Indications for Changes in DNA Composition Correlated with Early Embryonic Differentiation (Triturus vulgaris,Urodela)*

During early embryogenesis of the newt Triturus vulgaris (early gastrula to early neurula) the DNA was characterized by various methods of genome analysis. By preparative CsCl density gradient centrifugation an AT-rich satellite fraction (about 10% of total DNA) was found in all developmental stages studied. An additional GC-rich fraction, with a portion of about 3% of the genome, could be visualized only in the yolk plug stages. Filter hybridization experiments with labelled ribosomal RNA indicate that the number of rRNA cistrons is by a factor of 1.69±0.17 higher in the DNA of the mid-gastrula than of the tailbud stage. In ethidium bromide-CsCl gradients this additional (possibly amplified) rDNA bands at the same density as linear DNA. The analytical cleavage of DNA with ten restriction endonucleases reveals an extreme heterogeneity of the Triturus genome. The methylation pattern of DNA, studied with the aid of the isoschizomers Hpa II and Msp I, remains constant during early development. The reassociation kinetics of DNA, recorded spectrophotometrically, show that the portion of DNA that reassociates until Cot 10 increases significantly from 20% in the gastrula stages to 30% in the early neurula stage.