Intracellular RNA during meiosis in microsporangia of Taxus baccata

Using microfluorimetry the variations in the level of insoluble RNA in the nucleolus, nucleus and cytoplasm, have been followed during meiosis I in the microspore mother cells of Taxus baccata, in which meiosis extends over approximately six weeks. The concentration of RNA in nucleus and cytoplasm showed ordered changes during prophase, there being indications of a fall in the value in the nucleus at the beginning of prophase followed by a rise to a maximum at diakinesis. When the relative volumes of the cytoplasm were taken into account, and confidence limits applied, it became clear that there was no evidence of any significant loss of RNA from the cell during prophase. The amount in fact increased between zygotene and pachytene, and then showed little change until metaphase. With regard to movements between the compartments of the cell, the confidence limits do not allow firm conclusions, but the consistency of the data is in conformity with a slight movement from cytoplasm to nucleus at the time of formation of the synaptonemal complexes, followed by a fall in nuclear RNA and a rise in that of the cytoplasm. The data do not support the concept of a massive loss of RNA from the cytoplasm during prophase. The universality of this phenomenon is questioned, and the significance of the results is sought in the duration of meiosis I. THE CYTOPLASMIC changes which accompany meiosis have now been studied in a wide range of plants, and striking similarities have emerged. Examples are the fluctuation in ribosome frequency (and in the light microscope the intensity of the staining of the basophilia), and the transient dedifferentiation of the mitochondria and plastids. Investigation of the cytochemical features of these changes has been almost entirely confined to Lilium. The amount of ribonuclease-degradable RNA in the microspore mother cells, for example, has been estimated by Mackenzie, Heslop-Harrison and Dickinson (1967). Dickinson (1981) subsequently located RNA in the membrane-particle association found in the dedifferentiated plastids during prophase. Using high-resolution autoradiography, Porter, Bird and Dickinson (1982) investigated the incorporation of tritiated thymidine and uridine into the nuclei of spore mother cells, and Bird, Porter and Dickinson (1983) the incorporation of the same nucleosides into the cytoplasm. Using biochemical methods, and also 3H poly(U)+ as a radioactive probe, Porter, Parry and Dickinson (1983) have also studied fluctuations in poly(A)+ RNA during meiosis. The only other ' Received for publication 6 February 1986; revision accepted 19 August 1986. 2 Current address: Department of Cell Biology, John Innes Institute, Norwich NR4 70H, United Kingdom. genera investigated with respect to RNA appear to be Trillium and Cosmos. In Trillium the flux in RNA during meiosis parallels that seen in Lilium (Mackenzie et al., 1967). In Cosmos, Knox, Dickinson and Heslop-Harrison (1970) followed changes in the cytoplasmic RNA remaining after acetic-alcohol fixation, using cytophotometry following Pyronin Y staining. The results were similar to those obtained with Lilium, and the values showed a sharp fall between zygotene and pachytene. Evidence points to labelled nucleotides being unable to enter microspore mother cells in situ and this has been ascribed to the impermeability of the callosic wall to large molecules (Heslop-Harrison and Mackenzie, 1967). It seems likely, therefore, that RNA rendered mobile by partial or total degradation will be unable to leave the mother cell, and consequently depletion at one site may be accompanied by augmentation at another. It is significant that the fall in cytoplasmic RNA recorded in Lilium and Cosmos coincides with the contraction of the chromosomes and the appearance in the nucleus of the synaptonemal complexes. These are known to contain RNA (Esponda and Stockert, 1971; Westergaard and von Wettstein, 1972), and they are a significant component of the mid-prophase nucleus. Microsporogenesis in conifers is well suited to the exploration of possible movements of RNA within the meiotic cells since in some