A single-cell survey of Drosophila blood
Mechanisms controlling hematopoiesis are highly conserved in the fruit fly Drosophila. However, unlike in higher mammals, the fly blood system is composed of only three distinct hemocyte populations: 1) plasmatocytes (PM) 2) crystal cells (CC) and 3) lamellocytes (LM). These cell types have been extensively characterized based on the expression of a few marker genes and cellular morphology, which are inadequate to classify the complete hemocyte repertoire. In this study, we used single-cell RNA sequencing (scRNA-seq) to map the Drosophila larval blood cell types and their diverse states across various inflammatory conditions such as wounding and wasp infestation. Our scRNA-seq identified various states within PMs (PM1-12), CCs (CC1-2) and LMs (LM1-2) [BOX 1]. Using Monocle-3 based pseudotemporal ordering of cells, we propose that a decrease in the expression of cell cycle genes such as Polo as a potential mechanism of transdifferentiation [BOX 2]. Further, our scRNA-seq identified emergence of activated states of PMs (PM6-7) that enriched in several genes encoding antimicrobial peptides (AMP). Importantly, we discovered a novel AMP, CG43236, which is closely related to Metchnikowin (Mtk) [BOX 3]. Finally, we report that the FGF ligand branchless (bnl) and its receptor breathless (btl) are expressed in rare subsets of CCs and LMs, 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 [BOX 4]. Together, our scRNA-seq analysis reveals the diversity of Drosophila blood cell populations and provides a rich resource of gene expression profiles for a systems-level understanding of their functions.