Rapid chromosomal assignment of multiple genomic clones in tomato using primary trisomics

In order to rapidly assign relatively large numbers of tomato genomic clones to specific chromosomes, we have developed the following approach: groups of five to eight clones from a single copy tomato library are pooled, nick-translated, and utilized as probes against Southern blots consisting of a panel of trisomic DNAs. Since the trisomic DNAs are digested with the same enzyme used to produce the genomic library (PstI), each hybridizing band can be related to a specific genomic clone and the relative intensities of the bands can be used to assign each to a specific chromosome. With this technique, we have assigned 52 clones to specific chromosomes and verified the assignment of 21 out of 23 by genetic mapping in a segregating F2 population. In addition to selecting clones according to chromosome, the idea of using multiple clones or "molecular darts" may have broader applications, such as screening for differences between the genomes of nearly isogenic lines.