The resilience of the mitochondrial genome (mtDNA) to a high mutational pressure depends, in part, on negative purifying selection in the germline. A paradigm in the field has been that such selection, at least in part, takes place in primordial germ cells (PGCs). Specifically, Floros et al. (Nature Cell Biology 20: 144–51) reported an increase in the synonymity of mtDNA mutations (a sign of purifying selection) between early-stage and late-stage PGCs. We re-analyzed Floros’ et al. data and determined that their mutational dataset was significantly contaminated with single nucleotide variants (SNVs) derived from a nuclear sequence of mtDNA origin (NUMT) located on chromosome 5. Contamination was caused by co-amplification of the NUMT sequence by cross-specific PCR primers. Importantly, when we removed NUMT-derived SNVs, the evidence of purifying selection was abolished. In addition to bulk PGCs, Floros et al. reported the analysis of single-cell late-stage PGCs, which were amplified with different sets of PCR primers that cannot amplify the NUMT sequence. Accordingly, there were no NUMT-derived SNVs among single PGC mutations. Interestingly, single PGC mutations show a decrease of synonymity with increased intracellular mutant fraction. More specifically, nonsynonymous mutations show faster intracellular genetic drift towards higher mutant fraction than synonymous ones. This pattern is incompatible with predominantly negative selection. This suggests that germline selection of mtDNA mutations is a complex phenomenon and that the part of this process that takes place in PGCs may be predominantly positive. However counterintuitive, positive germline selection of detrimental mtDNA mutations has been reported previously and potentially may be evolutionarily advantageous.

线粒体基因组 (mtDNA) 对高突变压力的恢复能力部分取决于种系中的负纯化选择。该领域的一个范例是，这种选择至少部分地发生在原始生殖细胞（PGC）中。具体来说，弗洛罗斯等人。 (Nature Cell Biology 20: 144–51) 报道了早期和晚期 mtDNA 突变的同义性增加（纯化选择的标志）阶段PGC。我们重新分析了弗洛罗斯等人。数据并确定他们的突变数据集受到源自位于第 5 号染色体的 mtDNA 起源 (NUMT) 核序列 (NUMT) 的单核苷酸变异 (SNV) 的严重污染。污染是由交叉特异性 PCR 引物对 NUMT 序列进行共扩增引起的。重要的是，当我们去除 NUMT 衍生的 SNV 时，纯化选择的证据就被废除了。除了批量 PGC 之外，Floros 等人也发现了纯化选择的证据。报道了单细胞晚期PGC的分析，这些PGC使用不同组的PCR引物进行扩增，但不能扩增NUMT序列。因此，在单个 PGC 突变中不存在 NUMT 衍生的 SNV。有趣的是，单个 PGC 突变表现出同义性的减少以及细胞内突变分数的增加。更具体地说，非同义突变比同义突变表现出更快的细胞内遗传漂移，向更高的突变分数移动。这种模式与占主导地位的负选择不相容。这表明 mtDNA 突变的种系选择是一个复杂的现象，并且发生在 PGC 中的这一过程的部分可能主要是积极的。然而，违反直觉的、有害 mtDNA 突变的阳性种系选择此前已被报道过，可能在进化上是有利的。