Genetic architecture of qualitative and quantitative Melampsora larici-populina leaf rust resistance in hybrid poplar: genetic mapping and QTL detection

International audience; • In order to elucidate the genetic control of resistance to Melampsora larici-populina leaf rust in hybrid poplars, a Populus deltoides × P. trichocarpa F1 progeny was analysed for qualitative and quantitative rust resistances. • This progeny was evaluated for three components of quantitative resistance (latent period, uredinia number and uredinia size) to seven M. larici-populina strains in controlled conditions, and for one component of field susceptibility (rust colonization on the most infected leaf). • One qualitative resistance locus inherited from P. deltoides, R1, was localized on the genetic map. It segregates 1 : 1 in the F1 progeny and is effective against four of the studied strains. QTL analysis was performed separately on R1 and r1 genotype subsets. An additional detection was conducted on the entire F1 progeny for the three strains able to overcome R1 and for MAX2. A total of nine QTLs were detected. Two had large, broad-spectrum effects. One (RUS) is inherited from the P. trichocarpa parent; the other is inherited from P. deltoides and colocalized with R1. Seven QTLs had only limited and specific effects. Significant interaction effects were detected mainly between the two major QTLs. • Implications of these results for durable resistance breeding strategies, and possible benefits from the Populus genome sequence, are discussed.