Time course study of the chromosome-type breakage-fusion-bridge cycle in maize

The B chromosome of maize has been used in a study of dicentric chromosomes . TB-9Sb is a translocation between the B and chromosome 9. The B-9 of TB-9 Sb carries 60% of the short arm of 9. For construction of dicentrics, a mod ified B-9 chromosome was used, B-9-Dp9. It consists of the B-9 chromosome p lus a duplicated 9S region attached to the distal end. In meiosis, fold-bac k pairing and crossing over in the duplicated region gives a chromatid-type dicentric B-9 that subsequently initiates a chromatid-type breakage-fusion -bridge cycle. In the male, it forms a single bridge in anaphase II of meio sis and at the first pollen mitosis. However, the cycle is interrupted by n ondisjunction of the B centromere at the second pollen mitosis, which sends the B-9 dicentric to one pole and converts it from a chromatid dicentric t o a chromosome dicentric. As expected, the new dicentric undergoes the chro mosome-type breakage-fusion-bridge cycle and produces double bridges. A lar ge number of plants with chromosome dicentrics were produced in this way. T he presence of double bridges in the root cells of plants with a chromosome dicentric was studied during the first 10 wk of development. It was found that the number of plants and cells showing double bridges declined steadil y over the 10-wk, period. Several lines of evidence indicate that there was no specific developmental time for dicentric loss. "Healing" of broken chr omosomes produced by dicentric breakage accounted for much of the dicentric loss. Healing produced a wide range of derived B-9 chromosomes, some large and some small. A group of minichromosomes found in these experiments prob ably represents the small end of the scale for B-9 derivatives.