Construction of a high density linkage map and its application in the identification of QTLs for soluble sugar and organic acid components in apple

Soluble sugars and organic acids have a strong impact on the overall organoleptic quality of fruits. In this study, we report the identification of quantitative trait loci (QTLs) for individual sugars and organic acids in apple. A high density linkage map of apple was constructed using the 1536 EST-derived SNP GoldenGate genotyping platform. The linkage map consists of 601 molecular markers, including 540 single nucleotide polymorphisms (SNPs) and 61 simple sequence repeats (SSRs), spanning 1368.4 cM with an average of 2.28 cM per marker. The contents of soluble sugars, including sucrose, glucose, fructose, sorbitol, and organic acids, including malic acid and citric acid, were used as the phenotypic data in QTL analysis. Two QTLs for malic acid content were detected on linkage groups (LGs) 8 and 16, while no QTL was found for citric acid content. Four QTLs for the glucose, sucrose, fructose, and sorbitol content were found to be clustered in one region on LG 3. Moreover, an additional QTL for glucose content was detected on the LG 4. Our study not only expands our understanding of the genetic basis for fruit organoleptic quality but it also provides molecular markers that will aid in marker-assisted selection for fruit quality in apple breeding programs.