Structure and molecular evolution of the ribosomal DNA external transcribed spacer in the cockroach genus Blattella

The ribosomal DNA (rDNA) cluster of insects contains several hundred repeating structural-functional units and, therefore, is a typical example of a multigene family. Eukaryotic ribosomal RNA (rRNA) genes (18S, 5.8S, and 28S like) are arranged in tandemly repeated clusters in the nucleolus organizers, separated by several spacers, namely the nontranscribed spacer, the external transcribed spacer (ETS), and the internal transcribed spacers. The nucleotide sequences of the ETS of the three closely related Blattella cockroach species, Blattella germanica (Linnaeus, 1767), Blattella asahinai (Mizukubo, 1981), and Blattella lituricollis (Walker, 1868), were determined and compared. The three species had relatively similar ETS lengths, and sequence differences among them could be explained by two types of rearrangements, namely deletions of subrepeats and nucleotide substitutions. Minor ETS variants in B. germanica differed from the major variant in the same way that the major ETS variants of the three Blattella species differed from each other. Concerted evolution and the birth-and-death models, which are often invoked to explain the diversity and evolution of the multigene families of rDNA clusters, are discussed in the light of our data. A new model is proposed to explain the evolutionary reorganization of the ETS region: evolution of rDNA by "magnification-and-fixation" is characterized by magnification of minor subrepeats, which become adaptive in a new rapidly changed environment, and subsequent fixation of this variant type as a major component of the multigene family of a new species.