The level of redundancy of ribosomal genes, and the relationship of this level to nucleolar formation at different stages of embryonic development, have been examined in the Mexican axolotl, Ambystoma mexicanum. Individuals from 4 inbred stocks were examined, as well as descendants from 2 nucleolar variants which, in the heterozygous condition, are distinguished by exceptionally small nucleoli. Ribosomal RNA-DNA hybridization assays show that one of the 4 wild type lines has only about one-third as much ribosomal DNA (rDNA) as the other three. One of the nucleolar variants has the same level of rDNA as the larger wild-type level; the other variant has the same amount as the smaller ribosomal genome line. Both original nucleolar variants arose as F1 progeny of crosses between a large rDNA genome line and the small genome line. Cytological examination of pregastrula stage embryos from wild type and nucleolar variant lines show that the lengths of the nucleolar organizer regions (NOR) and the sizes of nucleoli formed, are directly correlated with the amount of rDNA present at the nucleolar locus. During gastrulation of the nucleolar variants, however, a transition appears to take place and the amount of rDNA ceases to be the determining factor in nucleolar size. After late gastrula, heterozygous progeny resulting from crosses of either large rDNA genome or small rDNA genome wild type individuals with either nucleolar variant line, have a small and a large nucleolus. The factor or factors associated with this apparent lack of competitive ability of the variant NOR, when opposed to a normal NOR, are unknown. It might be suggested that since the chromosomal alterations which produced the nucleolar variants in both cases eliminated the gene determining the dark colour pattern, they could at the same time have eliminated other genetic material.

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