RFLP and RAPD markers linked to the rosy leaf curling aphid resistance gene (Sd1) in apple

[Sd.sub.1] is a dominant gene for resistance to biotypes 1 and 2 of the rosy leaf curling aphid, Dysaphis devecta Wlk., which can cause economic damage to apple trees. This report describes the identification of three RFLP and four RAPD markers linked to [sd.sub.1] in a cross between the D. devecta susceptible variety 'Prima' ([sd.sub.1][sd.sub.1]) and the resistant variety 'Fiesta' ([Sd.sub.1][sd.sub.1]). Potted trees were artificially infested in the glasshouse, and the ratio of resistant:susceptible plants supported the hypothesis that the resistance was under the control of a single dominant gene. The position of the gene was mapped to a single locus on a 'Fiesta' chromosome, within 2 cM of three tightly linked RFLP markers (MC064a, 2B12a and MC029b); the four RAPD markers were located further away (between 13 and 46 cM). This is the first report of molecular markers for an aphid resistance gene in tree fruit crops. The potential application of these markers in a markerassisted resistance breeding programme is discussed.