BACKGROUND The origin of animals from their unicellular ancestor was one of the most important events in evolutionary history, but the nature and the order of events leading up to the emergence of multicellular animals are still highly uncertain. The diversity and biology of unicellular relatives of animals have strongly informed our understanding of the transition from single-celled organisms to the multicellular Metazoa. Here, we analyze the cellular structures and complex life cycles of the novel unicellular holozoans Pigoraptor and Syssomonas (Opisthokonta), and their implications for the origin of animals. RESULTS Syssomonas and Pigoraptor are characterized by complex life cycles with a variety of cell types including flagellates, amoeboflagellates, amoeboid non-flagellar cells, and spherical cysts. The life cycles also include the formation of multicellular aggregations and syncytium-like structures, and an unusual diet for single-celled opisthokonts (partial cell fusion and joint sucking of a large eukaryotic prey), all of which provide new insights into the origin of multicellularity in Metazoa. Several existing models explaining the origin of multicellular animals have been put forward, but these data are interestingly consistent with one, the "synzoospore hypothesis." CONCLUSIONS The feeding modes of the ancestral metazoan may have been more complex than previously thought, including not only bacterial prey, but also larger eukaryotic cells and organic structures. The ability to feed on large eukaryotic prey could have been a powerful trigger in the formation and development of both aggregative (e.g., joint feeding, which also implies signaling) and clonal (e.g., hypertrophic growth followed by palintomy) multicellular stages that played important roles in the emergence of multicellular animals.

中文翻译：

---

从动物的掠食性单细胞近亲了解后生动物多细胞性的起源。

背景技术动物的单细胞祖先起源是进化史上最重要的事件之一，但是导致多细胞动物出现的事件的性质和顺序仍然高度不确定。动物的单细胞近亲的多样性和生物学特性强烈地帮助我们了解了从单细胞生物到多细胞后生动物的转变。在这里，我们分析新型单细胞全人类的Pigoraptor和Syssomonas（Opisthokonta）的细胞结构和复杂的生命周期，以及它们对动物起源的影响。结果Syssomonas和Pigoraptor的生命周期复杂，具有多种细胞类型，包括鞭毛，变形鞭毛，非鞭毛类非鞭毛细胞和球形囊肿。生命周期还包括多细胞聚集体和合胞体样结构的形成，以及单细胞鞭毛虫的异常饮食（部分细胞融合和大真核生物的联合吮吸），所有这些都为多细胞起源的发展提供了新见识。在后生动物中。已经提出了几种解释多细胞动物起源的现有模型，但是有趣的是，这些数据与“同胞孢子假说”一致。结论祖先后生动物的进食方式可能比以前认为的要复杂，不仅包括细菌的猎物，还包括较大的真核细胞和有机结构。捕食大型真核生物的能力可能是两者聚集和形成（例如，共同喂养，