A SINGLE-GENE CLUSTER CONTROLS INCOMPATIBILITY AND PARTIAL RESISTANCETO VARIOUS MELAMPSORA-LARICI-POPULINA RACES IN HYBRID POPLARS

Complete cosegregation for race-specific incompatibility with three Me lampsora larici-populina rust races was observed in five F-l hybrid pr ogenies of Populus, with different patterns among the various progenie s. A single gene cluster could explain these segregations: one locus w ith multiple alleles or two tightly linked loci controlling complete r esistance to E1 and E3, and two tightly linked loci for E2. The random amplified polymorphic DNA marker OPM03/04_480 was linked to that clus ter in all families (<1 cM). This marker accounted for more than 70% o f the genetic variation for field resistance in each family (heritabil ity approximate to 0.40). The same marker accounted for up to 64% of t he clonal variation for growth in the nursery under natural inoculum p ressure; the weak tolerance to rust of F-1 interspecific hybrids was a ttributed to a genetic background effect. Partial resistance was split into epidemiological components (heritability ranged from 0.35 to 0.8 7). Genotypic correlations among resistance traits for the different r aces were high (0.73 to 0.90). However, correlations among different r esistance components for a single race were not all significant. A maj or quantitative trait locus for all components of partial resistance t o E2 was associated to the cluster controlling incompatibility to F-1 and E3 and marked by OPM03/04_480 (R-2 from 48 to 68%).