independence along the jaw. This polarity bias is antisymmetric at the cohort level, and is never observed to flip polarity over time. Taken together, these patterns favor the hypothesis of a tooth cycling process patterned during embryonic development and extremely robust to environmental and developmental perturbation.

DEN1-2  10:00 am  Early dental development and the origins of toothlessness in amniotes. Lainoff AJ*, University of California, San Francisco; Moustakas-Verho JE, University of Helsinki; Hu D, UC San Francisco; Kallonen A, University of Helsinki; Marcucio RS, University of California, San Francisco; Hlusko LJ, University of California, Berkeley
Abstract: A key question at the intersection of evolutionary and developmental biology is how complex structures such as teeth arise; identifying developmental discrepancies between taxa with and without these structures can help address this. Although BMP4 and FGF8 antagonistically initiate tooth development in mice, it is largely unknown whether odontogenesis is initiated the same way in other amniotes. Moreover, changes in BMP4-signaling have previously been implicated in evolutionary tooth loss in Aves. Here we demonstrate that Bmp4, Msx1, and Msx2 expression is limited proximally in the red-eared slider turtle (Trachemys scripta) mandible at stages equivalent to those at which odontogenesis is initiated in mice, similarly to previously reported results in chicks. To address whether the limited domains in the turtle and the chicken indicate an evolutionary molecular parallelism or simply an ancestral phenotype, we assessed gene expression in a toothed reptile (the American alligator, Alligator mississippiensis) and a toothed non-placental mammal (the gray short-tailed opossum, Monodelphis domestica). We demonstrate that the Bmp4 domain is limited proximally in M.domestica and that the Fgf8 domain is limited distally in A.mississippiensis just preceding odontogenesis. Further, we show that Msx1 and Msx2 expression patterns in these species differ from those found in mice. Our data suggest that a limited Bmp4 domain does not necessarily correlate with edentulism, and reveal that the initiation of odontogenesis in non-murine amniotes is more complex than previously imagined. Our data also suggest a partially conserved odontogenic program in T.scripta, as indicated by Pitx2, Pax9, and Barx1 expression and the presence of a Shh-expressing palatal epithelium, which we hypothesize may represent potential dental rudiments based on the fossil record.

DEN1-3  10:15 am  Crocodiles as perfect models to investigate the mechanisms of continuous dental replacement as it functioned in mammalian ancestors. Thivichon-Prince BTP*, IGFL, UMRCNRS 5242, ENS, University Lyon1, HCL, France; Bertin BT, IGFL, UMRCNRS 5242, ENS, University Lyon1,France; Tafforeau PT, European Synchrotron Radiation Facility, Grenoble, France; Richman JR, Life Sciences Institute, UBC, Vancouver, Canada; Viriot LV, IGFL, UMRCNRS 5242, ENS, University Lyon1,France
Abstract: As evidenced by the fossil record, toothed vertebrates originally were polyphyodont, which means that their dentition was continuously replaced throughout life. Although polyphyodonty still prevails in most extant non-mammalian toothed vertebrates, various reductive types emerged convergently a high number of times over evolution. The most emblematic trend in the reduction of dental generation number can be traced along the transition from pre-mammalian to mammalian synapsids, during which the continuous dental replacement slowed down into a maximum of two dental generations throughout life (diphyodonty). Since pre-mammalian synapsids do not have any present descendants, we selected crocodiles as the best living proxy to evaluate the situation of the dentition in pre-mammalian synapsids. Crocodiles have three characteristics that make them suitable for experimental studies: (1) Crocodiles are polyphyodont; (2) embryos can be procured on a seasonal basis from captive colonies; and (3) crocodiles are the only living non-mammalian tetrapods with thecodont dentitions. Thus tooth attachment in crocodiles is similar to mammals in that their teeth are anchored into bony sockets by a periodontal ligament. Using various techniques (histology, 3D X-ray microtomography and in situ hybridization) we characterized the pattern of dental replacement in Nile Crocodiles from La Ferme aux Crocodiles de Pierrelatte (France). Especially, Synchrotron tomography allowed us to 3D reconstruct spatial relationships of different dental generations, even at cellular level and revealed left-right symmetric tooth replacement pattern. These results suggest a spatio-temporal dynamic control. We discovered temporally and spatially restricted expression of Notch-Delta signaling molecules and target genes. These patterns suggest that a subset of Notch ligands is active during tooth development and replacement.

DEN1-4  10:30 am  Stem cells and molecular circuits in alligator tooth renewal. Wu P*, Department of Pathology, Keck School of Medicine, University of Southern California, Los Angeles, CA 90033; Tsai S, Department of Pathology, Keck School of Medicine, University of Southern California, Los Angeles, CA 90033; Abdelhamid A, Qassim College of Dentistry, Qassim University, Saudi Arabia; Widelitz R, Department of Pathology, Keck School of Medicine, University of Southern California, Los Angeles, CA 90033; Chuong CM, Department of Pathology, Keck School of Medicine, University of Southern California, Los Angeles, CA 90033
Abstract: Alligators have robust regenerative powers for tooth renewal. In contrast, extant mammals can either renew their teeth once (diphyodont dentition, such as in humans) or not at all (monophyodont dentition, such as in mice). Previously, we used multiple mitotic labeling to map putative stem cells in alligator dental laminae which contain quiescent odontogenic progenitors (Wu et al., 2013. Proc Natl Acad Sci U S A. 110:E2009-18). We demonstrated that tooth cycle initiation is related to β-catenin/Wnt pathway activity in the dental lamina bulge. Here we use transcriptome analysis to examine the molecular pathway related to the tooth renewal process. We collected juvenile alligator dental lamina at different cycling stages and performed RNA-sequencing. Our data shows that several pathways, such as Wnt, BMP, FGF and ECM/MMP pathways are activated at the transition from the pre-initiation stage to the initiation stage. In addition, we identified the molecular circuitry among different stages of tooth cycling. The Wnt pathway may play the most important role in the tooth cycling, accompanied by other moleculer pathways. We conclude that multiple pathways are involved in the molecular circuitry regulating tooth cycling. This result opens a possibility to apply this knowledge to mammalian tooth renewal.

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