Genetic mapping of fruit quality traits in apple (malus x domestica borkh.)
Apple fruit quality is of utmost importance to apple farmers and breeders in the selection and commercialization of new cultivars. Fruit size, colour, texture, firmness and taste are all traits that affect the quality of fruit. In this study the genetic contribution of these traits, and others were evaluated in order to generate the genetic markers required for the application of marker assisted selection in fruit quality breeding. Three mapping populations, ‘Prima’ x ‘Anna’, ‘Golden Delicious’ x ‘Priscilla’ and ‘Golden Delicious’ x ‘Anna’, consisting of 87, 87 and 141 respectively, were used in the study. Fruit samples were analysed, using a range of visual, physical and sensory measurements, over a period of three years, and the data was then correlated using statistical analysis. Traits analysed included stripe-ness, fruit colour, fruit size, fruit form, ground colour, russet, texture, fruit firmness, juiciness, sugar content, acidity, taste, skin toughness, %TSS, fruit mass and diameter. ANOVA detected significant levels of variation between the three families for all traits except taste and russet; while highly significant ‘within family’ variation was also observed for all traits in pre- and post-storage analyses, except for sugar content (sweetness) and fruit form. Within family variation also contributed the largest percentage towards the variance components of all traits. Heritability estimates found stripe-ness to be the most heritable trait, from subjective analyses, while heritability values ranged from 0.41 to 0.84 for instrumentally measured traits. The genetic maps for the three populations were generated using both published microsatellites and new EST-SSR and DART markers, using JoinMap 4.0". The integrated genetic linkage maps of ‘Prima’ x ‘Anna’, ‘Golden Delicious’ x ‘Priscilla’, ‘Golden Delicious’ x ‘Anna’ consisted of 398 (133 SSR and 265 DArT), 353 (80 SSR and 273 DArT) and 213 (87 SSR and 126 DArT) markers respectively. The maps were 1021.6cM, 1079cM and 1302.7cM in length, respectively. Location of quantitative trait loci (QTL) for 14 fruit quality traits was detected using MapQTL 5.0" and a total of 79 pre-storage and 60 poststorage QTLs were identified on the three mapping populations. Comparative genome analysis and the role of various genes on the outcome of fruit quality can now be investigated. Using the integrated genetic maps, and the QTLs identified, candidate markers associated with these QTL can be used for marker-assisted selection, to increase the speed and efficiency of the apple breeding program.