Impaired chromosome segregation in plant anaphase after moderate hypomethylation of DNA

10(-6) M and 10(-5) M 5-azacytidine, demethylated around 9% and 17% of the 5-methylcytosine residues found in Allium cepa L. native DNA, respectively. Both treatments stimulated RNA synthesis in the cells of root meristems. On the other hand, the 10(-5) M treatment gave rise to multiple chromosomal anomalies in mitosis before any fall in the mitotic index was detectable, but no chromosomal breaks were ever seen. Serious lesions involved in chromatids and segregation in anaphase were preferentially found after hypomethylation of DNA sequences replicated in the second half of the previous S period: (i) sister telomeres remained unresolved at the cell equator while kinetochores had reached the poles, (ii) whole unsegregated chromosomes were pulled to one of the poles by obviously disfunctional kinetochores, resulting in an unbalanced distribution of chromatids, (iii) unsegregated chromosomes in other cells remained at the spindle equator as if kinetochores were nonfunctional, while cytoplasmic division took place before their migration to the poles. Frequently, a growing cytokinetic plate randomly cut the unsegregated chromosomes, giving rise to aneuploid nuclei. These anaphase failures are a firm basis to explain why the 10(-5) M treatment selectively depressed the rate of cell proliferation in these cells in the long run. On the other hand, if hypomethylation occurred at the first half of the previous S period, enlarged chromosomal segments were evident in most metaphases, while chromosome laggards and bridges were recorded in anaphase at rather similar frequencies after the different 5-azacytidine treatments. These data were consistently obtained both in the native mononucleate cells of meristems and in one subpopulation of synchronous cells labelled as binucleate by 5 mM caffeine.