BACKGROUND Apicomplexans are the causative agents of major human diseases such as malaria and toxoplasmosis. A novel group of apicomplexans, recently named corallicolids, have been detected in corals inhabiting tropical shallow reefs. These apicomplexans may represent a transitional lifestyle between free-living phototrophs and obligate parasites. To shed light on the evolutionary history of apicomplexans and to investigate their ecology in association with corals, we screened scleractinians, antipatharians, alcyonaceans, and zoantharians from shallow, mesophotic, and deep-sea communities. We detected corallicolid plastids using 16S metabarcoding, sequenced the nuclear 18S rRNA gene of corallicolids from selected samples, assembled and annotated the plastid and mitochondrial genomes from a corallicolid that associates with a deep-sea coral, and screened the metagenomes of four coral species for corallicolids. RESULTS We detected 23 corallicolid plastotypes that were associated with 14 coral species from three orders and depths down to 1400 m. Individual plastotypes were restricted to coral hosts within a single depth zone and within a single taxonomic order of corals. Some clusters of closely related corallicolids were revealed that associated with closely related coral species. However, the presence of divergent corallicolid lineages that associated with similar coral species and depths suggests that corallicolid/coral relations are flexible over evolutionary timescales and that a large diversity of apicomplexans may remain undiscovered. The corallicolid plastid genome from a deep-sea coral contained four genes involved in chlorophyll biosynthesis: the three genes of the LIPOR complex and acsF. CONCLUSIONS The presence of corallicolid apicomplexans in corals below the photic zone demonstrates that they are not restricted to shallow-water reefs and are more general anthozoan symbionts. The presence of LIPOR genes in the deep-sea corallicolid precludes a role involving photosynthesis and suggests they may be involved in a different function. Thus, these genes may represent another set of genetic tools whose function was adapted from photosynthesis as the ancestors of apicomplexans evolved towards parasitic lifestyles. Video abstract.

中文翻译：

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深海珊瑚为生态学、进化以及质体在广泛的珊瑚虫顶端复合体共生体中的作用提供了新的见解。

背景技术顶复菌是主要人类疾病例如疟疾和弓形体病的病原体。在热带浅礁珊瑚中发现了一组新的顶端复合菌，最近被命名为珊瑚菌。这些顶复体可能代表了自由生活的光养生物和专性寄生虫之间的过渡生活方式。为了阐明顶端复门类的进化历史并研究其与珊瑚相关的生态学，我们从浅海、中光和深海群落中筛选了石珊瑚类、反链藻类、桫椤类和珊瑚藻类。我们使用 16S 元条形码检测了珊瑚质体，对选定样本中珊瑚质的核 18S rRNA 基因进行了测序，组装并注释了与深海珊瑚相关的珊瑚质的质体和线粒体基因组，并筛选了四种珊瑚物种的珊瑚质宏基因组。 。结果 我们检测到 23 种珊瑚质体型，与 14 个珊瑚物种相关，来自三个目，深度可达 1400 m。个体质体型仅限于单一深度区域内和单一珊瑚分类目内的珊瑚宿主。一些密切相关的珊瑚菌群被发现与密切相关的珊瑚物种有关。然而，与相似的珊瑚物种和深度相关的不同珊瑚胶谱系的存在表明珊瑚胶/珊瑚关系在进化时间尺度上是灵活的，并且大量的顶端复合体可能仍未被发现。来自深海珊瑚的珊瑚质体基因组包含四个参与叶绿素生物合成的基因：LIPOR 复合体和 acsF 的三个基因。结论 透光带以下的珊瑚中珊瑚胶顶端复合体的存在表明它们并不局限于浅水珊瑚礁，而是更普遍的珊瑚虫共生体。深海珊瑚胶中 LIPOR 基因的存在排除了涉及光合作用的作用，并表明它们可能参与不同的功能。因此，这些基因可能代表了另一组遗传工具，其功能是随着顶复门类的祖先向寄生生活方式进化而适应光合作用的。视频摘要。