Ribosomal (r)DNA is a highly dynamic, conserved, multigene family whose sequence homogeneity is thought to be maintained by intra- and interchromosomal recombination, which are capable of changing rDNA copy number. It is generally not known how environmental stress such as sublethal exposure to environmentally relevant concentrations of metals impacts rDNA copy number. To determine how chronic metal exposure affects rDNA, we measured copy number of the 18S rRNA gene in 355 copper and nickel-exposed samples and 132 metal-free samples derived from 325 mutation accumulation (MA) lines of two genetically distinct Daphnia pulex lineages. The MA lines were sampled at four time points over 100+ generations of clonal propagation. The copy number of rDNA was also measured in 15 individuals sampled from a metal-free non-MA control population established from the same progenitor as one of the MA lineages. We found that mean rDNA copy number fluctuated across lines exposed to metals with a tendency to decrease over time. In contrast, mean rDNA copy number in the metal-free control lines and the non-MA population remained stable over time. It is generally accepted that extreme rDNA loss results in the loss of organism fitness. Thus, fluctuations in rDNA copy number, including losses, could affect the long-term viability of natural populations of Daphnia in metal-contaminated habitats.

核糖体（r）DNA是一个高度动态的，保守的多基因家族，其序列同质性被认为是通过染色体内和染色体间重组得以维持，它们能够改变rDNA的拷贝数。通常尚不知道环境压力（例如，致死性暴露于与环境相关的金属浓度下）如何影响rDNA拷贝数。为了确定长期的金属暴露如何影响rDNA，我们测量了355种铜和镍暴露的样品以及132种无金属样品中18S rRNA基因的拷贝数，这些样品来自两种遗传上不同的水蚤的325个突变累积（MA）系血统。在超过100代的克隆繁殖的四个时间点对MA系进行了采样。还从15个无金属非MA对照人群中测量了rDNA的拷贝数，这些人群来自与MA谱系之一相同的祖先建立的非金属非MA对照人群。我们发现，平均rDNA拷贝数在暴露于金属的品系中波动，并随时间降低。相反，无金属对照品系和非MA群体中的平均rDNA拷贝数随时间保持稳定。人们普遍认为，极端的rDNA丧失会导致机体适应性丧失。因此，rDNA拷贝数的波动（包括损失）可能会影响金属污染栖息地中水蚤自然种群的长期生存能力。