MICRONUCLEOLI IN THE XENOPUS GERMINAL VESICLE

The germinal vesicle of the Xenopus oocyte contains 1500 or more extra chromosomal nucleoli that are assembled on amplified copies of the rRN A genes. Many of these nucleoli have diameters of 10-15 mu m, but some are much smaller, ranging down tn 1 mu m or less. Morphologically the smaller nucleoli or ''micronucleoli'' resemble the similarly sized B snurposomes, but they can be recognized with appropriate antibody prob es (e.g., anti-nucleolin and anti-fibrillarin). We describe here a sen sitive fluorescent staining technique that uses avidin and propidium i odide to visualize the rDNA in the amplified nucleoli. Many large nucl eoli stain about as brightly as haploid yeast nuclei on the same slide s. They presumably contain about 12 Mb of DNA, equivalent to 900 rDNA repeats. The smallest micronucleoli display only a tiny dot of stain, which must correspond to relatively few rDNA repeats.