Construction of a Genetic Linkage Map and QTL Analysis of Fruit-related Traits in an F1 Red Fuji x Hongrou Apple Hybrid

A genetic linkage map of the apple, composed of 175 SSR and 105 SRAP markers, has been constructed using 110 F1 individuals obtained from a cross between the 'Red Fuji' Malus domestica and 'Hongrou' Malus sieversii cultivars, which have relatively high levels of DNA marker polymorphism and differ remarkably in fruit-related traits. The linkage map comprised 17 linkage groups, covering 1299.67 cM with an average marker distance of 4.6 cM between adjacent markers, or approximately 91% of Malus genome. Linkage groups were well populated and, although marker density ranged from 2.1 to 9.5 cM, just 10 gaps of more than 15 cM were observed. Moreover, just 12.5% of markers displayed segregation distortion. The present genetic linkage map was used to identify quantitative trait loci (QTLs) affecting fruit-related traits. 23 QTLs for ten fruit traits were detected by multiple interval mapping: 3 QTLs for Vc content, One QTL for single fruit weight, 2 QTLs for peel-phenols content, 2 QTLs for flesh-hardness, 2 QTLs for diameter, 6 QTLs for acid content, 1 QTL for sugar content, 2 QTLs for soluble solids content, 2 QTLs for flesh-phenols and 2 QTLs for brittleness. These QTLs were located on linkage groups C1, C2, C3, C5, C6, C7, C9, C10, C14 and C17, respectively. The phenotypic variations exhibited by each QTL ranged from 2% to 72%, and their LOD values varied from 2.03 to 8.93, of which five QTLs were major effect genes (R2 >= 10%). The tight linkage markers (*me2em7-460f, *MS01a03-180m, *me1em6-307m, *CH05c06-102f, *me1em8-423f) would be helpful to elucidate the molecular mechanisms of apple domestication and breeding in the future.