We have recently completed studies on the inheritance and mapping of the sucrose accumulation trait from L. chmielewskii, as well as its introgression into tomato. All F1 L. esculentum x L. chmielewskii hybrids analyzed (148 plants) accumulate hexoses, hence sucrose accumulation is recessive in hybrids with tomato. In F2, BCF2 and backcrosses to the wild parent, the distribution of sucrose concentrations is bimodal, with low sucrose (high reducing sugar) and high sucrose (low reducing sugar) groups, indicating the trait can be treated in a qualitative fashion. Segregation for sugar type was consistent with monogenic control in the BC1F2 and RBC1 populations, whilst a deficiency of sucrose accumulators was noted in the F2 and BC2F2 populations (Table 1). Accordingly, the symbol sucr (sucrose accumulator) has been proposed for the locus. The map location of sucr was determined by scoring F2, F3 and BC1F2 populations for 95 RFLP and isozyme markers covering the genome. Only chromosome 3 markers showed association with sugar type, among which TG102 showed nearly perfect cosegregation, a fact confirmed in larger populations (Table 2). The molecular marker data reveals that the deficiency of sucrose accumulators in BC2F2 is due to distorted segregation at the locus rather than involvement of additional loci (Table 2). A cDNA for the tomato fruit acid invertase gene (TIV1) maps to the same region, with no recombination between it and TG102 detected in several populations representing over 1700 meiotic products. This suggests sucr represents an invertase allele with little or no expression in fruit, a conclusion supported by the aforementioned biochemical studies. Having introgressed sucr into cv. Hunt100, we are currently assessing its phenotypic effects. Our results so far indicate that sucrose accumulation increases soluble solids, although yield is depressed due to partial sterility. In the backcross to L. chmielewskii, the trait has no apparent effect on solids, while fruit size is reduced.
L. hirsutum is also a sucrose accumulating species (Miron and Schaffer 1991) and the trait is similarly under monogenic recessive control (see article in this volume by Stommel). To test if the same gene is involved, L. chmielewskii was hybridized with L. hirsutum (LA1777). The F1 hybrid accumulates sucrose to concentrations comparable to both parents, indicating allelism at the sucr locus. So far we have no evidence of differences between the two alleles, in fact we suspect a common origin since all of the green-fruited species studied to date accumulate sucrose, while the red-fruited ones accumulate hexose. Accordingly, distinguishing allele symbols would not seem appropriate at this time.
Table 1. Segregation for fruit sugar type in hybrids of domestic tomato (cv.UC204C) with the sucrose accumulating species L. chmielewskii accession LA1028. "RBC1" denotes the backcross of the F1 to the wild parent. Values represent the number of plants in each phenotypic category.
____________________________________________________ Fruit Sugar Type Ratio ________________ Generation Hexose Sucrose Tested Chi-square ____________________________________________________ F2a 85 6 3:1 15.5*** RBC1 133 120 1:1 0.6 BC1F2a 47 10 3:1 1.32 BC2F2 180 22 3:1 21.4*** ____________________________________________________ a Data from Yelle et al. (1991)Table 2. Association of fruit sugar type with the RFLP marker TG102 in L. esculentum x L. chmielewskii populations. (+ = esculentum allele, c = chmielewskii allele).
___________________________________________________ Fruit TG102 Genotype ___________________ Population Sugar +/+ +/c c/c ___________________________________________________ BC1F2 Hexose 12 35 0 Sucrose 0 0 10 BC2F2 Hexose 73 106 1 Sucrose 0 1 21 RBC1 Hexose - 133 0 Sucrose - 0 120 ___________________________________________________
Figure 1. Partial map of chromosome 3 showing the region surrounding the sucrose accumulator locus, sucr. Recombination estimates were obtained during introgression from L. chmielewskii into tomato. The markers are as follows: r, the yellow fruit flesh gene, an allele of which is present in L. chmielewskii; TIV1, the tomato fruit acid invertase gene (Klann et al. 1992); TG, tomato genomic probes obtained from Steve Tanksley (Cornell Univ.). Map distances are in cM.
Davies, J.N. (1966) Nature 209: 640-641.
Klann, E., S. Yelle, A.B. Bennett (1992) Plant Physiol. 99: 351-353.
Miron, D. and A.A. Schaffer (1991) Plant Physiol. 95: 623-627.
Yelle, S., R.T. Chetelat, M. Dorais, J.W. DeVerna, and A.B. Bennett (1991) Plant Physiol. 95: 1026-1035.
Yelle, S., J.D. Hewitt, N.L. Robinson, S. Damon, and A.B. Bennett (1988) Plant Physiol. 87: 737-740.