Dinotoms: a novel model system for plastidogenesis studies between kleptoplasty and endosymbiosis

人们普遍认为，所有的叶绿体最初都是独立的微藻，并且是由内生共生事件产生的；然而，由于几乎所有的光养生物都已经经历了进化发育阶段，许多叶绿体的发生过程仍然是一个谜。许多模式生物被用来阐明质体的发育过程，如海懒、纤毛虫、变形虫或变形虫。最尚未探索的可塑性发生问题是：自由生活的微藻如何转变为早期的内生共生体。然而，目前的模式生物都不能用来回答这个问题，因为它们要么仍处于暂时的(“盗贼”)阶段，要么已经处于永久性的内共生阶段。Dinotoms是一种将硅藻作为其光合作用内共生体的恐鞭毛类动物，它是一种突破性的有机体，允许我们通过实验测试这些其他方面的隐藏过程。它们的内共生硅藻经历了三个连续的进化阶段：一个可塑性阶段(D.capens)，在此阶段，摄食的硅藻保持了大约两个月；一个阶段，永久保持了多个内共生硅藻(D.kwazulunatalsis)；以及一个阶段，即单个硅藻永久保持(其他已知的恐龙)。建立了卡氏盘藻、克氏盘藻和硅藻Nitzschia cf.作为一种可以用现代分子生物学技术探索的模型系统，我和我的合作者目前正在生成他们的基因组和转录信息以及基因修改工具。在DFG资助的第一个时期，我专注于建立卡氏盘藻菌株，以阐明其进化阶段，并研究N.cf的生物活性。卡氏吸虫摄食后的吸毒症。这项建议旨在利用新建立的模型系统来阐明从眼睑成形术到早期永久性内共生的分子进化过程。基于我在第一个DFG时期的成就，我提出了三个假设。这些假说是：A)寄主甲藻严格控制其内共生硅藻，其代谢已经处于海藻可塑性阶段；B)存在或不存在两个系统，即控制硅藻细胞周期基因的表达和控制内共生硅藻的硝酸盐供应，从而将卡氏杜氏藻和夸祖鲁纳塔尔杜鹃之间的进化阶段分开；以及C)硅藻线粒体在海藻可塑性阶段就降低了它们的活动，其功能部分被甲藻线粒体所取代。这些项目将与已经建立的N.cf一起进行。Agnita转换工具和分析的恐龙转录数据。我的项目旨在阐明内共生硅藻在不同进化阶段被寄主整合和控制的程度。

It is widely accepted that all plastids were originally independent microalgae and arose by endosymbiotic events; however, many processes of plastidogenesis remain a mystery as almost all phototrophs already passed through the evolutionary developmental stages. Many model organisms are used to shed light on the developmental processes of plastids, e.g., sea slugs Elysia spp., ciliates Mesodinium spp., or the amoeba Paulinella chromatophore. The yet most unexplored question of plastidogenesis is: how do free-living microalgae transform into early-stage endosymbionts. However, none of the present model organisms can be applied to answer this question, because they are either still in the transient (“kleptoplastic”), or already in the permanent endosymbiotic stage. Dinotoms, dinoflagellates that harbour diatoms as their photosynthetic endosymbionts, are ground-breaking organisms that allow us to experimentally test these elsewise hidden processes. Their endosymbiotic diatoms exhibit three successive evolutionary stages: a kleptoplastic stage (D. capensis), in which ingested diatoms are maintained for approximately two months; a stage in which multiple endosymbiotic diatoms are permanently maintained (D. kwazulunatalensis); and a stage where a single diatom is permanently maintained (other known dinotoms). To establish D. capensis, D. kwazulunatalensis and the diatom Nitzschia cf. agnita, the plastidial source of D. capensis, as a model system that is explorable with modern molecular biology techniques, I and my collaborators are currently generating their genomic and transcriptomic information as well as genetic modification tools. During the first DFG-funded period, I focused on establishing the D. capensis strain to clarify its evolutionary stage and to investigate the bioactivity of N. cf. agnita after ingestion by D. capensis. This proposal aims to use the newly-established model system to elucidate the molecular evolutionary processes from kleptoplasty to early permanent endosymbiosis. I propose three hypotheses, based on my achievements of the first DFG period. These hypotheses are A) host dinoflagellates tightly control their endosymbiotic diatoms metabolically already at the kleptoplastic stage; B) the presence or absence of two systems, i.e., the control of diatom cell cycle gene expression and the control of nitrate supply for endosymbiotic diatoms, separates the evolutionary stages between D. capensis and D. kwazulunatalensis; and C) diatom mitochondria reduce their activity already at the kleptoplastic stage and the functions are partially replaced by dinoflagellate mitochondria. These projects will be conducted with the already-established N. cf. agnita transformation tools and the analysed dinotom transcriptomic data. My projects aim to elucidate how deeply endosymbiotic diatoms are integrated and controlled by the host at the different evolutionary stages of D. capensis and D. kwazulunatalensis.