| dc.description.abstracteng | Brains are the central organ integrating sensory information and orchestrating behaviour. Their function and development are of great scientific interest, but their enormous complexity makes them difficult to study. The reduced size and complexity of insect brains compared to mammal brains, together with the extraordinary experimental toolkits, make insect model organisms an excellent study case. Insect brains have a conserved structure of functional subunits, the neuropils, but at the same time, strong divergence across the large number of species is observed. These aspects offer the opportunity to study the developmental processes responsible for evolutionary divergence. The central complex (CX) is a very conserved neuropil responsible for navigation by orientation and motor control. A curious developmental divergence between insect species is a shift in developmental timing, so-called heterochrony: some species have a CX in the larva while others develop it during metamorphosis. It has remained enigmatic, how homologous cells develop divergently in different species to realise these evolutionary changes.
The conserved transcription factor (TF) retinal homeobox (rx) is expressed in the anterior brain of many animals and involved in brain development. In insects, Rx-positive cells contribute to the formation of the CX. In this study, I applied the concept of a genetic neural lineage (GNL) to study insect brains. A GNL includes all neurons expressing a certain developmental regulatory gene and is likely homologous between species, particularly GNLs of conserved TFs like rx. I genetically labelled the rx GNL in the beetle Tribolium castaneum using CRISPR/Cas9-mediated knock-in of the reporter GFP, as was previously done in Drosophila melanogaster. Using the two transgenic lines, I was able to compare Rx-positive cells of different developmental stages between the species, revealing conservation and divergence of the rx GNL. Cells were analysed in terms of number, cell body position, projection pattern and neurotransmitter content. Based on these factors, I defined expression domains and cell clusters and assessed conservation and divergence of this basically homologous cell population. The rx GNLs showed strong association with the CX, with most projections entering the upper unit of the central body and the noduli. Additionally, there is association with the lateral accessory lobe as well as involvement in the neuroendocrine system of the pars intercerebralis. I found several cases of evolutionary divergence in the rx GNLs between the species, including additional cell clusters, organisation of cell clusters in different arrangements, different numbers of cells and diverged roles in mushroom body development. I found that most Rx-positive cells do not express the neurotransmitters GABA, serotonin or dopamine, nonetheless, these data provided an additional framework for homologising cell clusters. Furthermore, I compared the brain volume of larval and adult brains of Drosophila melanogaster and Tribolium castaneum and discovered a set of serotonergic tangential cells of the lower unit of the central body in Tribolium castaneum, which was not described before. This work reveals interesting divergences of the insect brain, which now allows studying the developmental underpinnings of evolutionary divergence. Further, it shows the potential of GNLs as a powerful tool for evolutionary developmental biology. Expanding this approach to include more species could provide more insights into the evolution and development of brains. | de |