The insect brain is specified by a conserved transcriptional program distinct from the segmental CNS

Abstract

The brain is the central processing center that integrates information to guide animal behavior, while the posterior CNS performs less central functions. To what degree this functional distinction is reflected in divergence of the genetic regulation remains unexplored, as well as the hypothesis of a distinct evolutionary origin of brain and trunk CNS. In insects, neuroblast biology is similar across the CNS, but the presence of brain-specific neuroblast types and differences in the gene sets involved in their spatial and temporal specification indicate divergence. However, a comprehensive and unbiased comparison of the regulatory code of brain versus trunk neuroblasts has been lacking. Here, we reveal the transcription factors expressed in insect embryonic neuroblasts by a combined single-cell analysis from the fly Drosophila melanogaster and the beetle Tribolium castaneum, for which we established a Gal4 enhancer trap system to generate a line marking NBs with tGFP. From the combined dataset of nearly 70,000 neuroblasts, we identified around 23,000 brain neuroblasts, from which we identified cell populations and their transcriptional signatures. We found 188 brain-specific TFs, including seven spatially restricted transcription factors not previously associated with neuroblasts (Hmx, CG15696, CG32532, dmrt99B, fD59A, TfAP-2, and Fer1). In addition to the expected canonical Type II NBs, we found two related clusters with intriguing transcriptional divergence. The large number of brain-specific TFs supports a profound divergence between the insect brain and posterior CNS. Together with recent work in vertebrates, our data confirm an ancient divergence of the brain from the trunk.