Chromosomal regions carrying QTLs for sugar content or related traits (Brix°, fructose, glucose, or sucrose content), based on 14 populations involving eight different species (Patersonet al. 1988,1990,1991; Azanza et al. 1994; Eshed and Zamir 1995; Goldman etal. 1995; Grandillo and Tanksley 1996b; Tanksley etal. 1996; Fulton etal. 1997, 2000, 2002a; Bernacchi etal. 1998b; Chen etal. 1999; Saliba-Colombani et al. 2001; Doganlar et al. 2002c; Causse et al. 2004; Frary et al. 2004) are summarized in Fig. 7. From three to 19 QTLs were detected per progeny, with a total of 95 QTLs concentrated in 56 chromosomal regions. For the majority of QTLs the wild species alleles increased the trait value. In 28 regions, QTLs were detected in more than one population, and may possibly correspond to the same QTL. But the large number of regions involved suggests that manymech-anisms are responsible in increasing fruit sugar content. The same results were obtained for acid content (Causse et al. 2002, 2004; Fulton et al. 2002a), with only a few regions common to acid and sugar content. In contrast, frequent colocations between QTLs for sugar content and fruit weight (Grandillo etal. 1999) with opposite allelic effects could be detected, suggesting pleiotropic effects of some common QTLs.
Levin et al. (2000) described a locus, Fgr, that modulates the fructose-glucose ratio in mature fruit, with a S. habrochaites allele yielding a higher ratio. More recently, Levin et al. (2004) showed that alleles of S. habrochaites at two loci interacted to increase this ratio. These loci remain to be characterized.
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