Quantitative determination of amplified rDNA and its distribution during oogenesis in Xenopus laevis

The number of extra-chromosomal nucleoli and their rDNA content were determined during oogenesis in Xenopus laevis. The highly variable number of nucleoli (500 to 2,500) in oocytes of the same stage and from the same female or of different stages or from different females is not a measure of the extent of amplification. In all oocytes examined, a inversely proportional relation was found between the number of nucleoli in an oocyte and their mean rDNA content. These results indicate that there is no variation of the rDNA content of oocytes during oogenesis nor between oocytes of different females. The varying nucleolar numbers found in oocytes result thus from fusion and fission of pre-existing nucleoli. The determination of the rDNA content, in absolute units (35 pg), after amplification which occurs at the beginning of oogenesis, makes it possible to calculate the rDNA content of one nucleolus. This ranged from 0.7.10(-2) pg to 15.10(-2) pg, corresponding to about 500--11,000 cistrons of rDNA. No distinct size classes between these two extremes were observed.