Department of Biology, Indiana University Bloomington, Bloomington, Indiana 47403, USA.
BMC Biol. 2010 Dec 22;8:150. doi: 10.1186/1741-7007-8-150.
Horizontal gene transfer (HGT) is relatively common in plant mitochondrial genomes but the mechanisms, extent and consequences of transfer remain largely unknown. Previous results indicate that parasitic plants are often involved as either transfer donors or recipients, suggesting that direct contact between parasite and host facilitates genetic transfer among plants.
In order to uncover the mechanistic details of plant-to-plant HGT, the extent and evolutionary fate of transfer was investigated between two groups: the parasitic genus Cuscuta and a small clade of Plantago species. A broad polymerase chain reaction (PCR) survey of mitochondrial genes revealed that at least three genes (atp1, atp6 and matR) were recently transferred from Cuscuta to Plantago. Quantitative PCR assays show that these three genes have a mitochondrial location in the one species line of Plantago examined. Patterns of sequence evolution suggest that these foreign genes degraded into pseudogenes shortly after transfer and reverse transcription (RT)-PCR analyses demonstrate that none are detectably transcribed. Three cases of gene conversion were detected between native and foreign copies of the atp1 gene. The identical phylogenetic distribution of the three foreign genes within Plantago and the retention of cytidines at ancestral positions of RNA editing indicate that these genes were probably acquired via a single, DNA-mediated transfer event. However, samplings of multiple individuals from two of the three species in the recipient Plantago clade revealed complex and perplexing phylogenetic discrepancies and patterns of sequence divergence for all three of the foreign genes.
This study reports the best evidence to date that multiple mitochondrial genes can be transferred via a single HGT event and that transfer occurred via a strictly DNA-level intermediate. The discovery of gene conversion between co-resident foreign and native mitochondrial copies suggests that transferred genes may be evolutionarily important in generating mitochondrial genetic diversity. Finally, the complex relationships within each lineage of transferred genes imply a surprisingly complicated history of these genes in Plantago subsequent to their acquisition via HGT and this history probably involves some combination of additional transfers (including intracellular transfer), gene duplication, differential loss and mutation-rate variation. Unravelling this history will probably require sequencing multiple mitochondrial and nuclear genomes from Plantago. See Commentary: http://www.biomedcentral.com/1741-7007/8/147.
水平基因转移（HGT）在植物线粒体基因组中较为常见，但转移的机制、程度和后果在很大程度上仍不清楚。先前的研究结果表明，寄生植物通常作为转移供体或受体参与其中，这表明寄生虫与宿主之间的直接接触促进了植物之间的基因转移。
为了揭示植物间 HGT 的机制细节，研究了两个群体之间转移的程度和进化命运：寄生属菟丝子和一小部分车前属物种。广泛的聚合酶链反应（PCR）调查线粒体基因表明，至少有三个基因（atp1、atp6 和 matR）最近从菟丝子转移到车前属。定量 PCR 分析表明，在检查的车前属一个种系中，这三个基因具有线粒体位置。序列进化模式表明，这些外来基因在转移后不久就降解为假基因，逆转录（RT）-PCR 分析表明，这些基因都无法检测到转录。在 atp1 基因的本地和外来拷贝之间检测到三个基因转换的情况。三个外来基因在车前属内的相同系统发育分布以及 RNA 编辑的祖先位置处保留胞嘧啶表明，这些基因可能是通过单个 DNA 介导的转移事件获得的。然而，在受体车前属中三个物种中的两个物种的多个个体的采样显示，所有三个外来基因的系统发育差异和序列分歧模式都非常复杂和令人困惑。
本研究报告了迄今为止最好的证据，证明多个线粒体基因可以通过单个 HGT 事件转移，并且转移是通过严格的 DNA 水平中间产物进行的。在共存的外来和本地线粒体拷贝之间发现基因转换表明，转移基因可能在产生线粒体遗传多样性方面具有重要的进化意义。最后，每个转移基因谱系内的复杂关系意味着在通过 HGT 获得这些基因后，它们在车前属中的历史非常复杂，这可能涉及到其他转移（包括细胞内转移）、基因复制、差异丧失和突变率变化的某种组合。揭示这段历史可能需要从车前属中测序多个线粒体和核基因组。
