Observations on early germ cell development and premeiotic ribosomal DNA amplification in Xenopus laevis

The origin of premeiotic ribosomal DNA (rDNA) amplification in germ-line cells of Xenopus laevis has been examined using in situ RNA-DNA hybridization on cytological preparations, tritiated thymidine autoradiography, and isopycnic density gradient centrifugation. Primordial germ cells (PGC), from the time they first become localized in the genital ridge at day no. 4 of development, until approximately day no. 22, remain in an extended interphase condition. During this time PGC do not incorporate tritiated thymidine, have near diploid levels of rDNA as demonstrated by cytological RNA-DNA hybridization, and possess only one or two nucleoli. Starting on day no. 22-24, mitosis, sexual differentiation, and rDNA gene amplification all begin in the germ cells. Multiple nucleoli also make their appearance at this stage. Ribosomal DNA amplification continues in gonial cells as long as they remain mitotically active. Amplified copies of rDNA are lost from germ cells at the onset of meiotic prophase. This loss is probably permanent in the male germ line, but variable and temporary in the female germ line. Early gonial cells in the ovary have been deduced to have an average cycle time for each mitotic division of between 3.8 and 4.3 days at a temperature of 21 degrees C. Some oogonia appear to divide only four times before entering meiotic prophase, while the average during the initial wave of germ cell division is nine. Finally, a satellite DNA has been isolated from adult testes which has a density in neutral cesium chloride corresponding to the density of amplified oocyte rDNA. This satellite is not present in DNA isolated from somatic tissues of Xenopus.