Chytrid pseudopods are actin filled and require both actin polymerization and Arp2/3 activity. (A) Fixed chytrid cells with and without a flagellum (arrow). Staining with fluorescent phalloidin reveals a thin shell of cortical actin surrounding the cell body and a dense network of polymerized actin filling the pseudopods (brackets). DIC, differential interference contrast. (B) Two examples of chytrid cells with pseudopods that lose them when treated with 10 nm latrunculin B, an inhibitor of actin polymerization. Dynamic pseudopods (brackets) return after drug washout. (C) Quantification of reversible inhibition of pseudopods by latrunculin B (Lat B). Only cells that were making pseudopods before treatment and that were not washed away during the experiment were counted. (D) Two examples of chytrid cells with pseudopods that lose them when treated with 10 µm CK-666, an inhibitor of Arp2/3 activity. Pseudopods return after drug washout. (E) Quantification of reversible inhibition of pseudopods by CK-666. Only cells that were making pseudopods before treatment and that were not washed away during the experiment were counted. Symbols are means from three biological replicates, each with at least 29 (C) or 17 (E) cells. P-values were obtained from two-tailed paired t tests.