How do neuroblasts aquire their identity?

The insect brain forms from neural stem cells, the neuroblasts. The process leading to the selection of neuroblasts from neuroectoderm is well understood. Also how neuroblasts in the ventral nerve cord acquire their identity is described. It is not known, however, what makes brain neuroblasts different from each other.

In Drosophila, the brain is built by about 200.000 neurons that arise from about 100 neural stem cells (neuroblasts, NBs) in each hemisphere. Before and/or shortly after delamination each NB can be identified by a unique code of expressed transcription factors and this code is likely to direct their developmental fate. The offspring of a given NB follow an invariant developmental path that is different for each NB. Therefore, the early determinants and signals that provide NBs with distinct identities are essential and instructive for the formation of a functional brain. These determinants and signals are largely unknown but are different from those in the trunk.

Our aim is the identification of the early determinants and signals that give NBs their unique identity. For this, we use the red flour beetle Tribolium castaneum as a model system, because: 1) As most genes expressed in NBs are also required earlier in development, the late neural phenotype is superimposed and blurred by dramatic earlier defects in classical mutants. However, the strong systemic RNA interference (RNAi) in Tribolium allows us to knock out genes at later stages without compromising their early functions. 2) The central complex is not visible in Drosophila L1 larvae but it builds a discernible structure in Tribolium L1 larvae. Therefore, we focus on this brain part. 3) To identify conserved determinants and signals for NB identity in insects, comparative data from different species are required. The number of NBs in the Drosophila brain is smaller than in most other insects and the involution of its head leads to a derived head morphology. Tribolium provides a situation more typical for insects.

To adress this question, we are developing imaging systems for the brain of L1 larvae, describe the brain structures and identify regions missing in RNAi knock-downs. Genes expressed in the neuroectoderm potentially constitute the signals for NB identity specification. By knocking down these genes by RNAi we try to understand the signals that make one NB different from the other and to understand the consequences on brain architecture.

© Gregor Bucher, last update: july 07