Molecular Phylogeny and Description of the Novel Katablepharid <em>Roombia truncata</em> gen. et sp. nov., and Establishment of the Hacrobia Taxon nov

BackgroundPhotosynthetic eukaryotes with a secondary plastid of red algal origin (cryptophytes, haptophytes, stramenopiles, dinoflagellates, and apicomplexans) are hypothesized to share a single origin of plastid acquisition according to Chromalveolate hypothesis. Recent phylogenomic analyses suggest that photosynthetic “chromalveolates” form a large clade with inclusion of several non-photosynthetic protist lineages. Katablepharids are one such non-photosynthetic lineage closely related to cryptophytes. Despite their evolutionary and ecological importance, katablepharids are poorly investigated. Methodology/Principal FindingsHere, we report a newly discovered flagellate, Roombia truncata gen. et sp. nov., that is related to katablepharids, but is morphologically distinct from othermembers of the group in the following ways: (1) two flagella emerge from a papilla-like subapical protrusion, (2) conspicuous ejectisomes are aligned in multiple (5–11) rows, (3) each ejectisome increases in size towards the posterior end of the rows, and (4) upon feeding, a part of cytoplasm elastically stretch to engulf whole prey cell. Molecular phylogenies inferred from Hsp90, SSU rDNA, and LSU rDNA sequences consistently and strongly show R. truncata as the sister lineage to all other katablepharids, including lineages known only from environmental sequence surveys. A close association between katablepharids and cryptophytes was also recovered in most analyses. Katablepharids and cryptophytes are together part of a larger, more inclusive, group that also contains haptophytes, telonemids, centrohelids and perhaps biliphytes. The monophyly of this group is supported by several different molecular phylogenetic datasets and one shared lateral gene transfer; therefore, we formally establish this diverse clade as the “Hacrobia.” Conclusions/SignificanceOur discovery of R. truncata not only expands our knowledge in the less studied flagellate group, but provide a better understanding of phylogenetic relationship and evolutionary view of plastid acquisition/losses of Hacrobia. Being an ancestral to all katablepharids, and readily cultivable, R. truncata is a good candidate for multiple gene analyses that will contribute to future phylogenetic studies of Hacrobia.背景：根据色藻界假说（Chromalveolate hypothesis），拥有红藻起源次生质体的光合真核生物类群（隐藻（cryptophytes）、定鞭藻（haptophytes）、不等鞭毛类（stramenopiles）、甲藻（dinoflagellates）、顶复门原虫（apicomplexans））被推测具有单次统一的质体获得事件。近期的系统基因组学分析表明，光合型‘色藻类群’构成一个大型演化支，其中包含多个非光合的原生生物谱系。卡特藻类（katablepharids）便是此类与隐藻亲缘关系紧密的非光合谱系之一。尽管其在演化与生态学上具有重要意义，但目前对卡特藻类的研究仍较为匮乏。 方法学/主要研究结果：本研究报道了一株新发现的鞭毛虫类群Roombia truncata gen. et sp. nov.（截形屋氏鞭毛虫新属新种），其与卡特藻类亲缘关系相近，但在形态上与该类群其他成员存在显著差异，具体如下：(1) 两根鞭毛起源于乳突状的亚顶端突起；(2) 显著的弹射体（ejectisomes）呈多列（5~11列）排列；(3) 每列弹射体的尺寸沿纵向向后逐渐增大；(4) 摄食时，部分细胞质可弹性伸展以完整包裹猎物细胞。基于热休克蛋白90（Hsp90）、小亚基核糖体DNA（SSU rDNA）及大亚基核糖体DNA（LSU rDNA）序列构建的分子系统发育树均一致且强力支持：R. truncata是所有其他卡特藻类群的姊妹谱系，其中也包含仅通过环境序列调查发现的谱系。多数分析结果同样支持卡特藻类与隐藻之间存在紧密的亲缘关联。卡特藻类与隐藻共同隶属于一个范围更广的更大演化支，该支系还包含定鞭藻、端丝藻类（telonemids）、中阳虫类（centrohelids）以及可能的双鞭藻类（biliphytes）。该演化支的单系性得到了多组分子系统发育数据以及一次共享的水平基因转移事件的支持；因此，我们正式将这一多样的演化支命名为'Hacrobia'。 结论与意义：我们对R. truncata的发现不仅拓展了学界对研究较少的鞭毛虫类群的认知，同时也增进了我们对Hacrobia类群的系统发育关系以及质体获得与丢失演化历程的理解。作为所有卡特藻类群的姊妹谱系且易于培养，R. truncata是开展多基因分析的理想材料，将为未来Hacrobia类群的系统发育研究提供重要支撑。