Plum pox virus (PPV) is responsible for sharka disease, one of the most detrimental stone fruit diseases affecting Prunus trees worldwide. Only a few apricot cultivars have been described as resistant, most originating from North American breeding programmes. Several PPV resistance quantitative trait loci (QTLs) have been mapped in various progenies, consistently highlighting the contribution to the resistance of the upper part of linkage group 1 (LG1). However, to date, no consensus has been reached on the precise number of QTLs linked to the resistance to PPV in apricot and P. davidiana or on their accurate position on the genetic linkage map. In the present study, the quantitative resistance of cultivar 'Harlayne' was analysed over five growth periods in a large F1 population. Four QTLs were identified, three mapping on LG1, explaining between 5% and 39% of the observed phenotypic variance. In an effort to further this analysis of PPV resistance in apricot, these results were merged in a single QTL meta-analysis with those of five other PPV resistance analyses available in the literature. Three consensus QTL regions were identified on LG1 and a putative fourth region on LG3. QTL meta-analysis also revealed the contribution of each resistant cultivar to metaQTLs, providing interesting comparative data on the resistance factors shared between the resistance sources used in the various studies. Finally, it was shown that one of the metaQTLs co-localizes with the eukaryotic translation initiation factor eIF4E, thus providing new hypotheses on the mechanisms of PPV resistance in apricot.

A complex, polygenic resistance to Plum pox virus (PPV) was previously described in a wild peach-related species, Prunus davidiana clone P1908. In the current study, an analysis of quantitative trait loci (QTL) was performed on an F 2 population comprising 99 individuals obtained by selfing the F 1 individual #40 of an interspecific cross between susceptible nectarine cv. Summergrand and the resistant P. davidiana clone P1908. Six QTL were identified using both parametric and non-parametric methods of detection, individually explaining 5-28% of the phenotypic variance. The total phenotypic variation explained ranged from 29 to 58%. Alignment of the genetic map of the F 2 cross with the P. davidiana parent map showed consistency of QTL over generations, with three of the six QTL co-localizing at the 1-LOD interval and another one at the 2-LOD interval. Two of the QTL were mapped onto linkage group one, where resistance to PPV was previously mapped in apricot. Development and mapping of new microsatellite markers linked to candidate genes revealed a striking co-localization of three of the detected QTL with gene copies coding for eukaryotic translation initiation factors eIF4E and eIF(iso)4G. As co-localization of one QTL with candidate gene eIF(iso)4E was previously reported in the F 1 population, the results reported here strongly reinforce the idea that components of the eukaryotic translation initiation complex are correlated with resistance to PPV in P. davidiana P1908.

Plum pox virus (PPV) is a potyvirus that causes sharka disease in infested stone fruit trees (Prunus species, peach, apricot, plum). In apricots, the resistance is controlled by a major quantitative trait locus that explains up to 70% of the phenotypic variance; it is localised in the upper part of linkage group 1. In this report, we transformed candidate genes that mapped in the region of the apricot resistance locus into polymerase chain reaction markers (SSCP and SSR) and tested for their co-localisation with the major PPV resistance locus in related and unrelated populations. Three populations of F1 and F2 individuals issued from crosses between the PPV-resistant cultivar 'Stark Early Orange' or 'Goldrich' and three susceptible parents were used in this study. Molecular-marker data were collected to determine the linkage relationship between the PPV resistance locus in apricots and markers that target candidate disease-resistance genes. In addition, SSR markers linked to resistance-gene candidates were mapped to positions flanking the PPV resistance locus in different apricot populations. Therefore, we demonstrate that this strategy helps to saturate the major genomic region controlling resistance to PPV in apricot with valuable codominant markers.

Plum pox virus (PPV) is a devastating stone fruit disease of major importance, and better understanding of the genetic control of resistance to this trait would be useful for more efficient development of resistant cultivars. Previous studies have reported a locus of major effect from PPV resistance on linkage group 1. The current study confirms these results by mapping plum pox virus resistance in a F1 progeny issued from a cross between "Harlayne", as a PPV-resistant parent, and "Vestar" as a susceptible parent. The hybrids were grafted simultaneously and subsequently inoculated with the PPV-M and D strains.The symptom scoring on leaves was performed nine times over two vegetative cycles. Marker-trait associations were analyzed using the Kruskal-Wallis (KW) non-parametric test, and the PPV resistance loci were mapped using composite interval mapping (CIM). We show that both analyses (KW and CIM) highlighted the upper part of linkage group 1 of the apricot "Harlayne" genitor.

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