RESUMEN
A pseudogene bearing the bulk of the 18S RNA gene was detected outside the rDNA cluster. It comprised irregularly distributed nucleotide substitutions as well as short insertions and deletions. No sequence alterations were observed in the 5' region of the pseudogene, whereas the frequency of substitutions and alterations per nucleotide number in the 3' region and in the middle of the sequence was 7.6% and 1.8%, respectively. The observed sharp irregularity in distribution of substitutions and alterations was considered the result of successive recombinations between the functional 18S rRNA gene and its diverged or damaged variants. This phenomenon provides experimental evidence that recombinations between the pseudogene and functioning repeats of rDNA are implicated in the mechanism of rDNA sequence correction. A segment of the pseudogene sequence was shown to contain substitutions primarily in regions coding for single-strand parts of the RNA molecule. The same segment contained a deletion and an insertion of a nucleotide, approximating it to the most of the studied eukaryotic 18S rRNA sequences. These observations allowed us to supposed that a structural rDNA variant, a fragment of which appears in the pseudogene sequence, is present in the genome. The data obtained suggest both the presence of 18S rDNA variants, and recombination between them, determining the concerted evolution of rRNA genes.