Not rarely, we fail to understand a phenomenon alone from its current state; instead, we need to know how it came into being. The presence always encompasses the footprints of the past. This is prominently true for the evolution of cells. The way how organelles are organised, why they contain their own genomes and how they import most of the protein components by a regulated, complex and energy-consuming machinery is hard to derive from the current set-up of a cell. It is immediately understood, however, if described as remnants of an endosymbiotic event (Sagan 1967). To read a process from the footprints it left behind is not a trivial task, though, and requires a lot of sophisticated inference derived from a thorough knowledge of the current functional contexts. The discovery of “Lucy’s” footprints preserved in the fossil ashes of a volcano (Leakey and Hay 1979) allowed to conclude on the origin of human bipedal locomotion, but only because this type of movement was already well known from detailed studies of extant human movement. Two contributions to the current issue illustrate the art of tracking footprints by reading otherwise curious details of plastid structure and function as consequence of endosymbiotic evolution. The contribution by Kuroiwa et al. (2020) is using the organisation of the plastid genomes to infer on early eukaryotic evolution. Based on several decades of expertise in algal cell biology, the authors were able to define three classes of plastid genomes. The original, the so-called CN-type, showed one nucleoid in the chloroplast centre, while the SN-type, prevailing in green algae and land plants, exhibits DNA spread in many copies randomly all over the plastid. The third so-called CL-type harbours circular DNA in the plastid periphery and prevails in red and brown algae. The authors work on a primitive red algal model, Cyanidioschyzon merolae, which still displays the ancestral CN-type of plastid DNA. In this model organism, the plastid is still tightly coupled to the cell cycle of the “host cell”. For instance, during mitosis, the plastid nucleoid is stretched towards the spindle poles (Imoto et al. 2010). How this ancestral CN-type was later converted into the characteristic CL-type dominating in the red algae and their derivatives, the brown algae, has remained enigmatic. In the current work, they report that, surprisingly, the CL-type can be produced from the original CN-type by simple drying, meaning that the CL-type nucleoid has arisen by de-condensation of the ancestral CN-type. This transition is accompanied by a characteristic beads-on-a-string structure, which is also a peculiar feature of the CL-type nucleoids in brown algae. Thus, the red algal endosymbiont of brown algae seems to have been an organism very similar to the extant C. merolae. The contribution by Ohashi et al. (2020) asks the question: What happens with the protein filamentous temperaturesensitive Z (FtsZ) during development of the male gametophyte in higher plants? FtsZ executes cell division in prokaryotes and is still needed for the division of the plastid (although the gene is not any longer encoded on the plastid genome but was transferred to the nuclear genome of the “host”, such that the protein has to be imported back to the plastid). In the germinating pollen tube (which is an evolutionary remnant of an own, haploid, generation), the protein is found in the vegetative cell, while it is absent from the germ line (as are the plastids themselves). This might mean that the FtsZ protein is not translated even though mRNA accumulates in the male generative cell or that the transcript is exported into the vegetative cell, where it is translated, or that the transcripts are translated only after fertilisation, such that the FtsZ protein is acting in zygote or the endosperm. Using a GFP fusion of FtsZ in the model plant Arabidopsis thaliana (Fujiwara et al. 2010), they show that FtsZ protein is indeed hardly detectable in the germ line, while it is present in the vegetative cell. Furthermore, there is no evidence for a model, where the transcript is transmitted to zygote or endosperm and the protein functions after fertilisation. The finding that a factor crucial for plastid division is specifically suppressed in the male germ Handling Editor: Handling Editor: Peter Nick

中文翻译：

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跟踪质体进化的足迹

很多时候，我们无法仅从当前状态来理解一种现象；相反，我们需要知道它是如何产生的。存在总是包含过去的足迹。这对于细胞的进化尤其如此。细胞器的组织方式、为什么它们包含自己的基因组以及它们如何通过受监管、复杂且耗能的机器输入大部分蛋白质成分，很难从当前的细胞结构中推导出来。然而，如果将其描述为内共生事件的残余，则立即理解（Sagan 1967）。然而，从进程留下的足迹中读取进程并不是一项微不足道的任务，并且需要从对当前功能上下文的透彻了解中得出的大量复杂推理。保存在火山化石灰烬中的“露西”脚印的发现（Leakey 和 Hay 1979）让我们得出了人类双足运动起源的结论，但这只是因为这种类型的运动已经通过对现存人类运动的详细研究而广为人知. 对当前问题的两个贡献说明了通过阅读作为内共生进化结果的质体结构和功能的其他奇怪细节来追踪足迹的艺术。Kuroiwa 等人的贡献。(2020) 正在使用质体基因组的组织来推断早期真核生物进化。基于在藻类细胞生物学方面数十年的专业知识，作者能够定义三类质体基因组。原始的，即所谓的 CN 型，在叶绿体中心显示一个类核，而 SN 型，普遍存在于绿藻和陆生植物中，表现出以许多拷贝随机分布在整个质体中的 DNA。第三种所谓的 CL 型在质体外围含有环状 DNA，主要存在于红藻和褐藻中。作者研究了一种原始的红藻模型 Cyanidioschyzon merolae，该模型仍显示出原始 CN 型质体 DNA。在这种模式生物中，质体仍然与“宿主细胞”的细胞周期紧密耦合。例如，在有丝分裂期间，质体类核向纺锤体两极伸展（Imoto 等人，2010 年）。这种祖先的 CN 型后来如何转变为在红藻及其衍生物褐藻中占主导地位的特征性 CL 型仍然是个谜。在目前的工作中，他们报告说，令人惊讶的是，CL 型可以通过简单的干燥从原始 CN 型生产，这意味着 CL 型类核是通过祖先 CN 型的去凝聚而产生的。这种转变伴随着一种典型的串珠结构，这也是褐藻中 CL 型核素的一个特殊特征。因此，褐藻的红藻内共生体似乎是一种与现存的 C. merolae 非常相似的生物。Ohashi 等人的贡献。(2020) 提出了一个问题：在高等植物雄配子体发育过程中，蛋白质丝状温度敏感 Z (FtsZ) 会发生什么？FtsZ 在原核生物中执行细胞分裂，仍然需要质体的分裂（虽然基因不再编码在质体基因组上，而是转移到“宿主”的核基因组中，因此蛋白质必须输入回到质体）。在发芽的花粉管（它是自己的单倍体世代的进化残余物）中，蛋白质存在于营养细胞中，而它不存在于种系中（质体本身也是如此）。这可能意味着 FtsZ 蛋白不会被翻译，即使 mRNA 在雄性生殖细胞中积累，或者转录物被输出到营养细胞中，在那里它被翻译，或者转录物只在受精后才被翻译，这样 FtsZ 蛋白作用于受精卵或胚乳。在模式植物拟南芥中使用 FtsZ 的 GFP 融合（Fujiwara 等，2010），他们表明 FtsZ 蛋白确实在种系中几乎检测不到，而它存在于营养细胞中。此外，没有模型的证据，转录物被传递到受精卵或胚乳，蛋白质在受精后发挥作用。发现对质体分裂至关重要的因素在雄性细菌中被特别抑制处理编辑：处理编辑：彼得尼克