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

人们普遍认为，所有的质体最初都是独立的微藻，是由内共生事件产生的；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，许多塑性发生的过程仍然是一个谜。许多模式生物被用来阐明质体的发育过程，例如，海蛞蝓Elysia spp.，纤毛虫Mesodinium spp.或变形虫Paulinella chroophore。关于可塑性发生，最未被探索的问题是：自由生活的微藻如何转化为早期的内共生生物。然而，目前没有一种模式生物可以用来回答这个问题，因为它们要么仍处于短暂的（“盗贼生长”），要么已经处于永久的内共生阶段。以硅藻作为光合内共生体的鞭毛藻是一种开创性的生物，它使我们能够通过实验来测试这些原本隐藏的过程。它们的内共生硅藻表现出三个连续的进化阶段：一个窃液生长阶段（d.c apensis），在这个阶段，摄入的硅藻维持了大约两个月；永久维持多种内共生硅藻的阶段（D. kwazulunatalensis）；一个硅藻被永久维持的阶段（其他已知的生物）。为了建立D. capensis， D. kwazulunatalensis和硅藻Nitzschia cf. agnita （D. capensis的质体来源）作为一个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它们的基因组和转录组信息以及基因修饰工具。在dfg资助的第一个阶段，我重点建立了d.c apensis菌株，以明确其进化阶段，并研究了d.c apf.agnita被d.c apensis摄入后的生物活性。本研究旨在利用新建立的模型系统来阐明从窃听器成形术到早期永久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。这些假设是：A)宿主鞭毛藻严格控制其内共生硅藻的代谢已经在眼睑发育阶段；B)控制硅藻细胞周期基因表达和控制内共生硅藻硝酸盐供应这两个系统的存在与否区分了D. capensis和D. kwazulunatalensis的进化阶段；C)硅藻线粒体的活性在细胞生长阶段就已经降低，其功能部分被鞭毛藻线粒体所取代。这些项目将使用已经建立的N. cf. agnita转化工具和分析的恐龙转录组数据进行。我的项目旨在阐明D. capensis和D. kwazulunatalensis在不同进化阶段内共生硅藻被宿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.