Evolutionary variations in the expression of dorso-ventral patterning genes and the conservation of pioneer neurons in Tribolium castaneum

Insects are ideally suited for gaining insight into the evolutionary developmental mechanisms that have led to adaptive changes of the nervous system since the specific structure of the nervous system can be directly linked to the neural stem cell (neuroblast) lineages, which in turn can be traced back to the last common ancestor of insects. The recent comparative analysis of the Drosophila melanogaster and Tribolium castaneum neuroblast maps revealed substantial differences in the expression profiles of neuroblasts. Here we show that despite the overall conservation of the dorso-ventral expression domains of muscle segment homeobox, intermediate neuroblasts defective and ventral nervous system defective, the expression of these genes relative to the neuroblasts in the respective domains has changed considerably during insect evolution. Furthermore, functional studies show evolutionary changes in the requirement of ventral nervous system defective in the formation of neuroblast 1-1 and the correct differentiation of its presumptive progeny, the pioneer neurons aCC and pCC. The inclusion of the expression data of the dorso-ventral genes into the recently established T. castaneum neuroblast map further increases the differences in the neuroblast expression profiles between D. melanogaster and T. castaneum. Despite these molecular variations, the Even-skipped positive pioneer neurons show an invariant arrangement, except for an additional Even-skipped positive cluster that we discovered in T. castaneum. Given the importance of these pioneer neurons in establishing the intersegmental nerves and the longitudinal tracts, which are part of the conserved axonal scaffold of arthropods, we discuss internal buffering mechanisms that might ensure that neuroblast lineages invariantly generate pioneer neurons over a wide range of molecular variations.