Chromosome breakage-fusion-bridge-cycle and phenotypic instability in isochromosome lines of tomato.

Cultivated tomato lines with 7-9 copies of extra chromosomes, which were fully heterochromatic, were found to have normal growth and reproduction. Cytological evidence suggested that all extra chromosomes in the 24 plants investigated were derived from a single isochromosome of the short arm of chromosome 2. In spite of their common origin, the individual extra chromosomes within and between plants varied considerably with respect to size and morphology. The morphological variation of these extra chromosomes was found to result from chromosome-or chromatid breakage-fusion-bridge-cycle (BFBC). In order to verify whether BFBC could induce "genomic shock" and instability, the phenotypes of more than 10,000 seedlings from some of the progeny were examined. There was a 3-to 4-fold increase in the occurrence of the number of chlorophyll variegated sectors in the progeny of plants with BFBC when compared to the control variety.