Identification of a major QTL together with several minor additive or epistatic QTLs for resistance to fire blight in apple in two related progenies

Abstract

Although fire blight, caused by the bacterium Erwinia amylovora, is one of the most destructive diseases of apple (Malus · domestica) worldwide, no major, qualitative gene for resistance to this disease has been identified to date in apple. We conducted a quantitative trait locus (QTL) analysis in two F 1 progenies derived from crosses between the cultivars Fiesta and either Discovery or Prima. Both progenies were inoculated in the greenhouse with the same strain of E. amylovora, and the length of necrosis was scored 7 days and 14 days after inoculation. Additive QTLs were identified using the MAPQTL software, and digenic epistatic interactions, which are an indication of putative epistatic QTLs, were detected by two-way analyses of variance. A major QTL explaining 34.3-46.6% of the phenotypic variation was identified on linkage group (LG) 7 of Fiesta in both progenies at the same genetic position. Four minor QTLs were also identified on LGs 3, 12 and 13. In addition, several significant digenic interactions were identified in both progenies. These results confirm the complex polygenic nature of resistance to fire blight in the progenies studied and also reveal the existence of a major QTL on LG7 that is stable in two distinct genetic backgrounds. This QTL could be a valuable target in marker-assisted selection to obtain new, fire blight-resistant apple cultivars and forms a starting point for discovering the function of the genes underlying such QTLs involved in fire blight control.

QTL mapping of fire blight resistance in apple

“…The F7 QTL was identified at 13, 20 and 27 DAI, and the same QTL could also be identified with AUDPC values which might be explained by the strong correlation between PLL measurements and AUDPC. In contrast to Calenge et al (2005), we could not identify any QTL at one week after inoculation which could be due to different strains or may be due to different methods of inoculation used in both studies (cutting vs. injection). Lesion measurements and PLL at 6, 13, 20 and 27 DAI showed significant differences (p<0.05) between progenies therefore, different intervals after inoculation (6, 13, 20 and 27 DAI) can be very informative about the disease development and could be used to calculate the AUDPC (Jorge and Verdier 2002).…”

“…Therefore, the F7 QTL can be considered to be a stable QTL since it is consistent in different genetic backgrounds and at least for two different E. amylovora strains. Minor QTLs identified by Calenge et al (2005) on linkage group 3 of cultivar 'Fiesta' and linkage groups 12 and 13 of cultivar 'Discovery' could not be identified in our experiments. The phenotypic variation explained by F7 QTL (37.5 -38.6%) was less than the broad sense heritability, indicating that there are additional genetic elements contributing to fire blight resistance that were not identified in this study.…”

“…The data presented here together with those presented by Calenge et al (2005) identify a QTL on chromosome 7 of 'Fiesta' associated with resistance to fire blight. However, F7 QTL should be further validated in other genetic background and with other pathogen strains.…”

“…This explanation is supported by the results of Calenge et al (2004aCalenge et al ( , 2005, who identified a QTL in Fiesta in the same region of chromosome 7 using the bacterial strain CFBP 1430 in two different genetic backgrounds. The results reported here and the work of Calenge et al (2004aCalenge et al ( , 2005, shared a common SSR (CH04e05) on linkage group 7 of the map of cultivar 'Fiesta'. The distance of this SSR marker and the 2-LOD support interval of the QTL identified in both studies overlap suggesting they may be the same.…”

“…QTLs have also been found for apple scab resistance (Durel et al 2003;Liebhard et al 2003c). Recently the QTL mapping approach has been used to study fire blight resistance in pear (Dondini et al 2004) and apple (Calenge et al 2004a(Calenge et al , 2005. In this study, we applied the available linkage map data for the apple cross 'Fiesta'Â 'Discovery' to identify QTLs for fire blight resistance (Liebhard et al 2003b).…”