Conservation and evolutionary modifications of neuroblast expression patterns in insects.

One of the major questions in evolutionary developmental neurobiology is how neuronal networks have been adapted to different morphologies and behaviour during evolution. Analyses of neurogenesis in representatives of all arthropod species have revealed evolutionary modifications of various developmental mechanisms. Among others, variations can be seen in mechanisms that are associated with changes in neural progenitor identity, which in turn determines the neuronal subtype of their progeny. Comparative analyses of the molecular processes that underlie the generation of neuronal identity might therefore uncover the steps of evolutionary changes that eventually resulted in modifications in neuronal networks. Here we address this question in the flour beetle Tribolium castaneum by analyzing and comparing the development and expression profile of neural stem cells (neuroblasts) to the published neuroblast map of the fruit fly Drosophila melanogaster. We show that substantial changes in the identity of neuroblasts have occurred during insect evolution. In almost all neuroblasts the relative positions in the ventral hemi-neuromeres are conserved; however, in over half of the neuroblasts the time of formation as well as the gene expression profile has changed. The neuroblast map presented here can be used for future comparative studies on individual neuroblast lineages in D. melanogaster and T. castaneum and additional markers and information on lineages can be added. Our data suggest that evolutionary changes in the expression profile of individual neuroblasts might have contributed to the evolution of neural diversity and subsequently to changes in neuronal networks in arthropod.