A specific PCR assay for resistance to biotypes 1 and 2 of the rosy leaf curling aphid in apple based on an RFLP marker closely linked to the Sd1 gene

This report describes the conversion of a restriction fragment length polymorphism (RFLP) marker (the 2B12a locus), linked to the Sd(1) aphid resistance gene, to a polymerase chain reaction (PCR) based marker. A section of the 2B12 probe was sequenced and two primers were designed to amplify this sequence in the cultivars 'Prima' and 'Fiesta'; all the amplification products were the same size. After sequencing, two specific 24-mer oligonucleotides were synthesized (DdARM-5' and DdARM-3') to exploit a single base-pair difference. These primers were used to screen 44 plants from the 'Prima' x 'Fiesta' family and generated a single amplification product (196 bp), in approximately half of the seedlings, which was linked to the resistance gene Sd(1). The DdARM primer combination was used to evaluate a range of apple cultivars and selections, including some varieties derived from 'Cox' and alternative sources of resistance reported in the literature. In parallel with this work, the phenotypic response of the same genotypes was either confirmed or determined in replicated glasshouse tests. The sequence characterized amplified regions (SCAR) marker was amplified in all the resistant plants, with the exception of 'Northern Spy' and 3760 (the sources of Sd(2) and Sd(3) resistance, respectively), but never in the susceptible plants. The possible role of this marker in a marker-assisted breeding strategy, and its compatibility with a SCAR marker linked to the V-f gene for resistance to apple scab, is discussed.