Nucleolar evolution and coiled bodies during meiotic prophase in Olea europaea: differential localization of nucleic acids.

We studied the ultrastructural evolution of the nucleolus during meiotic prophase in olive microsporocytes. During prophase, nuclear bodies morphologically similar to coiled bodies were observed. The nucleic acid composition of these bodies was examined in microsporocytes using electron microscopic techniques with EDTA preferential ribonucleoprotein staining, anti-DNA immunolabeling, the in situ terminal deoxynucleotidyl transferase-immunogold technique, and in situ hybridization with 18S rRNA and U3 snoRNA digoxigenin-labeled probes. The ultrastructural appearance of the meiocyte nucleolus indicated a low level of activity from the early prophase stage: the granular component was practically absent and nucleoli were constituted almost exclusively by dense fibrillar component containing large fibrillar centers that lacked chromatin inclusions. However, the appearance of reactivation vacuoles in the nucleolus during zygotene and high levels of rRNA in the nucleoplasm during pachytene support the presence of a peak in rRNA synthesis. Our results also show that the nuclear bodies that appear during prophase I are ribonucleoproteinaceous in nature; neither DNA nor ribosomal RNA were detected. The presence of U3 snoRNA, as shown by in situ hybridization in nuclear bodies from plant material, is also evidence that these structures are coiled bodies. We suggest that coiled bodies are involved not only in pre- and post-splicing events but also in the storage, transport or recycling of rRNA maturation elements.