Breeding for fruit quality traits is complex due to the polygenic (quantitative) nature of the genetic control of these traits. Therefore, to improve the speed and efficiency of genotype selection, attention in recent years has focused on the identification of quantitative trait loci (QTLs) and molecular markers associated with these QTLs. However, despite the huge potential of molecular markers in breeding programmes, their implementation in practice has been limited by the lack of information on the stability of QTLs across different environments and within different genetic backgrounds. Here, we present the results from a comprehensive analysis of the inheritance of fruit quality traits within a population derived from a cross between the apple cultivars 'Telamon' and 'Braeburn' over two successive seasons. A total of 74 different QTLs were identified for all the major fruit physiological traits including fruit height, diameter, weight and stiffness, flesh firmness, rate of flesh browning, acidity, the ºBrix content and harvest date. Seventeen of these QTLs were 'major' QTLs, accounting for over 20% of the observed population variance of the trait. However, only one third (26) of the identified QTLs were stable over both harvest years, and of these year-stable QTLs only one was a major QTL. A direct comparison with published QTL results obtained using other populations (King et al., Theor Appl Genet 102:1227-1235, 2001 Liebhard et al., Plant Mol Biol 52:511-526, 2003) is difficult because the linkage maps do not share a sufficient number of common markers and due to differences in the trait evaluation protocols. Nonetheless, our results suggest that for the six fruit quality traits which were measured in all populations, nine out of a total of 45 QTLs were common or stable across all population × environments combinations. These results are discussed in the framework of the development and application of molecular markers for fruit quality trait improvement.

No abstract

An F 1 progeny derived from a cross between the apple (Malus x domestica) cultivars Telamon and Braeburn was used to identify quantitative trait loci (QTL) linked to the vitamin C (L-ascorbate [L-AA]) contents of fruit skin and flesh (cortex) tissues. We identified up to three highly significant QTLs for both the mean L-AA and the mean total L-AA contents of fruit flesh on both parental genetic linkage maps, confirming the quantitative nature of these traits. These QTLs account for up to a maximum of 60% of the total population variation observed in the progeny, and with a maximal individual contribution of 31% per QTL. QTLs common to both parents were identified on linkage groups (LGs) 6, 10, and 11 of the Malus reference map, while each parent also had additional unique QTLs on other LGs. Interestingly, one strong QTL on LG-17 of the Telamon linkage map colocalized with a highly significant QTL associated with flesh browning, and a minor QTL for dehydroascorbate content, supporting earlier work that links fruit L-AA contents with the susceptibility of hardfruit to postharvest browning. We also found significant minor QTLs for skin L-AA and total L-AA (L-AA 1 dehydroascorbate) contents in Telamon. Currently, little is known about the genetic determinants underlying tissue L-AA homeostasis, but the presence of major, highly significant QTL in both these apple genotypes under field conditions suggests the existence of common control mechanisms, allelic heterozygosity, and helps outline strategies and the potential for the molecular breeding of these traits.Vitamin C (L-ascorbic acid [L-AA]) is essential for all living plant tissues. Apart from well-known functions in oxidative stress defense, associated with its antioxidant properties and its abilities to detoxify reactive oxygen species, it also has important roles in the regulation of plant cell growth and expansion, photosynthesis, as well as hormone functions (for review, see Davey et al., 2000;Smirnoff, 2000). Even though nutritional deficiencies are rare in modern western cultures, it is generally recognized that dietary L-AA also has important health benefits for the consumer, and an increased intake of vitamin C has been associated with a decreased incidence of several important human diseases and disorders (Carr and Frei, 1999; DemmigAdams and Adams, 2002;Hancock and Viola, 2005). Furthermore, in meat-poor diets, dietary L-AA can contribute to the improved uptake of iron and zinc, which are the major micronutrient deficiencies worldwide (Frossard et al., 2000). In apple (Malus x domestica) and other fruit species, there are indications that increased antioxidant contents and in particular higher L-AA levels may be associated with improved fruit postharvest properties (Barden and Bramlage, 1994;Veltman et al., 1999Muckenschnabel et al., 2001;Venisse et al., 2001;Franck et al., 2003aFranck et al., , 2003bHodges et al., 2004;Hancock and Viola, 2005). In addition, L-AA has been implicated in resistance to a wide range of biotic and abiotic stresses (Davey ...

No abstract