The ability to detect and respond to the presence of pathogens is essential for survival and development of every living organism. In the two most famous branches of the eukaryotic tree complex immune responses have been described. Detection of a pathogen usually initiates a cell death reaction restricted to the infected cell by the inflammatory reaction in animals and the hypersensitive response in plants. In both cases the innate immune response is initiated when Pathogen Associated Molecular Patterns (PAMP) resulting from the presence of the pathogen are recognized by dedicated Pattern Recognition Receptors (PRR). Interestingly, PRRs in plants and animals share a common architecture associating a central nucleotide binding domain (NB) to an N terminal effecter domain and a C terminal protein protein interaction domain made of Leucine Rich Repeats (LRR). This organisation, characteristic of the STAND class of proteins, results from convergent evolution. In both plants and animals, mutations associated to PRRs can lead to auto-immune diseases resulting in the activation of defence mechanisms in absence of any pathogen. Surprisingly no immune system has been described in the fungal branch of the eukaryotic tree despite fungi being closely related to animals. In filamentous fungi a conspecific non self recognition process known as Vegetative Incompatibility (VI) is initiated after the somatic fusion of cells originating from genetically different strains. This recognition ensured by het genes triggers a cell death reaction restricted to the fusion and surrounding cells that is now well described. het genes have been cloned in the model species Podospora anserina. Alleles of het-c encoding for a glycolipid transfer protein (GLTP) can be incompatible with alleles from the NWD gene family encoding for STAND proteins. NWD family members have the ability to permanently generate a considerable number of variants differing by their protein-protein interaction domain (WD domain). We have recently hypothesized that the P. anserina NWD family members actually encode for PRRs and that the VI reaction is a pathological manifestation of the fungal immune response as a by-product of the pathogen driven diversification of the NWD genes. In addition we have identified a fungal species, Epicoccum nigrum that triggers a very strong reaction when confronted to WT P. anserina. On confrontation plates, E. nigrum filaments grow within P anserina territory, and in return, P. anserina hyphae appear to coil around and kill the E. nigrum cells. This P. anserina reaction is appears less efficient in P. anserina mutant strains that cannot proceed through the VI reaction, including suppressor mutants and the strain deleted for the GLTP encoding gene het-c does not react to the presence of this fungal species. We thus hypothesize that the HET-C protein and the NWD PRR receptors are involved in recognition of heterospecific non-self and induction of an appropriate response. Preliminary results indicate that this response includes many features of the VI reaction. The present project aims at characterizing the molecular components of the heterospecific non-self recognition machinery using the P. anserina/E. nigrum interaction. What recognizes what and how? We will investigate the role of the different molecular actors of the recognition process with a particular emphasis on the HET-C and NWD proteins. We will also try to confirm that the P. anserina response to the pathogen is comparable to the VI reaction. Finally, we will extend the description of fungal immune response to additional fungal host/Pathogen systems.