This project examines the nature of the ribosomal genome (rDNA). In particular, we are concerned with a) the range of variability of the level of rDNA redundancy in various organisms and the relationship of this variability to synthetic function, to cell and genome size and to other possible regulating factors, b) the relationship of the number of ribosomal genes at a particular site to the synthetic activity of that site, in particular, to see if competitive success in cells with multiple NORs is determined by a higher level of redundancy, c) the inheritance of redundancy level, especially in Ambystoma mexicanum crosses involves heterogeneity in rDNA amounts on the two NORs, d) the pattern of synthesis of rDNA in early development of A. mexicanum, e) the relationship of rDNA amount and location of the NOR in translocation mutants of Zea mays on the synthetic activity of the locus, as expressed by relative nucleolar sizes, and f) the base sequence divergence of rRNA genes (transcribed into 18 and 28 sRNA) as compared to the divergence of transcribed spacers and nontranscribed spacers in Drosophila species. Except for the latter two projects, most of the work will involve amphibians, primarily the Mexican axolotl, A. mexicanum. A portion of the work will involve the use of a nucleolar mutant to determine the factors which result in the formation of a small nucleolus in the presence of a normal amount of rDNA.