Cymbomonas tetramitiformis is a peculiar green alga that unites in one cell the abilities of photosynthesis and phagocytosis, which makes it a very useful model for the study of the evolution of plastid endosymbiosis. We have pondered over this issue and propose an evolutionary scenario of trophic strategies in eukaryotes, including primary and secondary plastid endosymbioses. C. tetramitiformis is a prototroph, just like the common ancestor of Archaeplastida was, and can synthesize most small organic molecules contrary to other eukaryotic phagotrophs, e.g. some metazoans, amoebozoans, and ciliates, which have not evolved tight endosymbiotic relationships. In order to establish a permanent photosynthetic endosymbiont they do not have to become prototrophs, but have to acquire the genes necessary for plastid retention via horizontal (including endosymbiotic) gene transfer. Such processes occurred successfully in the ancestors of eukaryotes with permanent secondary plastids and thus led to their great diversification. The preservation of phagocytosis in Cymbomonas (and some other prasinophytes as well) seems to result from nutrient deficiency in their oligotrophic habitats. This forces them to supplement their diet with phagocytized prey, in contrasts to the thecate amoeba Paulinella chromatophora, which also successfully transformed cyanobacteria into permanent organelles. Although Paulinella endosymbionts were acquired very recently in comparison to primary plastids, Paulinella has lost the ability to phagocytose, most probably due to the fact that it inhabits nutrient-rich environments, which renders the phagotrophy nonessential.

中文翻译：

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Cymbomonas tetramitiformis——一种具有细菌味道的特殊绿藻植物，为质体进化提供了线索。


Cymbomonas tetramitiformis是一种特殊的绿藻，它将光合作用和吞噬作用的能力结合在一个细胞中，这使其成为研究质体内共生进化的非常有用的模型。我们思考了这个问题，并提出了真核生物营养策略的进化方案，包括初级和次级质体内共生。 C. tetramitiformis是一种原养生物，就像古质体的共同祖先一样，并且可以合成大多数小有机分子，这与其他真核吞噬生物相反，例如一些后生动物、变形虫和纤毛虫，这些动物尚未进化出紧密的内共生关系。为了建立永久的光合作用内共生体，它们不必成为原养生物，但必须通过水平（包括内共生）基因转移获得质体保留所需的基因。这些过程成功地发生在具有永久次生质体的真核生物的祖先中，从而导致了它们的巨大多样化。香波单胞菌（以及其他一些草植物）吞噬作用的保留似乎是由于其寡营养栖息地的营养缺乏所致。这迫使它们用吞噬的猎物来补充饮食，这与绿藻阿米巴变形虫形成鲜明对比，后者也成功地将蓝细菌转化为永久性细胞器。尽管与初级质体相比， Paulinella内共生体是最近才获得的，但Paulinella已经失去了吞噬能力，​​这很可能是由于它栖息在营养丰富的环境中，这使得吞噬能力变得不必要。