Identification of QTLs for resistance to powdery mildew and SSR markers diagnostic for powdery mildew resistance genes in melon (Cucumis melo L.)

Abstract

Powdery mildew caused by Podosphaera xanthii is an important foliar disease in melon. To find molecular markers for marker-assisted selection, we constructed a genetic linkage map of melon based on a population of 93 recombinant inbred lines derived from crosses between highly resistant AR 5 and susceptible 'Earl's Favourite (Harukei 3)'. The map spans 877 cM and consists of 167 markers, comprising 157 simple sequence repeats (SSRs), 7 sequence characterized amplified region/cleavage amplified polymorphic sequence markers and 3 phenotypic markers segregating into 20 linkage groups. Among them, 37 SSRs and 6 other markers were common to previous maps. Quantitative trait locus (QTL) analysis identified two loci for resistance to powdery mildew. The effects of these QTLs varied depending on strain and plant stage. The percentage of phenotypic variance explained for resistance to the pxA strain was similar between QTLs (R (2) = 22-28%). For resistance to pxB strain, the QTL on linkage group (LG) XII was responsible for much more of the variance (41-46%) than that on LG IIA (12-13%). The QTL on LG IIA was located between two SSR markers. Using an independent population, we demonstrated the effectiveness of these markers. This is the first report of universal and effective markers linked to a gene for powdery mildew resistance in melon.

Mapping of resistance genes to races 1, 3 and 5 of Podosphaera xanthii in melon PI 414723

“…These include Pm-1 from PI 78374 (race 1) in linkage group IX (Teixeira et al 2008), Pm-w from WMR-29 (race 2-French) in LG V, and Pm-y from VA-435 (race 2-French) in LG XII (Pitrat 1991. Besides, resistance QTL were also mapped in LGs II (races 1, 3, and N1), V (1, 2, and 5), and XII (races 1, 5, and N1) (Perchepied et al 2005, Fukino et al 2008, Yuste-Lisbona et al 2011b). Thus, the diversity of resistance genes to different races of P. xanthii opens the possibility of combining them in single melon genotypes in order to achieve resistance to multiple races.…”

“…Forty seven microsatellite markers polymorphic between PI 414723 and Védrantais were selected from the linkage maps described by Fukino et al (2008) and Gonzalo et al (2005), in order to represent all linkage groups and provide a map framework. The PCR reactions consisted of 20 ng of DNA, 0.6 µM of each primer, 1X PCR Master Mix (Promega -M7505) and ultrapure sterile water to complete 12 µl.…”

“…Among the mapped loci, only one (AFLP) marker presented segregation distortion. 12 LG were named in accordance to Périn et al (2002), based on SSR markers mapped by Gonzalo et al (2005) and Fukino et al (2008 Figure 1. Linkage map of F 2 population (PI 414723 × Védrantais).…”

“…For this, a saturation of this linkage group with additional markers would be necessary. It is interesting to note that a resistance QTL to races 1 and N1 of P. xanthii from a different resistance source (AR-5) was found closely linked to the microsatellite loci CMBR8 and CMBR120 from LGII (Fukino et al 2008). The first marker was not mapped in the present study since it was monomorphic between the parents, but the second was located 32.6 cM from Pm-x1.5, and 27.5 cM from Pm-x3, initially suggesting that QTL is located in a distinct portion of LG and, therefore, do not correspond to neither of these two genes.…”

“…The first marker was not mapped in the present study since it was monomorphic between the parents, but the second was located 32.6 cM from Pm-x1.5, and 27.5 cM from Pm-x3, initially suggesting that QTL is located in a distinct portion of LG and, therefore, do not correspond to neither of these two genes. However, this conclusion needs further testing, given that the accurate location of QTLs depends largely on the density of markers and that LGII assembled by Fukino et al (2008) is comprised of only eight markers distributed in two unlinked sub-groups.…”