Aiming to extend the actual inheritance studies resistance of 10 interspecific progenies of Eucalyptus spp. developed in the Genolyptus Project were assessed by artificial inoculation. The segregation pattern of all ten families suggests the occurrence of double recessive epistasis. Rust resistance must then depends on two major effect genes, one that codes for an R protein, capable of recognize Puccinia psidii Avr proteins directly and the other that codes for an extremely important member of the transduction pathway, i.e. a protein kinase that acts downstream of R-Avr recognition. The lack of a signaling molecule would block the activation of the defense mechanisms in the same way that the lack of the R protein would result in susceptibility. Alternatively rust resistance in eucalypts might follow the guard hypothesis, with a classic R protein coded by one locus monitoring the protein that is target for pathogens effector molecules and that is coded by the second locus. In order to map resistance gene/QTL, 188 plants obtained from an interspecific cross [(E. dunnii x E. grandis 2) x (E. urophylla x E. globulus)] were assessed for resistance and genotyped with a set of microsatellites scanning all linkage group 3. Although the segregation pattern for rust resistance in that specific cross corresponds to that expect to a dominant monogenic trait, the resistance was treated as a quantitative trait and QTL analysis were performed to estimate the position and the effect of the locus involved in resistance expression. For QTL mapping in the integrated map an interval mapping strategy developed by Fulker & Cardon was used, and for QTL mapping in the pseudo-testcross maps an interval mapping strategy developed by Lander & Botstein was used. With these analysis it was detected a major effect QTL for rust resistance in linkage group 3. This QTL was detected with high confidence by the methodology of Fulker & Cardon in the region between markers Embra286 and Embra122 and by the strategy of Lander & Botstein in the region between markers Embra350 and Embra239. The estimates are that these QTLs explains near 71 and 42% of the phenotypic variation, respectively and since rust resistance inheritability in this cross was estimated in 84%, the detected QTLs should explain near 84% and 50% of rust resistance in this family, respectively. Hence these QTLs were detected in a same region of linkage group 3, they must correspond to the same locus. Aiming to obtain the exact position of the resistance locus on the integrated map previously constructed for (DxG2)x(UxGL) family, the resistance data was analyzed with an algorithm implemented in the GQMOL software. With this analysis it was possible to locate the resistance locus in the region between markers Embra286 and Embra239 in a genetic window of 14,61cM also confirming the hypothesis that both parents are heterozygotes for the resistance locus. The flanking markers of this gene/QTL could be used in introgression experiments with an expected efficiency near 99% for selecting resistant plants, assuming no recombination interference in the target genomic area.
