X-ray and UV-induced chromatid aberrations: evidence for polynemic chromosomes?

Ikushima and Wolff have recently interpreted both their observation of chromatid aberrations in second and third mitoses following X-irradiation and the production of chromatid type chromosomal aberrations by UV light administered during the G1 phase of the cell cycle in terms of a polyneme model of eukaryote chromosome structure. They were led to do so, however, largely because of their X-ray data, which the interpreted as evidence for the induction of sub-chromosomal lesions (by G1 irradiation; sub-chromatid for G2 irradiation) which appear as chromatid type aberrations only in later divisions. We here report data from similar X-ray experiments in which synchronized Chinese hamster tissue culture cells were irradiated in either G1 or G2 and then scored for chromatid aberrations in their first, second and third post-irradiation mitoses. Our results do not show the effect reported by Ikushima and Wolff. We conclude that all of the data available of aberration production is compatible with a simple mononeme model of eukaryote chromosome structure.