Figure 1.

Haematopoietic reaction to wasp infection in Drosophila larvae. When a parasitoid wasp lays an egg in a larva, it is detected by circulating blood cells (plasmatocytes), which probably signal to the haematopoietic tissues, including the lymph gland. This inhibits the JAK–STAT pathway in the precursor cells (prohaemocytes) of the medullary zone of the lymph gland, licencing them to exit the undifferentiated state. It also increases ROS levels in the PSC, inducing the transdifferentiation of haemocoelic plasmatocytes into lamellocytes and the differentiation of prohaemocytes to lamellocytes. A cue emitted by the PSC, proposed to be the EGFR ligand Spitz, is needed for circulating cells to adopt the lamellocyte fate.