QTL IDENTIFICATION AND DNA MARKER DEVELOPMENT FOR INDIVIDUAL SUGARS IN APPLE

Yingzhu Guan

Apple is one of the most popular fruits around the world, with an increasing demand for new cultivars with improved quality. Sweetness, along with texture, flavor and other attributes like price, determines the overall acceptability of apple. However, sensory sweetness is a complex trait, highly affected by other sensory sensations, e.g., aroma, texture, and the balance of sugars and acids. Three sugars (fructose, glucose, and sucrose) and the sugar alcohol sorbitol are present in apple, and concentrations for each are highly influenced by environment. Selecting this quantitative trait by phenotype-based selection in conventional breeding programs would thus be challenging, time-consuming, and labor-intensive. By comparison, marker-assisted selection (MAS) is more reliable, stable, and efficient. This study focused on the QTL detection and the development of DNA markers associated with QTL regions for individual sugars in apple to enable MAS and therefore improve the efficiency and creativity of breeding programs. The germplasm set studied consisted of 274 individuals with extensive yet breeding-relevant genetic variation. One thousand four hundred and sixteen polymorphic SNPs were used for genetic map construction. The sweetness traits (fructose, glucose, sucrose, sorbitol, soluble solids content (SSC), and sensory sweetness), malic acid, titratable acidity (TA) and pH were evaluated at harvest, after 10 weeks and 20 weeks of refrigerated storage with 1 week at room temperature in 2011 and 2012. A total of 30 QTLs was detected for fructose, glucose, sucrose, sorbitol, and SSC from harvest through storage in two years, and one QTL region on linkage group 1 was consistently highly associated with variation for fructose content from harvest through storage in both years. Sucrose, glucose, and sorbitol content were also associated with the region. Six polymorphic SNPs were chosen from that QTL region for functional haplotype development. Seven SNP haplotypes and 13 diplotypes were identified in WSU germplasm, of which six haplotypes and 11 diplotypes were analyzed for genetic effect. Association of these haplotypes and diplotypes with fructose content was found, documented and will provide the necessary foundation toward functional haplotype development to enable MAS in apple breeding programs for sweetness selection.

QTL IDENTIFICATION AND DNA MARKER DEVELOPMENT FOR INDIVIDUAL SUGARS IN APPLE

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