Towards the Identification of the Gene Conferring Resistance against the Rosy Apple Aphid (Dysaphis plantaginea) in Apple

Most of the apple cultivars are susceptible to rosy apple aphid (RAA, Dysaphis plantaginea) but resistance have also been described in apple germplasm laying the basis for the development of new resistant cultivars by breeding. The cultivar Florina, a Malus floribunda #821 derivative, is resistant to RAA and a single resistance gene (Dp-fl) has been mapped in a 330 Kb region on linkage group 8. In this work, a chromosome walking was performed by using a Florina Bacterial Artificial Chromosome (BAC) library to identify candidate resistance genes. A minimum tiling path of BACs covering regions from both ‘resistant’ and ‘susceptible’ chromosomes were identified and a 279 Kb resistance locus was fully sequenced. Through the development of new polymorphic markers, the resistance locus-mapping interval was narrowed down to a physical region of 56 Kb. During the fine-mapping process, two genotype-phenotype incongruences were identified. A single candidate gene, predicted to code for a protein similar to the Quirky gene of Arabidopsis, was identified. To understand the role of this gene, a gene expression analysis was performed on both Florina and Golden Delicious and the Quirky gene was found to be more expressed at 72 hours after the infestation only in Golden while in Florina the expression was generally very low. To validate the gene function a genetic transformation of Gala and Florina was started. Finally, to confirm the identification of the resistance locus, large progenies derived from Malus floribunda were screened to identify more recombinants. This analysis extended the resistance region at the bottom of chromosome 8. Various genes putatively involved in defense response were also identified in the GDDH13 genome sequence. Thus, the presence of additional genes involved in RAA resistance cannot be excluded and a further step of chromosome walking would be necessary for the identification of new candidate genes.