Molecular Mapping of QTL for Sugars in Melon

Sucrose, fructose, glucose, and total sugars (TS) are major factors determining mature melon (Cucumis melo L.) fruit sweetness. The ratio of individual sugar compositions is also an important fruit sweetness trait. Molecular markers linked to genes regulating synthesis of sugar components may improve the breeder’s ability to recover high sugar genotypes and aid in the development of high sugar cultivars. Our objective was to identify molecular markers linked to quantitative trait loci (QTL) for sucrose, fructose, glucose, three individual sugar ratios, TS, and total soluble solids (TSS) in a genetic linkage map by means of an F2 population from the ‘Deltex’ (high TS) x TGR 1551 (low TS) cross. The parents and F2 plants were grown in two separate greenhouse and field experiments. Continuous frequency distributions for eight sugar traits were observed in greenhouse and field populations indicating quantitative inheritance for the traits. Sucrose was positively correlated with TS and TSS, whereas it was negatively correlated with fructose and glucose. Of 350 markers on the linkage map, 115 were significantly associated with at least one of the eight sugar traits in the greenhouse population based on simple linear regression. Eight QTL affecting sucrose were found on linkage groups (LGs) 4, 5, 6, 7, 8, and 9. Six QTL on different LGs were significant in a stepwise multiple regression (SMR) analysis where the full model explained 30% of the total variation for sucrose. We detected 12 QTL for each of two individual sugar components, fructose and glucose, on the map. The SMR analysis indicated that eight and six QTL accounted for 41% and 42% of the total fructose and glucose variation, respectively. Nine QTL for TS were observed on LGs 1, 4, 6, 7, 8, and 9. Markers associated with five QTL on LGs 4, 6, 8, and 9 were significant in the SMR, and explained 31% of the total variation for TS. Regions on LGs 1, 2, 4, 5, 6, 7, and 8 were related to four or more sugar traits, and confirmed in the field population. This suggests that in this cross, sugar traits are controlled partially by the same QTL. We identified common sugar QTL on LGs 2, 3, 4, 6, and 11 derived from ‘Deltex’ and ‘Dulce’ and unique sugar QTL derived only from ‘Deltex’. These markers linked to the sugar synthesis QTL could be useful in melon breeding for improving the mature fruit sweetness.