Differential synthesis of the genes for ribosomal RNA during amphibian oögenesis.

Molecular hybridization experiments have shown that the genes for ribosomal RNA are located in or near the nucleolus organizer in Drosophila melanogasterl' 2 and in the toad Xenopus laevis.3' 4 Under normal circumstances the number of organizers per genome will be characteristic of the organism, and DNA from various tissues should contain the same proportion of ribosomal genes. This conclusion has been confirmed for several tissues of the chicken.2 However, cytological evidence has long suggested that the nucleolus organizer undergoes a differential replication in oocytes of certain animals. Data supporting this view will be summarized later in this article. If such a differential replication occurs, oocyte DNA should be enriched with respect to the ribosomal genes. The cytological picture is particularly striking in ovaries of the toads Bufo and Xenopus, in which the differential synthesis occurs during pachytene. In recently metamorphosed toads the ovary contains a sufficiently high proportion of pachytene o6cytes to permit detection of the differential synthesis by biochemical means.5 This communication describes the ovarian DNA of Xenopus and demonstrates that it contains an excess of sequences coding for rRNA. Recently Brown and co-workers6' 7 have studied the DNA from older oocytes of Xenopus and have reached similar conclusions.