Cytoplasmic Nucleoloids of Lilium Male Reproductive Cells Contain rDNA Transcripts and Share Features of Development with Nucleoli

Abstract

Nucleolus-like inclusions in the meiocytes of Lilium sp. have been investigated at the light and electronmicroscope levels, with respect both to their composition and their formation during the meiotic divisions. Those present in the cytoplasm (cytoplasmic nucleoloids) first appear as small structures late in anaphase I. Coincidentally, small nuclear nucleolus like inclusions (NLBs) develop on the surface of the chromosomes, and sometimes in the region between chromosomes as they are carried into the daughter nuclei. The cytoplasmic nucleoloids increase in size from late anaphase I to the dyad stage, but disappear before metaphase II. They redevelop late in anaphase II and persist, increasing in size, until the tetrad stage. The NLBs also increase in size over this period. Ultrastructural investigation indicates that both the nascent nucleoloids and NLBs are fibrous in texture as they arise in late anaphase. Elsewhere in the cytoplasm, however, large accumulations of amorphous electron-opaque material also aggregate. This material becomes associated with the nucleoloids and the NLBs during the period of their enlargement. Nucleoloids late in development, and post-telophase nucleoli are not invested by these amorphous masses. Interestingly, cytoplasmic nucleoloids and NLBs react in an identical fashion to a range of cytochemical treatments and, using in situ hybridisation at the electron-microscope level, they have been demonstrated to contain RNA sequences homologous with a wheat ribosomal gene probe. Cytoplasmic nucleoloids are thus interpreted as following an identical developmental pathway to nucleoli except in that, perhaps owing to high levels of rDNA transcription during meiosis, they condense in the cytoplasm rather than the nucleoplasm.