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 tomat o fruits is described. A backcross breeding program based on the interspeci fic cross of Lycoparsicon hirsutum and L. esculentum yielded stable genotyp es with a high ratio of fructose to glucose (>1.5:1) compared with the appr oximately equimolar ratios found in L. esculentum. Two inter-simple-sequenc e repeat (ISSR) DNA sequences, highly associated (20 <LOD score <21) with t he trait, were identified. The markers were found to be less associated wit h either glucose or fructose levels individually (2 <LOD score <3) and were statistically unlinked to total sugars and total soluble solids (TSS). The se two ISSR bands segregated in a dominant fashion and were found to be all elic to each other, one associated in coupling and the other in repulsion w ith the trait of high fructose to glucose ratio. Both ISSR markers were map ped 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 po pulations from the cross between genotypes having high and low fructose to glucose ratios, suggested that the L. hirsutum-derived allele (Fgr(H)), whi ch increases the fructose to glucose ratio, is partially dominant. Fgi(H) l eads to an increase in fructose levels and a subsequent decrease in glucose levels, with no effect on total hexose levels. Accordingly, we conclude th at the Fgr locus modulates the partitioning of hexose sugars between fructo se and glucose, with no effect on total sugars or TSS.