A single-cell survey of Drosophila blood

Drosophila blood cells, called the hemocytes, are composed of three distinct cell populations: plasmatocytes (PM), crystal cells (CC), and lamellocytes (LM). These cell types have been extensively characterized based on the expression of a few marker genes and cell morphologies, which are inadequate to classify the complete hemocyte repertoire. Further, intermediate states along the course of maturation of these cell types have been poorly characterized. Here, we used single-cell RNA sequencing (scRNA-seq) to map the Drosophila larval hemocytes across different inflammatory conditions. We identified all known cell types, resolved PMs into different states based on the expression of genes involved in cell cycle, antimicrobial response, and metabolism together with the detection of CC and LM intermediates. Further, we discovered rare cells within CC and LM populations that express the fibroblast growth factor (FGF) ligand branchless (bnl) and receptor breathless (btl), respectively. We demonstrate that both bnl and btl are required for mediating effective immune responses against parasitoid wasp eggs in vivo, highlighting a novel role for FGF signaling in inter-hemocyte crosstalk. Our scRNA-seq analysis reveals the diversity of hemocyte populations and provides a rich resource of gene expression profiles for a systems-level understanding of their functions. inDrops, 10x Genomics Chromium, and Drop-seq based single-cell RNA-sequencing of ~20K hemocytes from the Drosophila larvae across different inflammatory conditions