Figure 1. Phylogeny and flagellar features of the choanoflagellate S.rosetta.

(A) Phylogenetic tree showing major eukaryotic suprakingdoms (colored blocks) stemming from the last common eukaryotic ancestor (LECA). Suprakingdoms with representatives that have been imaged using cryo-ET are labeled (i.e. Alveolata, Opisthokonta, Excavata, and Archaeplastida) with example species. Choanoflagellates are part of the Opisthokonta branch and form a sister group with metazoans, having shared a last common unicellular ancestor more than 600 million years ago. Whereas metazoans are multicellular animals, the choanoflagellates have remained unicellular/colonial. (B) Fixed Salpingoeca rosetta cell (a marine choanoflagellate). A short movie of an S. rosetta cell swimming and additional images of selected S. rosetta cell types can be found in Figure 1—video 1 and Figure 1—figure supplement 1, respectively. (C) Overview cartoon of the choanoflagellate cell architecture, including the cell body and the ring of actin-based microvilli comprising the collar, which surrounds a single flagellum with a flagellar vane. (D) Cross-sectional diagram of the choanoflagellate flagellum indicating known flagellar components. The cross-section in this figure and throughout the paper are viewed from proximal towards the distal tip of the flagellum, and the longitudinal sections are shown with proximal on the left unless otherwise indicated. Labels: CPC, central pair complex; DMT, doublet microtubule; IDA and ODA, inner and outer dynein arm; N-DRC, nexin-dynein regulatory complex; RS, radial spoke. Scale bar: 10 µm (B).

Figure 1—figure supplement 1. Morphology of a subset of S.rosetta cell types.

Examples of choanoflagellate cell morphology from a single culture. Slow swimmers have a relatively large, rounded cell body, whereas fast swimmers have a smaller, pointier cell body, shorter collar microvilli, and a longer flagellum. The cluster shown here is not a true ‘rosette’, but rather represents linked cells that likely recently divided. Cells were fixed with 2% glutaraldehyde and imaged at 60 x magnification using differential interference contrast light microscopy. Scale bar, 10 µm.

Figure 1—video 1. Choanoflagellate cell swimming in culture imaged by light microscopy at ×40 magnification.

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Propelled by its single flagellum, a slow swimming S. rosetta cell (dark/larger oval) navigates through a field of bacterial prey (small circles). Captured bacteria also line the ring of collar microvilli at the base of the cell body. A recently divided doublet cell enters the field of view from the lower right near the end of the video.