The Drosophila lymph gland (LG) is a model system for studying hematopoiesis and blood cell homeostasis. Here, we investigated the patterns of division and differentiation of pro-hemocytes in normal developmental conditions and response to wasp parasitism, by combining lineage analyses and molecular markers for each of the three hemocyte types. Our results show that the embryonic LG contains primordial hematopoietic cells which actively divide to give rise to a pool of pro-hemocytes. We found no evidence for the existence of bona fide stem cells and rather suggest that Drosophila pro-hemocytes are regulated as a group of cells, rather than individual stem cells. The fate-restriction of plasmatocyte and crystal cell progenitors occurs between the end of embryogenesis and the end of the first larval instar, while Notch activity is required for the differentiation of crystal cells in third instar larvae only. Upon parasitism, lamellocyte differentiation prevents crystal cell differentiation and lowers plasmatocyte production. We also found that a new population of intermediate progenitors appears at the onset of hemocyte differentiation and accounts for the increasing number of differentiated hemocytes in the third larval instar. These findings provide a new framework to identify parameters of developmental plasticity of the Drosophila lymph gland and hemocyte homeostasis in physiological conditions and in response to immunological cues.

The Drosophila melanogaster larval hematopoietic organ, the lymph gland, is a model to study in vivo the function of the hematopoietic niche. A small cluster of cells in the lymph gland, the posterior signaling center (PSC), maintains the balance between hematopoietic progenitors (prohemocytes) and their differentiation into specialized blood cells (hemocytes). Here, we show that Decapentaplegic/bone morphogenetic protein (Dpp/BMP) signaling activity in PSC cells controls niche size. In the absence of BMP signaling, the number of PSC cells increases. Correlatively, no hemocytes differentiate. Controlling PSC size is, thus, essential for normal blood cell homeostasis. Activation of BMP signaling in the PSC requires expression of the Dally-like heparan-sulfate proteoglycan, under the control of the Collier/early B-cell factor (EBF) transcription factor. A Dpp > dpp autoregulatory loop maintains BMP signaling, which limits PSC cell proliferation by repressing the protooncogene dmyc. Dpp antagonizes activity of wingless (Wg)/ Wnt signaling, which positively regulates the number of PSC cells via the control of Dmyc expression. Together, our data show that Collier controls hemocyte homeostasis via coordinate regulation of PSC cell number and PSC signaling to prohemocytes. In mouse, EBF2, BMP, and Wnt signaling in osteoblasts is required for the proper number of niche and hematopoietic stem cells. Our findings bring insights to niche size control and draw parallels between Drosophila and mammalian hematopoiesis.TGF-β | hematopoiesis | myc