High-resolution genetic analysis of the Sd-1 aphid resistance locus in Malus spp

Aphids cause serious physical and economic damage to most major crops throughout the world through feeding damage, with consequent symptom development and virus transmission. The rosy leaf-curling aphid ( Dysaphis devecta Wlk.) is a pest of apple ( Malus spp.) which displays an exceptionally clear phenotype with respect to susceptible and resistant symptoms. The Sd-1 locus for resistance to D. devecta biotypes 1 and 2 is present in Cox's Orange Pippin and its progeny and had previously been mapped to the top of linkage group 7. Detailed fine mapping of the locus was initiated with AFLP bulked segregant analysis of both pedigree and segregating bulks, which identified three new marker loci. Preliminary marker order in the Sd-1 region was established through mapping in a family derived from Prima x Fiesta, with additional segregation analysis on a Fiesta x Golden Delicious family. Previous recombinant data was re-evaluated and corrected. Two co-segregating AFLP fragments were found to contain a common (GA)(23) repeat, from which a PCR-based simple sequence repeat (SSR) assay was developed. A high-resolution map around the Sd-1 region was established by analysing a large meta-population of Sd-1 recombinants using 759 additional individuals from different families. The Sd-1 gene has been located within a 1.3-cM interval flanked by the molecular markers SdSSRa and 2B12a and co-locates with the RFLP marker MC064. Allelism between Sd-1 and Sd-2 resistant sources was tested. Molecular markers tightly linked to Sd-1 were shown to be co-segregating with the Sd-2 locus, which indicated that Sd-1 and Sd-2 loci are at least tightly linked and, probably, allelic.