Early neurogenesis in the flour beetle Tribolium castaneum
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Insects exhibit considerable variety in their morphology and can be found in many diverse habitats. Despite these variations, early neurogenesis seems to be conserved in insects. In all species investigated to date it begins with the formation of neural stem cells (neuroblasts), which establish a distinct internal layer and produce a fixed number of neurons and glial cells. The neuronal cells then form a characteristic rope ladder-like axonal scaffold. However, it is evident that the composition or identity of the individual neurons must have changed during insect evolution to allow for variations in neuronal networks. This raises questions regarding which developmental steps have been changed and the manner in which they have been modified. In order to address these questions, early neurogenesis was analysed in the flour beetle Tribolium castaneum and the results were compared to the well-studied fruit fly Drosophila melanogaster. Initially a map of trunk neuroblasts in T. castaneum was established, which revealed a high degree of conservation in the arrangement of individual neuroblasts compared to D. melanogaster. However, a comparison of the expression patterns of genes that confer regional identity to neuroblasts showed considerable variations. Significant differences in the expression patterns of the segment polarity gene wingless and the columnar gene ventral nerve cord defective (vnd) were found. Furthermore, the impact these changes in neuroblast identity have on the composition and identity of their respective progeny was analysed. As a result changes in the number of Even-skipped and Tailup expressing neurons in T. castaneum embryos were found, with three-fold more Tailup expressing neurons compared to D. melanogaster. To further analyse the role of the neuroblast identity gene vnd in the formation of Even-skipped positive neurons, RNAi gene silencing studies were performed, resulting in the loss of neurons and changes in neuronal migration pattern. In summary, the results demonstrate that evolutionary changes in neuronal networks result from changes in neuroblast identity, which in turn have an impact on the composition of neuronal lineages.
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