Mapping of the apple scab-resistance gene Vb

Abstract

Apple scab, caused by the fungus Venturia inaequalis, is the major production constraint in temperate zones with humid springs. Normally, its control relies on frequent and regular fungicide applications. Because this control strategy has come under increasing criticism, major efforts are being directed toward the breeding of scab-resistant apple cultivars. Modern apple breeding programs include the use of molecular markers, making it possible to combine several different scab-resistance genes in 1 apple cultivar (pyramiding) and to speed up the breeding process. The apple scab-resistance gene Vb is derived from the Siberian crab apple 'Hansen's baccata #2', and is 1 of the 6 ''historical'' major apple scab-resistance genes (Vf, Va, Vr, Vbj, Vm, and Vb). Molecular markers have been published for all these genes, except Vr. In testcross experiments conducted in the 1960s, it was reported that Vb segregated independently from 3 other major resistance genes, including Vf. Recently, however, Vb and Vf have both been mapped on linkage group 1, a result that contrasts with the findings from former testcross experiments. In this study, simple sequence repeat (SSR) markers were used to identify the precise position of Vb in a cross of 'Golden Delicious' (vbvb) and 'Hansen's baccata #2' (Vbvb). A genome scanning approach, a fast method already used to map apple scab-resistance genes Vr 2 and Vm, was used, and the Vb locus was identified on linkage group 12, between the SSR markers Hi02d05 and Hi07f01. This finding confirms the independent segregation of Vb from Vf. With the identification of SSR markers linked to Vb, another major apple scab-resistance gene has become available; breeders can use it to develop durable resistant cultivars with several different resistance genes.Résumé : La tavelure du pommier, causée par le champignon Venturia inaequalis, constitue la principale contrainte à la production dans les zones tempérées aux printemps humides. Habituellement, la lutte repose sur des pulvérisations réguliè-res et fréquentes avec de fongicides. Une telle méthode étant de plus en plus critiquée, des efforts importants sont dirigés du côté de la sélection de cultivars résistants à la tavelure. Les programmes modernes d'amélioration génétique du pommier incluent l'emploi de marqueurs moléculaires, rendant ainsi possible la combinaison (pyramidage) de plusieurs gènes de résistance au sein d'un seul cultivar et accélérant le processus de sélection. Le gène de résistance Vb est issu du pommetier de Sibérie 'Hansen's baccata #2' et il est un de six gènes majeurs « historiques » de résistance à la tavelure (Vf, Va, Vr, Vbj, Vm et Vb). Des marqueurs moléculaires ont été rapportés pour tous ces gènes sauf Vr. Lors de croisements test effectués dans les années '60, il avait été conclu que Vb affichait une ségrégation indépendante de trois autres gènes majeurs dont Vf. Récemment, cependant, Vf et Vb ont tous deux été assignés au groupe de liaison 1, un résultat qui va à l'encontre des résultats des croisements test. Dans ce trav...

Identification and mapping of the novel apple scab resistance gene Vd3

“…Patocchi et al (1999a) suggested two hypotheses to explain this situation; the first one is that these plants are double recombinants, and the second one is that they were wrongly classified. As reported by these authors (Patocchi et al 1999a;Gygax et al 2004;Erdin et al 2006), the presence of double recombinants is quite unlikely because of the distance of the flanking markers in coupling with Vd3. In our case, we expect a frequency of double recombinants of 0.6% and not the 3% observed.…”

“…In both situations, the gene is mapped between the same markers, and the order of the markers in the map is not altered, but in the case of the exclusion of the GPI plants, the distance between Vd3 and the Vf locus was reduced from 2.9 to 1 cM. As was reported by other authors (Patocchi et al 1999a;Gygax et al 2004;Erdin et al 2006), we found difficulties determining the correct position of Vd3, as the phenotypic data were not as precise as the molecular data. During the analysis of the population 2000-012C, we classified three plants (3%) as susceptible, but the flanking marker alleles were in coupling with the resistance.…”

“…In the mapping of the Vbj gene, Gygax et al (2004) found 13 resistant plants, out of a population of 173 individuals, that did not have the marker alleles in coupling with the gene and 12 susceptible ones that had them. Finally, Erdin et al (2006), during the mapping of the Vb gene in the population Golden Delicious × "Hansen's baccata #2," identified six GPI plants. After the exclusion of the resistance data of these plants, the tension that they observed in the map disappeared.…”

Microsatellite markers spanning the apple (Malus x domestica Borkh.) genome

“…In only one case, Z71981 SSR (LG15), is the approximate map position still to be determined. Some SSRs of the Hi set have already been mapped in other crosses than Fiesta × Discovery, and their map position relative to other SSRs was confirmed (Patocchi et al 2005;Gardiner et al 2006;Erdin et al 2006;Durel, unpublished).…”

“…Firstly, the Genome Scanning Approach (GSA, Patocchi and Gessler 2003) allows efficient mapping of major genes. This procedure was successfully applied to map the apple scab resistance genes Vr2, Vm Patocchi et al 2005), and Vb (Erdin et al 2006). Secondly, the Pedigree Genotyping concept was developed , which allows the exploitation of breeding material in the assessment of marker-trait associations and in allele mining by using multiple pedigreed plant populations, which can be any combination of crosses, cultivars, and breeding lines.…”