| dc.description.abstracteng | How morphological traits originate and diversify is a central question in evolutionary biology. Insects are the most diverse group of animals on the planet and over 80% of insect species belong to the subgroup of holometabola. The shape of a holometabolous insect experiences a striking change during metamorphosis, which allowed the evolution of an overwhelming morphological diversity. Hence, this process provides excellent samples to study the evolution of morphological innovation and diversity. Among insects, the developmental and genetic mechanisms of epidermal patterning are well understood in the model organism, Drosophila melanogaster. However, this highly derived Dipteran species does not show a typical metamorphosis. Drosophila replaces all larval epidermal cells by imaginal cells to form the adult epidermis. Instead, most holometabolous insects re-use larval cells to generate the adult epidermis, In contrast to Drosophila, the red flour beetle, Tribolium castaneum, shows a more typical mode of metamorphosis. Importantly, unbiased large scale RNA interference screening (iBeetle-screen) in Tribolium allows identifying and investigating gene sets involved in the process of morphological innovation and diversification independently from Drosophila knowledge.
In the first part of this thesis, the gin-trap was used as a study case to explore how a morphologically novel structure evolved during metamorphosis in Tribolium. Firstly, the wing genes known from Drosophila were investigated for their potential functions in gin-trap formation. The results showed that a large part of the upstream genes but much few downstream genes of the wing gene network were co-opted into gin-trap formation. Secondly, novel genes required for gin-trap development were searched in the iBeetle database. Ten genes were confirmed for their functions in gin-trap formation, most of which were required for wing formation as well. The only gin-trap specific gene, Tc-caspar, which was recruited from another biological context, was required for establishment of the anterior-posterior symmetry of the gin-traps. This is an innovation to this structure. Taken together, these data suggested that gin-traps evolved by co-option of a pruned wing gene regulatory network and a low level of gene recruitment from a distinct biological context.
In the second part, novel genes from iBeetle screen were identified and analyzed on antenna metamorphosis in Tribolium. Of the ten confirmed genes, half belonged to the new classifications which were not reported to be associated with antenna patterning in Drosophila. Interestingly, four genes were related to pre-mRNA splicing, indicating the potential role of this process for antenna remodeling. One taxonomically restricted gene was found to affect a specific region of the antenna. And then, I optimized a protocol for whole mount in situ hybridization of pre-pupal antennae and the expression patterns of novel genes showed that the expression patterns were consistent with a role of these genes in antenna remodeling. Finally, I compared the gene sets between antenna and leg development and verified a complex mix of divergence and constraint among these serially homologous appendages.
The data obtained in this thesis provide new insight into the morphological innovation and diversification during metamorphosis and are the basis for future studies. | de |