Abstract
Choanoflagellates are unicellular and colonial aquatic microeukaryotes that capture bacteria using an apical flagellum surrounded by a feeding collar composed of actin-filled microvilli. Flow produced by the apical flagellum drives prey bacteria to the feeding collar for phagocytosis. We report here on the cell biology of prey capture in rosette-shaped colonies and unicellular “thecate” or substrate attached cells from the choanoflagellate S. rosetta. In thecate cells and rosette colonies, phagocytosis initially involves fusion of multiple microvilli, followed by remodeling of the collar membrane to engulf the prey, and transport of engulfed bacteria into the cell. Although both thecate cells and rosette colony cells produce ∼70 nm “collar links” that connect and potentially stabilize adjacent microvilli, only thecate cells were observed to produce a lamellipod-like “collar skirt” that encircles the base of the collar. This study offers insight into the process of prey ingestion by S. rosetta, and provides a context within which to consider potential ecological differences between solitary cells and colonies in choanoflagellates.
Highlights
The closest living relatives of animals, the choanoflagellates, offer an opportunity to investigate the potential connections between prey capture, multicellularity, and animal origins [1,2,3,4]
Prior studies of the choanoflagellates Codosiga gracilis and Choanoeca perplexa have suggested that captured bacteria are encased in pseudopods [7,17,18], it is uncertain whether the pseudopods originate solely from the cell body or whether collar microvilli might contribute to the formation of phagocytic structures in choanoflagellates
Through direct observation of prey capture in S. rosetta thecate cells, we find that the process reproducibly involves four steps: (1)
Summary
The closest living relatives of animals, the choanoflagellates, offer an opportunity to investigate the potential connections between prey capture, multicellularity, and animal origins [1,2,3,4]. Prior studies of the choanoflagellates Codosiga gracilis and Choanoeca perplexa have suggested that captured bacteria are encased in pseudopods [7,17,18], it is uncertain whether the pseudopods originate solely from the cell body or whether collar microvilli might contribute to the formation of phagocytic structures in choanoflagellates. It is unknown whether the mechanisms of prey capture in these two species are conserved in other choanoflagellates
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