On the origins of heterodonty in elasmobranchs
Bioinformatics and Theoretical Biology
Developmental Biology
Evolution, Anthropology
Final Report Abstract
The DFG-funded research project “Über den Ursprung der Heterodontie in Haien und Rochen“ was conducted in 2020-21 for the duration of two years at the IGFL, Lyon. This project aimed at documenting diversity and patterns of tooth shape variation in elasmobranchs (sharks and rays) and at elucidating the underlying developmental mechanisms, taking advantage of morphometric and in silico modelling approaches. Two different model frameworks were used, one representing the folding of the epithelial-mesenchymal interface of the developing tooth, the other representing developing dental tissues at a quasi-cellular resolution. In collaboration with an experimental research group, we proposed a hypothesis about how teeth develop in sharks, including conserved commonalities with the better investigated mammalian tooth development, as well as elasmobranch-specific mechanisms. I implemented our hypothesis into a mathematical model and successfully demonstrated that it is capable of reproducing a large part of tooth shape diversity seen in catsharks. By exploring further this model, I associated specific developmental mechanisms with specific tooth shape transitions. However, I also found that there are always multiple, asymmetric, ways to achieve the same morphological changes, suggesting a relatively unconstrained pool of alternative paths for convergent evolution. Second, I found within-individual tooth shape diversity to be common across sharks. Using a rich data set of shark dentitions, I identified common patterns of how tooth morphologies tend to vary along and between the jaws. While some of these patterns turned out to reflect phylogenetic position, others tended to be general. By comparing these patterns with a set of mammalian dentitions, I could trace some of these patterns even further, suggesting some of those to represent specific biases specific to tooth development in vertebrates. I also compared these patterns to the consequences of modification of some developmental mechanisms in silico, which suggested a causal link to certain developmental features of tooth development. Third, I correlated variation in tissue geometry at the onset of tooth development with tooth crown shape variation. This suggests that the spatial initial conditions may harbor a yet under-appreciated potential for generating variation. Thus, this latter finding may have specific importance for the wider theory of development of biological structures, but has to be analyzed in different, and more general, systems. Together with international collaborators, I participated in establishing a novel framework for comprehensively connecting diverse phenomena in evo-devo, based on the idea of independence, but causal equivalence, of genetic, environmental, and material-structural factors that, together, build phenotypes via unfolding developmental dynamics. While the scope and detail of this project was constrained by the limited possibilities of testing hypotheses experimentally, it successfully explored the morphogenetic potential of an explicit model of shark tooth development, discovered therein pervasive patterns of shape variation, some with potential importance throughout vertebrates, and linked these to developmental mechanisms. Thus, the project contributed valuably to both tooth development and general evo-devo theory and may have potential to instigate novel research questions and avenues in the future.
