A Drosophila glial cell atlas reveals that transcriptionally defined cell types can be morphologically diverse

Abstract

AbstractMorphology is a key defining feature of neuronal identity. Like neurons, glia display diverse morphologies, both across and within glial classes. In the Drosophila central nervous system, glia are categorized into five main classes (outer and inner surface glia, cortex glia, ensheathing glia, and astrocytes), which also show within-class morphological diversity (morphotypes). Whether morphological differences reflect underlying transcriptional heterogeneity is unclear. We analysed and validated single cell RNA sequencing data of Drosophila glia in two well-characterized tissues from distinct developmental stages, containing distinct circuit types: the embryonic ventral nerve cord (motor) and the adult optic lobes (sensory). Our analysis identified a new morphologically and transcriptionally distinct surface glial population in the ventral nerve cord. However, many glial morphotypes could not be distinguished transcriptionally, and indeed, embryonic and adult astrocytes were transcriptionally analogous despite differences in developmental stage and circuit type. While we did detect extensive within-class transcriptomic diversity for optic lobe glia, this could be explained entirely by glial residence in the most superficial neuropil (lamina) and an associated enrichment for immune-related functions. In summary, we generated the first single-cell transcriptomic atlas of glia in Drosophila, and our extensive in vivo validation suggests that morphology is not set by an intrinsic transcriptional program. Instead, we propose that glia adopt morphological and functional states in response to their local environment. This atlas will serve as a resource for the community to probe glial diversity and function.