Molecular evolution of ribosomal DNA in Daphnia

已经观察到，基因组(多基因家族)中多次出现的基因副本之间的序列相似性在物种内非常高，即使物种之间的差异非常大。这种现象被称为“协同进化”。核糖体DNA(RDNA)是一个多基因家族，在植物和动物细胞中以数百到数千个拷贝存在，编码组成核糖体的RNA分子(RRNA)：细胞蛋白质合成的场所。一个rDNA单位由3个rRNA基因组成，由“间隔区”隔开。间隔区含有控制rRNA产生速率的序列。RDNA单位的一些区域在不同物种之间分化非常缓慢，而其他区域，如间隔区，即使在关系密切的物种之间也会非常迅速地分化。尽管这种快速分化，这些序列仍然保留了它们的功能。

It has been observed that sequence similarity between copies of genes that are present many times within genomes (multigene families) is very high within species, even when divergence is very high between species. This phenomenon is known as "concerted evolution". Ribosomal DNA (rDNA) is a multigene family that is present in hundreds to thousands of copies in plants and animal cells, and encodes the RNA molecules (rRNA) that make up ribosomes: the site of cellular protein synthesis. One rDNA unit consists of 3 rRNA genes separated by "spacers". The spacers contain sequences that control the rate of rRNA production. Some regions of the rDNA unit diverge between species very slowly while others, such as the spacers, diverge very rapidly, even among closely related species. Despite this rapid divergence, the sequences still retain their function.