Exploration of Populus nigra L. genetic variation for partial resistance against the co-adapted pathogen Melampsora larici-populina

Breeding poplars for more durable resistance to Melampsora larici-populina (Mlp) leaf rust is a real challenge, especially in an european context where Mlp populations exhibit high genetic diversity in black poplar-larch sympatric areas and where selective pressures from the domesticated interspecific host (P. xeuramericana and P.xinteramericana) are both rapid and strong. Several approaches combining field evaluation under natural infection and artificial inoculation tests with control of the pathogen diversity were more recently directed at the study of genetic variation for partial resistance (QR), in the co-evolved parental species P. nigra L. Quantitative and molecular genetic studies of the genetic control of partial resistance were developed on a 4x3 factorial mating design, one enlarged F1 mapping pedigree and one association population composed of more than 800 clones and genotyped with more than 8,000 SNPs. The existence of a large reservoir of variation for QR in different natural populations of P.nigra was demonstrated for several epidemiological parameters, with some evidence of geographical patterns. Significant variation for strain-specificity was observed at clonal level within a subset of high field-resistant natural poplar genotypes. From classical linkage analysis, a list of twenty QTL distributed over the poplar genome explained between 2% to 60% of the phenotypic variation observed in artificial inoculation tests. Most of these QTL were trait- or strain-specific. The genomic mining of the detected QTL regions revealed close vicinity to clusters of NB-LRR genes and to other QTL associated to Salix resistance to Melampsora larici-epitea. The characterization of selection pressures exerted by host resistance on pathogen population provides useful insights into the dynamics of host-parasite co-evolution processes and is crucial for the design of both breeding and deployment strategies. Preliminary experiments suggested local adaptation of Mlp populations to partial resistances present in P.nigra populations. All these results will be discussed in the framework of poplar breeding strategies and poplar cultivar deployment