Fig 1.

A colonial choanoflagellate isolated from Mono Lake. (A) Choanoflagellates were collected from three sampling sites (asterisks) near the shore of Mono Lake, California (modified from a map in the public domain, formatted as USGS Imagery Only; https://www.usgs.gov/volcanoes/long-valley-caldera) (B) B. monosierra forms large colonies (differential interference contrast image). Scale bar = 50 µm. (C) B. monosierra (shown in bold) is a craspedid choanoflagellate closely related to S. rosetta and Microstomoeca roanoka. Phylogeny based on sequences of three genes: 18S rRNA, EFL, and HSP90. Metazoa (seven species) were collapsed to save space. Bayesian posterior probabilities are indicated above each internal branch, and maximum likelihood bootstrap values are below. (A “—” value indicates a bifurcation lacking support or not present in one of the two reconstructions.) Also see Fig. S1C for further phylogenetic analyses. (D and E) Two colonies from the ML2.1G culture (Fig. S1, Box2) reveal the extremes of the B. monosierra colony size range (D, 58 µm diameter; E, 19 µm diameter; scale bar = 20 µm). In B. monosierra colonies, each cell is oriented with its apical flagellum (white; labeled with anti-tubulin antibody) and the apical collar of microvilli (red; stained with phalloidin) pointing out. Nuclei (cyan) were visualized with the DNA-stain Hoechst 33342. (F) Colonies of B. monosierra span from 10 µm in diameter, a size comparable to that of small S. rosetta colonies, to 120 µm, over an order of magnitude larger. The diameters of B. monosierra and S. rosetta colonies were plotted as a violin plot; the median is indicated as a thick black line. Diameters of the colonies in panels D and E are indicated as colored bars behind the violin plot (D, red bar; E, blue bar).