SYNOPSIS. Studies on the nudeohis in various species of Amphibia, including the Mexican axolotl, have contributed to an understanding of the function of the nucleolar organizing region. In the first part of this paper we have reviewed some of the research on nucleolar variants in the axolotl. These variants were recognized in different strains of laboratory reared animals. The phenotypes include diminished nucleolar size and possible alterations in the levels of rDNA. The expression of several

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... f these variant nucleolar phenotypes does not, however, reflect a simple deletion of rRNA genes. Evidence from analyses of several mutant and apparently normal strains indicate that some animals have relatively high levels of rRNA redundancy, while others, including some animals which display variant nucleolar sizes, display "normal" levels. Data on the inheritance of rRNA genome sizes in the axolotl is reviewed. The observation that intermediate levels of redundancy were detected in some crosses is discussed in terms of inheritance data from other organisms. In the second part of this review a variety of information, obtained from studies of various organisms, on nucleolar expression is discussed.