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

Abstract

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 These rounded skeins are tentatively assumed to represent the withdrawn organizers.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.