RIBOSOMAL-RNA 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 characteri ze rRNA metabolism during the development of microspore mother cells i n the sugar beet anther (Beta vulgaris L.). In the course of meiotic p rophase and early postmeiotic interphase the morphological characteris tics of the nucleoli are typical of low or no transcriptional activity and a low rate of rRNA processing. However, we found evidence of an a pparent increase in the relative numbers of 18 S rRNA transcripts in s ome stages of microsporogenesis. This was found in both the nucleoli a nd cytoplasm of pachytene meiocytes, and in later stages there was a s pectacular accumulation of rRNA transcripts in nucleoli of the tetrad cells. Quantitative data are analyzed in the light of morphometric fin dings in the cells and their compartments to elucidate the degree to w hich changes in cell size are related to changes in labeling density a nd 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.