Résumé
Poplar leaf rusts caused by Melampsora spp. are considered as being among the most crucial sanitary problems threatening poplar stands worldwide. Several major resistance factors specific to poplar rusts have been identified, each of them discovered using a single Melampsora species.Most of them are inherited from Populus species that did not co-evolved with the pathogen, hence questioning the origin and the function of such "exapted" resistance.The purpose of this presentation is to compare the genetics of exapted and co-adapted resistance to M. medusae fsp. deltoidae (Mmd) and to M. larici-populina (Mlp), respectively. To achieve this, resistance QTL against Mlp was studied in two pedigrees. The first, an F2 P. trichocarpa xP. deltoides family, was previously evaluated for resistance to M medusae fsp. deltoidae, and a major gene for resistance Mmdl inherited from the P. trichocarpa grandparent was mapped. The second pedigree was a F1 P.nigra family.In the F2 pedigree, 17 QTLs were detected that are involved in the three components of quantitative resistance to the five Mlp strains inoculated in laboratory. These QTLs explain between 3.1 and 30.4% of the phenotypic variance. None ofthese QTLs have a broad range effect. No major resistance factor against Mlp was identified in this pedigree showing that Mmdl is not functional against Mlp strains tested.In the P. nigra family, 11 QTL explaining between 2% and 60 % of the phenotypic variation observed after inoculation with the two Mlp strains were detected. Most of these QTL were trait- or strain-specific except one. A major QTL inherited from the most resistant parent and located at the end of LG I explained more than 60% of uredinia size variation after inoculation with one Mlp strain and showed a moderate effect after inoculation with the other strain. The genomic mining of the QTL region revealed a cluster of 25 NB-LRR genes. Those results confirm in P.nigra the gene-for-gene model suggesting specific interactions between QTLs for quantitative resistance and Mlp strains.The results presented will be compared and discussed together with results of previous studies on Melampsora/Salicaceae pathosystems.