Induction of Univalent Chromosomes in Explanted Lily Microsporocytes by Inhibitors of DNA Synthesis

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 G2 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.