Kernels were imbibed in distilled water for 24 hr, then treated with 3 x 10-3 M methylmethanesulphonate (MMS) for 1, 2 or 4 hr separately. Anaphase bridges and/or fragments were found 48-96 hr from the end of treatment (Table 1) and even after 96 hr recovery from 4 hr treatment, 11.8% anaphase configurations were seen to be aberrant. These are chromatid changes, present in meristems many of whose cells have undergone 2-3 cycles since MMS treatment. Evidence that at least 2 cell cycles have occurred since treatment is this: i) metaphases/anaphases with non-congressed chromosomes were present 44 hr after treatment; ii) 24-72 hr later metaphases with 17-22 chromosomes, i.e. numerical aneuploids, were present (Table 2). The aneuploid cells had undergone a replication cycle and were dividing again. The presence of 2n + 1 and 2n-1 metaphases, in approximately equal numbers, confirms that the cells with non-congressed chromosomes divide again, and accordingly they may be capable of initiating stable lineages of aneuploid cells. Many reports emphasize one particular advantage of chemical mutagens; i.e., they induce high frequencies of point mutations and low frequencies of chromosome aberrations. The opposite result occurs with ionizing radiations. The results from the present study show that chromatid aberrations and chromosome non-congression are frequent after MMS treatment. This response may be specific to root meristems and may involve a stage that is particularly sensitive to MMS; i.e., the formation of alkylated bases such as 06-methylguanine in DNA of meristematic cells as they are proceeding from G1 into S of the first cell cycle in the root of germinating corn. Continued formation, over 2-3 cell cycles, of cells with aneuploid complements means that the root meristems are, at least for some days, complex chimeras. Some complements will lead to cell death or to failure to proliferate: this will result in diplontic selection and therefore to a reorganization of meristems. Cells capable of initiating lineages will be able to repopulate disrupted meristems and to produce new growth. The chromosome changes seen in meristems 2-3 cell cycles after treatment provide a basis for diplontic selection; for the establishment of a new meristem from, perhaps, a few initial cells.
Ji-Ping Zhao and Douglas Davidson
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