Shirzadegan M, Palmer J D, Christey M, Earle E D
Department of Biology, University of Michigan, Ann Arbor 48109.
Plant Mol Biol. 1991 Jan;16(1):21-37. doi: 10.1007/BF00017914.
We previously showed that the mitochondrial DNA (mtDNA) of a Brassica campestris callus culture had undergone extensive rearrangements (i.e. large inversions and a duplication) relative to DNA of the control plant [54]. In this study we observed that after continued growth, the mtDNA of this culture continues to change, with rearranged forms amplifying and diminishing to varying proportions. Strikingly similar changes were detected in the mtDNA profiles of a variety of other long- and short-term callus and cell suspension lines. However, the proportions of parental ('unrearranged') and novel ('rearranged') forms varied in different cultured cell mtDNAs. To address the source of this heterogeneity, we compared the mtDNA organization of 28 individual plants from the parental seed stock. With the exception of one plant containing high levels of a novel plasmid-like mtDNA molecule, no significant variation was detected among individual plants and therefore source plant variation is unlikely to have contributed to the diversity of mitochondrial genomes observed in cultured cells. The source of this culture-induced heterogeneity was also investigated in 16 clones derived from single protoplasts. A mixed population of unrearranged and rearranged mtDNA molecules was apparent in each protoclone, suggesting that the observed heterogeneity in various cultures might reflect the genomic composition of each individual cell; however, the induction of an intercellular heterogeneity subsequent to the protoplast isolation was not tested and therefore cannot be ruled out. The results of this study support our earlier model that the rapid structural alteration of B. campestris mtDNA in vitro results from preferential amplification and reassortment of minor pre-existing forms of the genome rather than de novo rearrangement. Infrequent recombination between short dispersed repeated elements is proposed as the underlying mechanism for the formation of these minor mtDNA molecules.
我们之前的研究表明，相对于对照植物的DNA，油菜愈伤组织培养物的线粒体DNA（mtDNA）经历了广泛的重排（即大的倒位和一次重复）[54]。在本研究中，我们观察到，在持续生长后，该培养物的mtDNA继续发生变化，重排形式以不同比例扩增和减少。在各种其他长期和短期愈伤组织及细胞悬浮系的mtDNA图谱中也检测到了惊人相似的变化。然而，亲本（“未重排”）和新出现（“重排”）形式的比例在不同的培养细胞mtDNA中有所不同。为了探究这种异质性的来源，我们比较了来自亲本种子库的28株单株植物的mtDNA组织。除了一株含有高水平新型质粒样mtDNA分子的植物外，未在单株植物之间检测到显著差异，因此来源植物的差异不太可能导致培养细胞中观察到的线粒体基因组多样性。我们还对源自单个原生质体的16个克隆进行了研究，以探究这种培养诱导的异质性的来源。在每个原生质体克隆中都明显存在未重排和重排的mtDNA分子混合群体，这表明在各种培养物中观察到的异质性可能反映了每个细胞的基因组组成；然而，原生质体分离后细胞间异质性的诱导未进行测试，因此不能排除这种可能性。本研究结果支持了我们早期的模型，即油菜mtDNA在体外的快速结构改变是由基因组中预先存在的少量形式的优先扩增和重排引起的，而不是从头重排。短分散重复元件之间的罕见重组被认为是这些少量mtDNA分子形成的潜在机制。
