Eickbush T H, Burke W D, Eickbush D G, Lathe W C
Department of Biology, University of Rochester, NY 14627, USA.
Genetica. 1997;100(1-3):49-61.
R1 and R2 are non-long terminal repeat (non-LTR) retrotransposable elements that specifically insert in the 28S ribosomal RNA (rRNA) genes of insects. Using the Drosophila genus, which includes some of the best characterized insect taxa, we have conducted a number of studies on the evolution of these elements. We find that R1 and R2 are subject to the same recombinational forces that give rise to the concerted evolution of the rDNA units. The turnover of R1 and R2 elements can be readily documented in different strains of D. melanogaster using 5' truncated elements as restriction-length polymorphisms. This turnover leads to uniform populations of elements with nucleotide sequence divergence of different copies averaging only 0.23% for the R2 and 0.47% for the R1 elements. Molecular phylogenetic analysis of elements from 16 different species of Drosophila suggests that these elements have been stable components of the rDNA locus for the 50-70 million year history of the Drosophila genus. Using changes at synonymous positions within the protein-encoding regions as estimates of the baseline substitution rate, it could be shown that R1 and R2 are evolving at rates similar to that of typical protein encoding genes provided corrections are made for the low codon bias of the elements. R1 and R2 are clearly well-adapted for their existence in the rDNA units of their host.
R1和R2是非长末端重复（non-LTR）反转录转座元件，它们特异性地插入昆虫的28S核糖体RNA（rRNA）基因中。利用果蝇属（其中包括一些特征最明确的昆虫分类群），我们对这些元件的进化进行了多项研究。我们发现，R1和R2受到与导致rDNA单元协同进化相同的重组力影响。利用5'截短元件作为限制性长度多态性，可以很容易地在不同品系的黑腹果蝇中记录R1和R2元件的更替情况。这种更替导致元件群体均匀一致，不同拷贝的核苷酸序列差异平均R2元件仅为0.23%，R1元件为0.47%。对来自16种不同果蝇的元件进行分子系统发育分析表明，在果蝇属5000万至7000万年的历史中，这些元件一直是rDNA位点的稳定组成部分。利用蛋白质编码区域内同义位置的变化作为基线替换率的估计，可以表明，如果对元件的低密码子偏好进行校正，R1和R2的进化速率与典型蛋白质编码基因的进化速率相似。R1和R2显然非常适应它们在宿主rDNA单元中的存在。
