Composition and Behavior of the Nucleolar Organizer of the Nucleolar Variant of Axolotls; Review and Comparison to Other Systems

SYNOPSIS. Studies on the nudeohis in various species of Amphibia,including the Mexican axolotl, have contributed to an understandingof the function of the nucleolar organizing region. In the firstpart of this paper we have reviewed some of the research onnucleolar variants in the axolotl. These variants were recognizedin different strains of laboratory reared animals. The phenotypesinclude diminished nucleolar size and possible alterations inthe levels of rDNA. The expression of several of these variantnucleolar phenotypes does not, however, reflect a simple deletionof rRNA genes. Evidence from analyses of several mutant andapparently normal strains indicate that some animals have relativelyhigh levels of rRNA redundancy, while others, including someanimals which display variant nucleolar sizes, display "normal"levels. Data on the inheritance of rRNA genome sizes in the axolotlis reviewed. The observation that intermediate levels of redundancywere detected in some crosses is discussed in terms of inheritancedata from other organisms. In the second part of this reviewa variety of information, obtained from studies of various organisms,on nucleolar expression is discussed.