Data from: Regular bottlenecks and restrictions to somatic fusion prevent the accumulation of mitochondrial defects in Neurospora

The replication and segregation of the multi-copy mitochondrial DNA (mtDNA) are not under strict control of the nuclear DNA. Within-cell selection may thus favour variants with an intracellular selective advantage but a detrimental effect on cell fitness. High relatedness among the mtDNA variants of an individual is predicted to disfavour such deleterious selfish genetic elements, but experimental evidence for this hypothesis is scarce. We studied the effect of mtDNA relatedness on the opportunities for suppressive mtDNA variants in the fungus Neurospora carrying the mitochondrial mutator plasmid pKALILO (pKAL). During growth, this plasmid integrates into the mitochondrial genome, generating suppressive mtDNA variants. These mtDNA variants gradually replace the wild-type mtDNA, ultimately culminating in growth arrest and death. We show that regular sequestration of mtDNA variation is required for effective selection against suppressive mtDNA variants. First, bottlenecks in the number of mtDNA copies from which a ‘Kalilo’ culture started significantly increased the maximum lifespan and variation in life span among cultures. Second, restrictions to somatic fusion among fungal individuals, either by using anastomosis-deficient mutants or by generating allotype diversity, prevented the accumulation of suppressive mtDNA variants. We discuss the implications of these results for the somatic accumulation of mitochondrial defects during ageing.多拷贝线粒体DNA（mtDNA）的复制与分离并不受核DNA（nuclear DNA）的严格调控。因此，胞内选择可能会青睐那些具备胞内选择优势、却对细胞适合度（cell fitness）产生有害影响的变异体。理论预测，单个个体内的线粒体DNA变异体间高亲缘性会抑制这类有害自私遗传元件的扩散，但目前支持该假说的实验证据仍较为匮乏。 本研究以携带线粒体突变性质粒pKALILO（pKAL）的真菌脉孢菌（Neurospora）为模型，探讨了线粒体DNA亲缘性对抑制型线粒体DNA变异体（suppressive mtDNA variants）出现概率的影响。该质粒在菌株生长过程中会整合至线粒体基因组，进而产生抑制型线粒体DNA变异体；此类变异体可逐步取代野生型线粒体DNA（wild-type mtDNA），最终引发生长停滞（growth arrest）与个体死亡。 研究结果证实，若要对抑制型线粒体DNA变异体实施有效选择，需对线粒体DNA变异进行规律性隔离。其一，以少量线粒体DNA拷贝作为"Kalilo"菌株培养起始种群所产生的瓶颈效应，可显著提升菌株的最长寿命，并增大不同培养物间的寿命差异；其二，通过使用菌丝融合缺陷突变体（anastomosis-deficient mutants）或构建异型多样性（allotype diversity）来限制真菌个体间的体细胞融合，可有效阻断抑制型线粒体DNA变异体的积累。 我们还讨论了本研究结果对于衰老（ageing）过程中线粒体缺陷体细胞积累相关研究的启示意义。