Nucleolar DNA in oocytes of Salmo irideus (Gibbons).

The amplification of ribosomal genes has been studied in oocytes from Salmo irideus. In situ nucleic acid hybridization showed that the synthesis of nucleolar DNA begins in oogonium and proceeds slowly through leptotene and zygotene when a small amount of extrachromosomal nucleolar DNA is produced. In early pachytene there is a rapid build-up of nucleolar DNA demonstrable by rapid incorporation of tritiated thymidine. Synthesis stops completely in early diplotene when nucleolar DNA becomes dispersed over the inner surface of the nuclear envelope in the form of small Feulgen-positive granules. Photometric measurements of Feulgen stained nuclei showed that the final amount of amplified nucleolar DNA synthesized in each oocyte is approximately 20 mug. The amplified DNA does not form a heterochromatic mass. The buoyant density of the amplified nucleolar DNA calculated from analytical centrifuge tracings in relation to DNA from Micrococcus luteus (rho = 1.731 g cm-3) is 1.715 g cm-3 and corresponds to a G+C content of 57%. There are indications that the buoyant density of the somatic nucleolar DNA is lower than that of amplified nucleolar DNA. Similarities and differences between ribosomal gene amplifications in oocytes of Salmo irideus and the corresponding process in Xenopus are discussed.