Evolutionary developmental biology (Evo-Devo) attempts to trace modifications in development that have lead to evolutionary novelty. Some important concepts in Evo-Devo are GENE DUPLICATION, SUBFUNCTIONALIZATION, MODULARITY, CO-OPTION, CIS-REGULATORY EVOLUTION, GENE NETWORKS and HUBS. This thesis consists of two case studies on genes in the beetle Tribolium castaneum which is thought to represent a more ancestral mode of development than the well-studied fruit fly Drosophila melanogaster. In the first case study, the expression and function of the two zerknüllt genes (Tc-zen1 and Tc-zen2) were investigated. Drosophila-zen is, initially under control of Dorsal, expressed at the dorsal side and specifies a single extraembryonic membrane, the amnioserosa. Tribolium possesses two extraembryonic membranes: an outer serosa and an inner amnion. Tc-zen1 displays early, anterior expression and specifies the serosa. Tc-zen1 knock-down enlarges the germ rudiment and deletes the serosa, but the remaining dorsal amnion allows relatively normal further development. Thus, in absence of Tc-zen1, Tribolium acquires features of Drosophila development. Tc-zen2 is expressed later and mediates the amnion-serosa fusion necessary for dorsal closure. Tc-zen2 depletion prevents this fusion and leads to completely everted (inside-out) larvae. In the second case study, Decapentaplegic (Dpp, a BMP ligand) and Short gastrulation (Sog, a BMP inhibitor) were examined. In Drosophila and vertebrates, BMP signaling plays a major role in dorsoventral patterning. In contrast to Drosophila, where Dm-dpp expression is restricted to the dorsal side by Dorsal, Tribolium-dpp shows differential expression along the anteroposterior axis. However, Tc-Sog is expressed in a ventral domain and establishes a dorsoventral Dpp activity gradient by transporting Dpp towards the dorsal side. Tc-sog RNAi abolishes neurogenesis and normal dorsoventral polarity in the ectoderm. Tc-dpp RNAi leads to the loss of dorsal cell fates. Surprisingly, similar to vertebrates but in contrast to Drosophila, Tribolium BMP knock-down enlarges the head, while knock-down of BMP antagonism deletes the head. Possibly, Sog/Chordin was already CO-OPTED for head formation in an ancestor of all bilateria. Besides CO-OPTION, the evolutionary implications of these case studies illustrate other concepts of Evo-Devo. For example, the DUPLICATION of zen in an ancestor of Tribolium was followed by SUBFUNCTIONALIZATION. Dpp belongs to a signaling MODULE that is repeatedly employed in different contexts. The CIS-REGULATORY SEQUENCES of dpp and zen acquired binding sites for Dorsal which became a HUB in the dorsoventral regulatory GENE NETWORK of Drosophila
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