Drosophila blood cells, called hemocytes, are classified into plasmatocytes, crystal
cells, and lamellocytes based on the expression of a few marker genes and cell morphologies,
which are inadequate to classify the complete hemocyte repertoire. Here, we used single-cell
RNA sequencing (scRNA-seq) to map hemocytes across different inflammatory conditions
in larvae. We resolved plasmatocytes into different states based on the expression
of genes involved in cell cycle, antimicrobial response, and metabolism together with
the identification of intermediate states. Further, we discovered rare subsets within
crystal cells and lamellocytes that express fibroblast growth factor (FGF) ligand
branchless and receptor breathless, respectively. We demonstrate that these FGF components
are required for mediating effective immune responses against parasitoid wasp eggs,
highlighting a novel role for FGF signaling in inter-hemocyte crosstalk. Our scRNA-seq
analysis reveals the diversity of hemocytes and provides a rich resource of gene expression
profiles for a systems-level understanding of their functions.
