Resident stem cells in adult organs play an essential role in tissue homeostasis. The tissue environment (niche) in which stem/progenitor cells reside plays a decisive role in providing the structural, trophic support and appropriate signals to regulate stem cell function. Genetic and molecular studies in mice have uncovered many signalling pathways involved in the communication between HSCs and their niche. However, our knowledge of the haematopoietic niche plasticity and function remain very fragmentary. Discovery, including by us, of a Drosophila haematopoietic niche, termed PSC, in the larval hematopoietic organ (the lymph gland, LG), has opened the prospect of using this model organism to decipher the basic rules of haematopoiesis and haematopoietic niche function. The PSC is a genetically tractable model to study how different signalling pathways and gene regulatory networks are integrated for maintenance of multipotent progenitors, with developmental and evolutionary perspectives. Not surprisingly, the number of excellent European and non-European laboratories studying Drosophila haematopoiesis has rapidly increased during the last few years. We need to face this intense competition in order to keep a leadership position. Our previous work has shown that Collier, the Drosophila ortholog of mammalian Early B-Cell Factor (EBF) specifies the PSC. We then showed that PSC cells act, in a non cell autonomous manner, to maintain JAK/STAT signalling in pro-hemocytes, thereby preventing their premature differentiation and preserving their multipotent character. It revealed that haemocyte homeostasis is dependent upon interactions between haematopoietic progenitors and their micro-environment in Drosophila, as is the case in vertebrates. Our most recent data (ANR prodrohemo) identified a new mode of regulation of JAK/STAT signalling in pro-hemocytes, an unexpected heterogeneity of these progenitors and the key role of Dpp/BMP signalling in maintaining the PSC size. BMP signalling was previously shown to control the number of osteoblasts, a major component of the HSC niche in the mammalian bone marrow; very recently EBF2 was shown to be required in immature osteoblasts. The striking parallels between the Drosophila PSC and the mammalian HSC niche raise fascinating questions. Our project has three main objectives: A molecular dissection of the PSC maintenance and function in physiological and parasitism conditions. A molecular characterisation of pro-haermocytes and the role of JAK/SAT signalling. A global, integrated view of the reciprocal communications between the PSC and pro-haemocytes. Methodology: Our project will take advantage of the combination of the Drosophila genetic and genomic resources, including dsRNA lines for gene extinction, imaging with new fluorescent reporter proteins, and novel methodologies : Laser micro-dissection of the LG cells and RNA TU-Tagging, followed by high-throughput RNA sequencing will allow to determine with unprecedented precision the molecular signatures of PSC cells and pro-haemocytes in wt larvae, mutant contexts and following parasitism; systematic integration of vertebrate and Drosophila datasets will be performed through external collaborations. Conclusion Deciphering molecular networks involved in maintaining a functional niche and characterizing, at the molecular level, cellular communications involved in the control of Drosophila haematopoiesis should provide important new knowledge on a primary aspect of innate immunity, the control of quiescence/proliferation of haematopoietic progenitors in physiological conditions and in response to immune threats. Together with ongoing studies on the role of EBF2 and various signaling pathways involved in the communications between osteoblasts and HSCs in vertebrates, our new research program should shed novel light on the emergence and diversification of haematopoietic microenvironments during evolution.