Advanced Initiation of Synchronous Mitoses in Physarum polycephalum Following UV-irradiation

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Abstract
Irradiation of multinuclear synchronous plasmodia of Physarum polycephalum with UV-light (2000 to 7000 ergs/mm2, λ max 2537 Å) at any time during the first two thirds of the mitotic cycle (S-period and early G2-period) delays the onset of the next nuclear mitosis by 1 to 3 hours (10 to 30% of the normal cycle). The initial delay of the mitotic schedule is overcompensated by the advanced initiation of subsequent mitoses. This remarkable shortening of the nuclear division cycle very likely is related to the selective destruction of some irradiated nuclei during the first postirradiation S-period. Since the increase of total protein is little affected, the ratio of protein/DNA (cytoplasm/nuclei) is increased after the UV treatment. A model is discussed suggesting that the onset of nuclear mitosis is controlled by a cytoplasmic factor which accumulates during interphase at a rate proportional to the increase of the total plasmodial mass.

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