Insights into the origin of metazoan multicellularity from predatory unicellular relatives of animals

Background: The origin of animals from their unicellular ancestor was one of the most important events in evolutionary history, bx it 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 induding flagellates, amoeboflagellates, amoeboid non-flagellar cells, and spherical cysts. The life cycles also include the formation of multicellular aggregations arid 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 donal (e.g., hypertrophic growth followed by palintomy) multicellular stages that played important roles in the emergence of multicellular animals.