Photosymbiosis, a mode of mixotrophy by algal endosymbiosis, provides key advantage to pelagic life in oligotrophic oceans. Despite its ecological importance, mechanisms underlying its emergence and association with the evolutionary success of photosymbiotic lineages remain unclear. We used planktonic foraminifera, a group of pelagic test-forming protists with an excellent fossil record, to reveal the history of symbiont acquisition among their three main extant clades. We used single-cell 18S rRNA gene amplicon sequencing to reveal symbiont identity and mapped the symbiosis on a phylogeny time-calibrated by fossil data. We show that the highly specific symbiotic interaction with dinoflagellates emerged in the wake of a major extinction of symbiont-bearing taxa at the end of the Eocene. In contrast, less specific and low-light adapted symbioses with pelagophytes emerged 20 million years later, in multiple independent lineages in the Late Neogene, at a time when the vertical structure of pelagic ecosystems was transformed by global cooling. We infer that in foraminifera, photosymbiosis can evolve easily and that its establishment leads to diversification and ecological dominance to such extent, that the proliferation of new symbioses is prevented by the incumbent lineages.

中文翻译：

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由生态学现状解释的现存浮游有孔虫的两波光共生获得


光共生是一种通过藻类内共生实现的混合营养模式，为贫营养海洋中的中上层生物提供了关键优势。尽管它在生态学上很重要，但其出现的机制以及与光共生谱系进化成功的关联仍不清楚。我们使用浮游有孔虫，一组具有出色化石记录的中上层测试形成原生生物，揭示了它们现存三个主要分支之间共生获得的历史。我们使用单细胞 18S rRNA 基因扩增子测序来揭示共生体身份，并在由化石数据校准的系统发育上绘制共生关系。我们表明，与甲藻的高度特异性共生相互作用是在始新世末期携带共生体的类群大规模灭绝之后出现的。相比之下，2000 万年后，在新近纪晚期的多个独立谱系中出现了与中上层植物的特异性较低和低光适应的共生体，当时中上层生态系统的垂直结构因全球变冷而发生转变。我们推断，在有孔虫中，光共生很容易进化，它的建立导致多样化和生态优势，以至于现有谱系阻止了新共生的增殖。