Phenotypic and fine genetic characterization of the D locus controlling fruit acidity in peach

Background: Acidity is an essential component of the organoleptic quality of fleshy fruits. However, in these fruits, the physiological and molecular mechanisms that control fruit acidity remain unclear. In peach the D locus controls fruit acidity; low-acidity is determined by the dominant allele. Using a peach progeny of 208 F-2 trees, the D locus was mapped to the proximal end of linkage group 5 and co-localized with major QTLs involved in the control of fruit pH, titratable acidity and organic acid concentration and small QTLs for sugar concentration. To investigate the molecular basis of fruit acidity in peach we initiated the map-based cloning of the D locus. Results: In order to generate a high-resolution linkage map in the vicinity of the D locus, 1,024 AFLP primer combinations were screened using DNA of bulked acid and low-acid segregants. We also screened a segregating population of 1,718 individuals for chromosomal recombination events linked to the D locus and identified 308 individuals with recombination events close to D. Using these recombinant individuals we delimited the D locus to a genetic interval of 0.4 cM. We also constructed a peach BAC library of 52,000 clones with a mean insert size of 90 kb. The screening of the BAC library with markers tightly linked to D locus indicated that 1 cM corresponds to 250 kb at the vicinity of the D locus. Conclusion: In the present work we presented the first high-resolution genetic map of D locus in peach. We also constructed a peach BAC library of approximately 15 x genome equivalent. This fine genetic and physical characterization of the D locus is the first step towards the isolation of the gene(s) underlying fruit acidity in peach.