Inactivation of the nucleolar apparatus during meiotic prophase in corn anthers

Additional nucleoli and NOR activity during meiotic prophase I in larch (Larix decidua Mill.)

Transcriptional activity was investigated in successive stages of prophase I (male meiosis) of larch meiocytes. Br-UTP incorporated into RNA was detected by light and electron microscopy. Two peaks of RNA synthesis were identified in the nucleolus. The first occurred during the zygotene-pachytene stage and the second (not previously described in plant meiocytes) in the diplotene. These processes correlated with a considerable increase in nucleolus volume during these periods. At the end of the zygotene, several perinucleolar structures lying close to each other and containing rRNA, argyrophilic proteins, U3 small nucleolar RNA, and fibrillarin were observed. The occurrence of newly formed RNA was also observed in these structures. This suggests that the observed perinucleolar structures correspond to the additional nucleoli known from animals.

rRNA distribution during microspore development in anthers of Beta vulgaris L. quantitative in situ hybridization analysis

We related changes in the ultrastructural organization of the nucleoli with the results of quantitative in situ hybridizations to characterize rRNA metabolism during the development of microspore mother cells in the sugar beet anther (Beta vulgaris L.). In the course of meiotic prophase and early postmeiotic interphase the morphological characteristics of the nucleoli are typical of low or no transcriptional activity and a low rate of rRNA processing. However, we found evidence of an apparent increase in the relative numbers of 18 S rRNA transcripts in some stages of microsporogenesis. This was found in both the nucleoli and cytoplasm of pachytene meiocytes, and in later stages there was a spectacular accumulation of rRNA transcripts in nucleoli of the tetrad cells. Quantitative data are analyzed in the light of morphometric findings in the cell and their compartments to elucidate the degree to which changes in cell size are related to changes in labeling density and distribution. The results are discussed in terms of rRNA synthesis, transport and degradation as processes involved in the regulation of rRNA within microsporocytes and microspores.

The nucleolus at prophase of meiosis in three plants: an ultrastructural study

The nucleolus and nucleolar organizing region have been studied with the electron microscope in respect of (a) the fine structure of the nucleolar organizer during zygotene in pollen mother cells ofPhaedranassa viridiflora, as compared with that seen in interphase nuclei of ovary wall and tapetum of this plant; (b) in relation to premature dissolution of the nucleolus at zygotene in pollen mother cells of an F1 hybrid ofLilium aureliensis×Lilium henryiand the hybrid lily ‘Formobel’; (c) in respect of pseudonucleoli found in association with the nucleolus in pollen mother cells of all three plants. In the pollen mother cells of thePhaedranassa, as distinct from a simple arrangement of intranucleolar chromatin observed in the nucleoli in interphase nuclei of tapetum and ovary wall, the organizer is shown to consist of rounded electron-opaque filamentous aggregations about 40-50 nm in diameter, joined by a fine filament seemingly as in a string of beads, and, toward the periphery of the nucleolus, arranged equidistant from each other in columns originating from the organizing chromosomes. Within the core of the nucleolus the aggregations, though still joined, were widely dispersed and contained within lacunae. Axial cores were closely aligned with the organizer, but the completion of the synaptonemal complex in the organizing region was apparently delayed. The premature dissolution of the nucleoli in the pollen mother cells of the two lilies led to a displacement of their characteristic zones: the peripheral particulate zones were dispersed and the weakly staining organizing region became external to therelicnucleolar cores. The relegated organizing regions, fringed lightly at the sides by chromatin, invariably contained 1-2 rounded areas comprised of fine filaments with an electron opacity equivalent to that of the coarser fibrils in the chromatin of the organizing chromosomes to which they were seen to be joined. These rounded skeins are tentatively assumed to represent the withdrawn organizers. No synaptonemal complexes or relics of its components were observed in the organizing regions. Round bodies (pseudonucleoli) with a maximum observed incidence of 2 per nucleus inPhaedranassaand 1 per nucleus in the lilies were frequently seen. They were either totally enclosed within the nucleolus, as was usually the rule in the former, or partly engulfed by it as in the latter. They were composed of filaments about 25 nm wide and their diameter ranged between 0.6-2.5 μm. Their close association with the nucleolus and confinement to meiosis, suggests that they are formed on the nucleolar chromosomes at loci close to the nucleolar organizer, which may be metabolically active only at prophase of meiosis. Various aspects concerning the nucleolar organizing region and organizer are discussed in the light of the present observations.