Molecular characterization of RAPD and SCAR markers linked to the Tm-1 locus in tomato

We have cloned and sequenced six RAPD fragments tightly linked to the Tm-1 gene which confers tomato mosaic virus (ToMV) resistance in tomato. The terminal ten bases in each of these clones exactly matched the sequence of the primer for amplifying the corresponding RAPD marker, except for one in which the 5'-endmost two nucleotides were different from those of the primer. These RAPD clones did not cross-hybridize with each other, suggesting that they were derived from different loci. From Southern-hybridization experiments, five out of the six RAPD clones were estimated to be derived from middle- or high-repetitive sequences, but not from any parts of the ribosomal RNA genes (rDNA), which are known to be tightly linked with the Tm-1 locus. The remaining clone appeared to be derived from a DNA family consisting of a few copies. These six RAPD fragments were converted to sequence characterized amplified region (SCAR) markers, each of which was detectable using a pair of primers having the same sequence as that at either end of the corresponding RAPD clone. All pairs of SCAR primers amplified distinct single bands whose sizes were the same as those of the RAPD clones. In four cases, the SCAR markers were present in the line with Tm-1 but absent in the line without it, as were the corresponding RAPD markers. In the two other cases, the products of the same size were amplified in both lines. When these SCAR products were digested with different restriction endonucleases which recognize 4-bp sequences, however, polymorphisms in fragment length were found between the two lines. These co-dominant markers are useful for differentiating heterozygotes from both types of homozygote.