A comprehensive temporal patterning gene network controls developmental timing in Drosophila medulla neuroblasts
Authors: Hailun Zhu 1; Sihai Dave Zhao 2; Alokananda Ray 1; Yu Zhang 1; Xin Li 1
Affiliations: 1) University of Illinois at Urbana-Champaign, Department of Cell and Developmental Biology, Urbana, IL; 2) University of Illinois at Urbana-Champaign, Department of Statistics, Urbana, IL
Keywords: o. stem cells; n. networks
During development, neural progenitors are temporally patterned to sequentially generate distinct neural types. Previous studies showed that sequential expression of five temporal transcription factors (TTFs), Homothorax (Hth), Eyeless (Ey), Sloppy paired (Slp), Dichaete (D) and Tailless(Tll), in medulla neuroblasts (NBs) of Drosophila larval brain is necessary for generating the full spectrum of neurons in a defined order. However, these TTFs identified through candidate antibody screening may not compose the complete TTF sequence, and several gaps remained unfilled concerning the initiation, termination and regulation of this temporal cascade. To address these questions, we applied single cell RNA sequencing (scRNA-seq) to our model system to discover all unknown TTFs and additional determinants, as well as to get a global view of the dynamic temporal patterning process of medulla neuroblasts. After analyzing the scRNA-seq data of medulla neuroblasts, two sets of genes showed high to low or low to high gradients of expression as neuroblasts age. With known TTFs as markers, we identified a list of novel TTF candidates, among which, SoxNeuro (SoxN), doublesex-Mab related 99B (Dmrt99B), Odd paired (Opa), Earmuff (Erm), Scarecrow (Scro), BarH1, BarH2 and Glial cells missing (Gcm) are necessary for medulla temporal patterning. We identified extensive cross-regulations among these novel TTFs and known TTFs, that generally follow the rule that a previous TTF is required to activate a later TTF, while a later TTF would repress a previous TTF. Our study revealed a comprehensive temporal patterning cascade: Hth + SoxN + dmrt99B -> Opa -> Ey+Erm -> Ey+Opa -> Slp+Scro -> D -> B-H1&2->Tll, Gcm, which controls the sequential generation of different neural types by regulating the expression of specific neuronal TFs. With Dmrt99B, Opa and Gcm discovered as TTFs, the mechanisms for the initiation and termination of the temporal cascade were uncovered. Moreover, in pursuit of the mechanism behind the regulation of the temporal cascade, we found that the initiation, progression and termination of the TTF temporal cascade require genes differentially expressed along the differentiation axis (NBs -> -> neurons) including Lola and Nerfin-1, providing clues as to why the temporal progression only happens in neuroblasts but not their differentiated progeny. In summary, our study revealed a comprehensive temporal gene network that controls developmental timing in neural progenitors.