Year: 2019
Type: dissertation
Abstract: The turtle carapace has been considered one of the classic examples of an evolutionary novelty. It is diagnostic of the turtle clade and probably largely responsible for its success in evolution. In more detail, it consists of ossified dermal plates overlaying laterally extended ribs, as well as a regular array of epidermal scutes. Interestingly, the unique and complex mosaic pattern formed by the carapacial scutes is highly conserved amongst almost all extant turtle taxa. Since the morphogenesis of this remarkable pattern has not been studied much before, I present, in this dissertation, a novel computational approach to elucidating carapacial scute development. In a mathematical model, a combination of two mechanistically different reaction-diffusion systems together with growth reproduces scute formation in the carapace. In order to disentangle the underlying gene regulatory network, we disturb carapace development experimentally in embryos of Trachemys scripta and use the model to understand the resulting phenotypes. We show how the carapacial ridge acts as a signaling center which is required for the onset of scute development. Furthermore, the model suggests how phenotypic differences between species might arise and explains, in line with statistical data from different turtle species, how environmental stress, rather than genetic mutations, leads to the very specific patterns of variation found within turtle populations. Thus, our results suggest that environmental stress might be an underrated source of phenotypic variation within species. Developmental insight helps us understand why this particular variation occurs and at what frequencies. In order to further understand how environmental factors might interfere with development and to be able to account for more realistic tissue growth and biomechanics, I present a more general multiscale model of animal development that integrates biomechanics, GRN dynamics and cell behaviours in different, interacting cell types and tissues. Thus, this model does not only allow an advancement of our understanding of mechanisms by which phenotypic variation in the developing turtle scute might be generated, it is also a useful tool to understand more general features of development and variation. Finally, I discuss my work in the context of previous hypotheses about the origins of turtle scutes as well as their variation. By a comparison with the development of other ectodermal organs in different vertebrates, I try to integrate turtle scutes into a wider evolutionary background.
Source: Työväentutkimus Vuosikirja
Author: Roland Zimm
Institution: University of Helsinki
Language: English
Subfield: Molecular Biology
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W2945765326