A comparison of the ribosomal DNA's of Xenopus laevis and Xenopus mulleri: the evolution of tandem genes

Abstract

The ribosomal DNA's of Xenopus laevis and Xenopus mulleri differ in their nucleotide sequences. Their base compositions are 67 and 65% G + C, respectively. The nucleotide sequences which code for 28 s and 18 s ribosomal RNA in the rDNA of the two species could not be distinguished by their size, base composition, or fidelity of hybridization. The difference between the two ribosomal DNA's lies in their spacer regions. Complementary RNA transcribed from spacer regions hybridized to heterologous rDNA at less than 20% of the level with which it hybridized to homologous rDNA. Partly denatured molecules of the two rDNA's were further compared by “denaturation mapping” in the electron microscope. The repeat length in X. mulleri rDNA is about the same as that in X. laevis rDNA. While the regions presumed to code for the 40 s precursor of rRNA have very similar denaturation patterns in the ribosomal DNA of X. laevis and X. mulleri, the patterns in the spacer regions are very different. Both physico-chemical and electron microscopic analyses show that the repeating sequences in X. mulleri ribosomal DNA are very similar, if not identical, to each other with respect to size and nucleotide sequence, a fact previously demonstrated for the rDNA of X. laevis. The multiple spacer sequences interspersed between the 18 s and 28 s gene regions have evolved together; the 18 s and 28 s genes have remained unchanged. Although the spacer regions differ greatly between species, they are very similar if not identical within each species. We conclude that a “correction” mechanism must operate to spread a mutation in one spacer sequence to the neighboring spacers faster than new changes arise in these genes. This phenomenon, which occurs within a single organism, is referred to as “horizontal” evolution, in contrast to “vertical” evolution which is the spread of a mutation through a breeding population.