Fgr, a major locus that modulates the fructose to glucose ratio in mature tomato fruits

Agenetic trait determining the ratio of fructose to glucose in mature tomato fruits is described. A backcross breeding program based on the interspecific cross of Lycoparsicon hirsutum and L. esculentum yielded stable genotypes with a high ratio of fructose to glucose (>1.5:1) compared with the approximately equimolar ratios found in L. esculentum. Two inter-simple-sequence repeat (ISSR) DNA sequences, highly associated (20 <LOD score <21) with the trait, were identified. The markers were found to be less associated with either glucose or fructose levels individually (2 <LOD score <3) and were statistically unlinked to total sugars and total soluble solids (TSS). These two ISSR bands segregated in a dominant fashion and were found to be allelic to each other, one associated in coupling and the other in repulsion with the trait of high fructose to glucose ratio. Both ISSR markers were mapped to the centromeric region of tomato chromosome 4. Quantitative analysis of the identified locus, based on data from segregating F-2, BC and F-3 populations from the cross between genotypes having high and low fructose to glucose ratios, suggested that the L. hirsutum-derived allele (Fgr(H)), which increases the fructose to glucose ratio, is partially dominant. Fgi(H) leads to an increase in fructose levels and a subsequent decrease in glucose levels, with no effect on total hexose levels. Accordingly, we conclude that the Fgr locus modulates the partitioning of hexose sugars between fructose and glucose, with no effect on total sugars or TSS.