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：

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从动物的掠食性单细胞亲属中洞察后生动物多细胞性的起源。


背景技术动物从其单细胞祖先的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。动物单细胞近亲的多样性和生物学特征极大地促进了我们对从单细胞生物到多细胞后生动物的转变的理解。在这里，我们分析了新型单细胞全生动物 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 的特点是具有复杂的生命周期，具有多种细胞类型，包括鞭毛虫、变形虫鞭毛虫、变形虫非鞭毛细胞和球形包囊。生命周期还包括多细胞聚集体和合胞体样结构的形成，以及单细胞后康生物的不寻常饮食（部分细胞融合和大型真核猎物的联合吸吮），所有这些都为多细胞起源提供了新的见解在后生动物中。已经提出了几种解释多细胞动物起源的现有模型，但有趣的是，这些数据与“合生孢子假说”一致。结论 祖先后生动物的摄食模式可能比之前想象的更为复杂，不仅包括细菌猎物，还包括更大的真核细胞和有机结构。以大型真核生物猎物为食的能力可能是聚集性（例如联合进食，这也意味着信号传导）和克隆性（例如，肥大生长，随后是palintomy）多细胞阶段，在多细胞动物的出现中发挥了重要作用。