DIFFERENTIAL INITIATION OF rRNA GENE ACTIVITY IN PROGENIES OF DIFFERENT BLASTOMERES OF EARLY XENOPUS EMBRYOS: EVIDENCE FOR REGULATED SYNTHESIS OF rRNA.

The dorsal and ventral blastomeres of 4-celled embryos of Xenopus luevis were separated. During the next 14 hr in cultnre, the cell numbers of the progeny cell aggregates of the dorsal and ventral blastomeres respectively, were the same. Synthesis of 4 S RNA started after about 4 hr of culture in both kinds of progeny cell aggregates. However, a clear-cut difference was found in the time when rRNA synthesis started in these aggregates: it began after 10 hr in cell aggregates derived from the dorsal blastomeres, and after about 14 hr in those derived from the ventral blastomeres. After this, rRNA synthesis became more and more active in both types of aggregates. This is the first demonstration of differential initiation of rRNA synthesis in aggregates derived from different blastomeres but containing the same number of cells. The results provide unequivocal evidence for developmental regulation of rRNA synthesis.