Evidence for Heterochromatin Involvement in Chromosome Breakage in Maize Callus Culture

ng from delayed separation of chromatids and typical bridges were observed in Feulgen preparations. The analysis of C-banded anaphases showed that delayed chromatids were held together at heterochromatic knob sites (primary event), and the presence of typical bridges with and without bands corresponding to knobs. These events suggest the occurrence of breakage-fusion-bridge (BFB) cycles initiated by chromosome arms broken during the primary event. Additional evidence for such a mechanism was the presence of gross aberrations involving chromosome 7, detected in several C-banded metaphases of some cultures. It is hypothesized that such aberrations are duplication deficiencies produced by BFB cycles and chromosome healing that would have occurred after some cell divisions.