Fig. 2
Zoospores and chitin-bearing infective cysts of Paraphelidium tribonemae. a Filament of Tribonema gayanum infected by P. tribonemae under optical microscopy. Many cysts can be seen attached to the filament surface, many (small) zoospores can be seen surrounding the filament before encystment; trophonts are present within Tribonema cell-wall delimited compartments. b The same filament as in a stained with fluorescent wheat germ agglutinin (WGA, red color) showing the presence of chitin in infective cysts (arrows show some examples) but not the zoospores (asterisks show some examples) under epifluorescence microscopy. c False-colored scanning-electron microscopy image of a filament infected by several P. tribonemae cysts (pedunculated rounded structures). The algal host filament is colored in green and parasite cysts in pink. Note that one cyst germ tube is penetrating the host cell by the space between the two algal cell-wall halves (white arrowhead) and two cysts are broken (black arrowheads), showing the penetration channel. d Ameboid zoospore (infrequent). e Amoeboflagellated zoospore. Scale bar: A = 5 µm, B–F = 1 µm. Phylogenetic trees related to chitin and other cell-wall synthesis and degradation-related enzymes are shown in Supplementary Fig. 2. Zoospore motility can be seen in Supplementary Video 1. For original fluorescent microscopy images, see Supplementary Data 7
