Sequence alignments of Corallicolids, apicomplexan symbionts of coral

AbstractCorals (Metazoa; Cnidaria; Anthozoa) have recently been shown to play host to a widespread and diverse group of intracellular symbionts of the phylum Apicomplexa. These symbionts, colloquially called ‘corallicolids’, are mostly known through molecular analyses, and no formal taxonomy has been proposed. Another apicomplexan, Gemmocystis cylindrus (described from the coral Dendrogyra cylindrus), may be related to corallicolids, but lacks molecular data. Here, we isolate and describe motile trophozoite (feeding) corallicolids cells using microscopic (light, SEM, and TEM) and molecular phylogenetic analysis to provide the basis for a formal description. Phylogenetic analyses using nuclear and plastid rRNA operons, and three mitochondrial protein sequences derived from single-cell transcriptomes, all confirm that these organisms fall into a discrete deep-branching clade within the Apicomplexa not closely related to any known species or major subgroup. As a result, we assign this clade to a new order, Corallicolida ord. nov., and family, Corallicolidae fam. nov. We describe a type species, Corallicola aquarius gen. nov. sp. nov. from its Rhodactis sp. host, and also describe a second species, Anthozoaphila gnarlus gen. nov. sp. nov., from the coral host Madracis mirabilis. Finally, we propose reassigning the incertae sedis taxon G. cylindrus from the order Agamococcidiorida to the Corallicolida, based on similarities in morphology and host localization to that of the corallicolids.

摘要：珊瑚（Metazoa，后生动物门；Cnidaria，刺胞动物门；Anthozoa，珊瑚虫纲）现已被发现可宿主一类广泛分布且多样的顶复门（Apicomplexa）细胞内共生体。这类共生体被俗称为corallicolids（珊瑚殖虫类），目前相关研究大多基于分子分析手段，尚未有正式的分类学界定。另一种顶复门生物——柱形芽胞虫（Gemmocystis cylindrus，最初从柱形木珊瑚Dendrogyra cylindrus中分离描述）——可能与珊瑚殖虫类存在亲缘关系，但缺乏分子数据支撑。本研究通过显微镜观察（光学显微镜、扫描电镜SEM及透射电镜TEM）与分子系统发育分析，分离并描述了具有运动能力的滋养体（营养摄食阶段）珊瑚殖虫类细胞，为其正式分类学描述提供依据。基于细胞核与质体rRNA操纵子，以及从单细胞转录组中获取的3个线粒体蛋白序列的系统发育分析均证实，此类生物在顶复门中形成一个独立的深分支演化支，与任何已知物种或主要类群的亲缘关系均较远。据此，我们将该演化支归入新建立的目：珊瑚殖虫目（Corallicolida ord. nov.），以及新建立的科：珊瑚殖虫科（Corallicolidae fam. nov.）。我们从宿主肉质海葵属（Rhodactis sp.）未定种中描述了模式种：海洋珊瑚殖虫（Corallicola aquarius gen. nov. sp. nov.）；同时从宿主奇妙石珊瑚（Madracis mirabilis）中描述了第二个物种：珊瑚虫栖居虫（Anthozoaphila gnarlus gen. nov. sp. nov.）。最后，基于形态学特征与宿主定位模式与珊瑚殖虫类的相似性，我们提议将分类位置未定的柱形芽胞虫（G. cylindrus）从无配子殖虫目（Agamococcidiorida）重新归入珊瑚殖虫目。