Dipartimento di Biologia e Protezione delle Piante, Università di Udine, via Scienze 208, 33100 Udine, Italy.
BMC Evol Biol. 2011 Jun 28;11:185. doi: 10.1186/1471-2148-11-185.
Several phytoplasmas, wall-less phloem limited plant pathogenic bacteria, have been shown to contain extrachromosomal DNA (EcDNA) molecules encoding a replication associated protein (Rep) similar to that of geminiviruses, a major group of single stranded (ss) DNA plant viruses. On the basis of that observation and of structural similarities between the capsid proteins of geminiviruses and the Satellite tobacco necrosis virus, it has been recently proposed that geminiviruses evolved from phytoplasmal EcDNAs by acquiring a capsid protein coding gene from a co-invading plant RNA virus.
Here we show that this hypothesis has to be rejected because (i) the EcDNA encoded Rep is not of phytoplasmal origin but has been acquired by phytoplasmas through horizontal transfer from a geminivirus or its ancestor; and (ii) the evolution of geminivirus capsid protein in land plants implies missing links, while the analysis of metagenomic data suggests an alternative scenario implying a more ancient evolution in marine environments.
The hypothesis of geminiviruses evolving in plants from DNA molecules of phytoplasma origin contrasts with other findings. An alternative scenario concerning the origin and spread of Rep coding phytoplasmal EcDNA is presented and its implications on the epidemiology of phytoplasmas are discussed.
几种植原体，即无细胞壁、局限在韧皮部的植物病原细菌，已被证明含有质外体 DNA（EcDNA）分子，这些 EcDNA 分子编码类似于朊病毒的复制相关蛋白（Rep），朊病毒是一类单链（ss）DNA 植物病毒的主要群体。基于这一观察结果以及朊病毒和卫星烟草坏死病毒衣壳蛋白之间的结构相似性，最近有人提出，朊病毒是通过从共同入侵的植物 RNA 病毒中获取衣壳蛋白编码基因，从植原体 EcDNA 进化而来的。
在这里，我们表明，这个假设必须被拒绝，因为（i）EcDNA 编码的 Rep 不是植原体的起源，而是通过植原体从朊病毒或其祖先的水平转移获得的；（ii）在陆地植物中，朊病毒衣壳蛋白的进化存在缺失环节，而对宏基因组数据的分析表明了一个替代方案，暗示着在海洋环境中存在更早的进化。
朊病毒从植原体 DNA 分子进化而来的假设与其他发现相矛盾。提出了一个关于 Rep 编码植原体 EcDNA 起源和传播的替代方案，并讨论了其对植原体流行病学的影响。
