Transcriptional complexity in the insect central complex: single nuclei RNA sequencing of adult brain neurons derived from type 2 neuroblasts

Abstract

AbstractIn both invertebrates such asDrosophilaand vertebrates such as mouse or human, the brain contains the most diverse population of cell types of any tissue. It is generally accepted that transcriptional diversity is an early step in generating neuronal and glial diversity, followed by the establishment of a unique gene expression profile that determines neuron morphology, connectivity, and function. InDrosophila, there are two types of neural stem cells, called Type 1 (T1) neuroblasts and Type 2 neuroblasts (T2). In contrast to T1 neuroblasts, T2 neuroblasts generate Intermediate Neural Progenitors (INPs) that expand the number and diversity of neurons. The diversity of T2 neuroblast-derived neurons contributes a large portion of the central complex (CX), a conserved brain region that plays a role in sensorimotor integration, including celestial navigation. Recent work has revealed much of the connectome of the CX, but how this connectome is assembled remains unclear. Mapping the transcriptional diversity of T2 neuroblast-derived neurons, including those projecting to the CX, is a necessary step in linking transcriptional profile to the assembly of the connectome. Here we use single nuclei RNA sequencing of T2 neuroblast- derived adult neurons to identify over 150 distinct cell clusters. We map neurotransmitter and neuropeptide expression and identify unique transcription factor combinatorial codes for each cluster (presumptive neuron subtype). This is a necessary step that directs functional studies to determine whether each transcription factor combinatorial code specifies a distinct neuron type within the central complex. We map several well- characterized columnar neuron subtypes to distinct clusters, and two neuronal classes map to a single cluster. Our data support the hypothesis that each cluster represents a one or a few closely related neuron classes.