Hundreds of tomato cultivars are grown worldwide. To protect the breeders rights the identification of cultivars is necessary. As the number of cultivars is increasing each year and the differences between cultivars are decreasing, ways to unambiguously identify cultivars should be developed. Molecular markers are indicated as tools for fingerprinting cultivars. Miller and Tanksley (1990) described the usefulness of RFLPs for identifying differences among genotypes.
Among cultivars only 6 % of the 40 probes tested were polymorphic on average. Still all nine cultivars in this study could be distinguished on the basis of one or more unique RFLPs. We investigated RFLPs between two old true breeding cultivars Moneymaker and Premier. Out of 95 probes tested with six restriction enzymes only one RFLP between the two cultivars was established (not shown). This low level of DNA variation limits the value of single copy probes for cultivar identification.
In eucaryotes, stretches of short, tandemly repetitive DNA frequently occur. These can be used as probes to detect repeat RFLPs. With such repeat probes hundreds of individual spots of human or mammalian DNA, separated by two dimensional gel electrophoresis, have been detected. The pattern of spots is unique for each individual (Uitterlinden et al., 1989 & 1991).
Three repeat probes were used to detect DNA polymorphisms between the tomato cultivars 'Premier' and 'Moneymaker'. After Southern hybridization with repetitive probes on average 19 bands in the region of 3-15 kbp were detected of which two were specific for 'Premier' and one for 'Moneymaker' (see Figure 1). The smaller fragments of 0.3-3 kbp were subjected to two dimensional electrophoresis which includes a separation of DNA fragments on the basis of length and base composition. On average per probe 115 spots were detected of which seven were unique to 'Premier' and two to 'Moneymaker'.
These results indicate the potential power of these short tandemly repetitive DNA probes to identify rare DNA variation between related tomato cultivars.
Literature Cited:
Miller J.C. & Tanksley S.D. (1990) Theor. Appl. Genet. 80,437-448
Uitterlinden, A.G., Slagboom, E.P., Knook, D.L. & Vijg, J. (1989) Proc. Natl. Acad. Sci. 86,2742-2746
Uitterlinden, A.G., Slagboom, E.P., Mullaart, E., Meulenbelt, I. & Vijg, J. (1991) Electrophoresis, in press.
Figure 1. Southern hybridization of genomic DNA from the tomato cultivars Moneymaker (MM) and Premier (PM) with three short, tandemly repetitive probes: 33.15, (CAC)n and 33.6. RFLP's are indicated by arrows.