Petersen Jörn, Ludewig Ann-Kathrin, Michael Victoria, Bunk Boyke, Jarek Michael, Baurain Denis, Brinkmann Henner
Leibniz-Institut DSMZ-Deutsche Sammlung von Mikroorganismen und Zellkulturen GmbH, Braunschweig, Germany.
Genome Biol Evol. 2014 Mar;6(3):666-84. doi: 10.1093/gbe/evu043.
The discovery of Chromera velia, a free-living photosynthetic relative of apicomplexan pathogens, has provided an unexpected opportunity to study the algal ancestry of malaria parasites. In this work, we compared the molecular footprints of a eukaryote-to-eukaryote endosymbiosis in C. velia to their equivalents in peridinin-containing dinoflagellates (PCD) to reevaluate recent claims in favor of a common ancestry of their plastids. To this end, we established the draft genome and a set of full-length cDNA sequences from C. velia via next-generation sequencing. We documented the presence of a single coxI gene in the mitochondrial genome, which thus represents the genetically most reduced aerobic organelle identified so far, but focused our analyses on five "lucky genes" of the Calvin cycle. These were selected because of their known support for a common origin of complex plastids from cryptophytes, alveolates (represented by PCDs), stramenopiles, and haptophytes (CASH) via a single secondary endosymbiosis with a red alga. As expected, our broadly sampled phylogenies of the nuclear-encoded Calvin cycle markers support a rhodophycean origin for the complex plastid of Chromera. However, they also suggest an independent origin of apicomplexan and dinophycean (PCD) plastids via two eukaryote-to-eukaryote endosymbioses. Although at odds with the current view of a common photosynthetic ancestry for alveolates, this conclusion is nonetheless in line with the deviant plastome architecture in dinoflagellates and the morphological paradox of four versus three plastid membranes in the respective lineages. Further support for independent endosymbioses is provided by analysis of five additional markers, four of them involved in the plastid protein import machinery. Finally, we introduce the "rhodoplex hypothesis" as a convenient way to designate evolutionary scenarios where CASH plastids are ultimately the product of a single secondary endosymbiosis with a red alga but were subsequently horizontally spread via higher-order eukaryote-to-eukaryote endosymbioses.
发现了一种自由生活的光合生物——维氏色虫（Chromera velia），它是顶复门病原体的近亲，这为研究疟原虫的藻类起源提供了一个意想不到的机会。在这项研究中，我们将维氏色虫中真核生物到真核生物内共生的分子印记与其在含多甲藻素的甲藻（PCD）中的对应印记进行了比较，以重新评估最近关于其质体具有共同祖先的说法。为此，我们通过下一代测序建立了维氏色虫的基因组草图和一组全长cDNA序列。我们记录了线粒体基因组中单个细胞色素氧化酶亚基I（coxI）基因的存在，因此它代表了迄今为止在遗传上最简化的需氧细胞器，但我们的分析集中在卡尔文循环的五个“幸运基因”上。选择这些基因是因为它们已知支持通过与红藻的一次次生内共生，使隐藻、囊泡藻（以PCD为代表）、硅藻和定鞭藻（CASH）的复合质体具有共同起源。不出所料，我们对核编码的卡尔文循环标记进行的广泛采样系统发育分析支持了色虫复合质体起源于红藻的观点。然而，它们也表明顶复门和甲藻（PCD）质体是通过两次真核生物到真核生物的内共生独立起源的。尽管这与目前关于囊泡藻具有共同光合祖先的观点不一致，但这一结论与甲藻中异常的质体基因组结构以及各谱系中分别为四层与三层质体膜的形态学悖论是一致的。对另外五个标记的分析进一步支持了独立内共生的观点，其中四个标记参与质体蛋白导入机制。最后，我们引入“红藻复合体假说”，作为一种方便的方式来描述进化场景，即CASH质体最终是与红藻一次次生内共生的产物，但随后通过高阶真核生物到真核生物的内共生水平传播。
