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ABSTRACT. The cytological effects of hydroxyurea (HU), an inhibitor of DNA synthesis, were examined on mitotic and meiotic divisions in lily microsporocytes. Microsporocytes in premeiotic stages and meiotic prophase were cultured in vitro for discrete periods in the presence of the inhibitor at various concentrations.Hydroxyurea interfered with mitotic development if administered to cells during the late S or early G2 phase of premeiosis, when cells undergo mitotic division by explantation. When applied during leptonema, the main effects were the suppression of meiotic development and the production of sticky chromosomes.The affected cells progressed through early prophase without showing any gross cytological abnormalities ; the inhibition of DNA synthesis by hydroxyurea during zygonema did not interfere with synapsis. On further culture, cells did not progress beyond prophase; but remained suspended in some stage of late prophase. Chromosome stickiness was the response of cells cultured with hydroxyurea during pachynema for a prolonged period. The relevance of these observations to microsporocyte development is discussed.

ABSTRACT. Using toyocamycin (TYM), an antimetabolite of RNA synthesis, we examined the significance of RNA synthesis during meiotic prophase. Lily microsporocytes at various stages of meiotic prophase were cultured for discrete intervals in the presence of the inhibitor at various concentrations.Cytological observations showed strong interference with the meiotic process of the inhibition of RNA synthesis at all meiotic stages up to early diakinesis.The main effects of inhibiting RNA synthesis were the suppression of meiotic development and abnormal chromosome segregation.When this inhibitor was applied to cells in leptonema or zygonema, zygonema arrest occurred. When it was applied in pachynema or diplonema, the cells did not progress beyond prophase, but remained suspended in some stage of late prophase. When the inhibitor was administered at half the concentration that induced suppression of meiotic development, the response of the cells was a delay in meiotic progression, but chiasma formation and chromosome segregation were almost always affected. These results demonstrate that RNA synthesis during meiotic prophase is essential to meiotic development.The purpose of this paper is to examine the relationship between RNA synthesis and the behavior of meiotic chromosomes. The occurrence of RNA synthesis during meiotic prophase was first demonstrated autoradiographically by Taylor (15), and his conclusion subsequently was confirmed by biological analysis (2). These and other studies, however, have not been concerned specifically with two distinctive events in meiosis, chromosome pairing and crossing over; thus, it is necessary to establish whether the RNA synthesized during meiotic prophase has an essential role in the meiotic process.Attempts have been made to survey the effects of the inhibitors of RNA synthesis on explanted lily microsporocytes at different stages of meiosis. An outline of the cytological effects was reported in an earlier publication (10). Among the results, the effect of toyocamycin (TYM) on meiotic cells was noteworthy because of its apparent effectiveness at all stages of meiotic prophase and its distinctive effect on the synaptic event. We therefore examined the cytological effect of TYM on microsporocytes during meiotic prophase more thoroughly with special reference to

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ABSTRACT. The cytological effects of nalidixic acid and novobiocin, inhibitors of DNA synthesis, on meiotic division in explanted microsporocytes of Lilium longiflorum were examined. Microsporocytes in the late G2 phase of premeiosis and in the early meiotic prophase were cultured in vitro for discrete periods in the presence of various concentrations of one of these inhibitors. The main effects of 0.4 mM nalidixic acid applied during leptonema and zygonema were suppression of meiotic development and the production of sticky chromosomes. At 0.1 to 0.2 mM, the meiotic rate was greatly reduced, and an application to cells in late G 2 phase or early leptonema greatly affected chromosome pairing and chiasma formation, eventually giving rise to univalent or highly achiasmatic chromosomes.Electron microscopy showed that nalidixic acid does not interfere with ongoing synapsis, but has its effect on chiasma formation by suppressing the initiation of formation of synaptinemal complexes. These results are evidence that this drug interferes with the mechanism of initiation of synapsis, but not with the maintenance of chromosome integrity. Similar results were obtained with microsporocytes exposed to novobiocin at concentrations of 0.005 to 0.02 mM. The significance of DNA synthesis during the zygotene stage is discussed on the basis of these observations.