Ribosomal RNA gene number and sequence divergence in the diploid-tetraploid species pair of North American hylid tree frogs.

Hyla chrysoscelis (2n = 24) and H. versicolor (2n = 48) are a diploid-tetraploid species pair of tree frogs. Hybridization saturation of isolated 125I-labeled ribosomal RNAs (rRNAs) with filter-immobilized DNA shows that there are twice as many rRNA genes in the tetraploid as in the diploid. For the diploid, saturation occurs at 0.037%, from which it is calculated that there are about 618 copies of the (18 S + 28 S) rRNA genes per haploid genome. Analysis of the extent of hybridization and also the thermal stability of homologous and heterologous hybrids shows that considerably more base substitutions have occurred in the tetraploid rDNA genes than in the diploid since their divergence. This is interpreted to reflect either a relaxation of the gene regulatory "correction" mechanism hypothesized to be responsible for the maintenance of identical tandem rRNA genes in the tetraploid or a release of one gene set from the normal selective constraints.