The Behavior of F-Actin during the Zoosporic Phases of the Chytridiomycete Gut Fungi Neocallimastix and Orpinomyces

Li, J., and Heath, I. B. 1994. The behavior of F-actin during the zoosporic phases of the chytridiomycete gut fungi Neocallimastix and Orpinomyces; Experimental Mycology 18, 57-69. The changing patterns of F-actin were shown with rhodamine-labeled phalloidin staining of cells fixed at different stages of their development cycle. Zoospores were permeated by diffuse F-actin which was more concentrated at the plasmalemma and in amoeboid projections, suggestive of a role in zoospore morphogenesis. Upon encystment, nuclei became enclosed in a perinuclear shell of F-actin which persisted throughout vegetative growth, including mitosis. We suggest that this shell, and cytoplasmic F-actin filaments, function in nuclear motility and positioning, and the former may also generate nuclear shape at mitosis. Also present during growth were peripheral F-actin plaques specifically associated with both diffuse cell wall deposition over the entire surface of the expanding cell body and localized deposition at rhizoid tips. Similar plaques are previously reported associated with localized wall synthesis in other fungi; their occurrence is now extended to the chytridiomycetes with their diffuse pattern of wall synthesis. During zoosporogenesis, cytoplasmic cleavage vacuoles were associated with extensive F-actin sheets which presumably generate or direct their expansion. Presumptive initials of these vacuoles are also F-actin-rich, consistent with an F-actin role in their initial clustering around the centrioles. Comparison of these observations with previous ones on similar stages in the unrelated oomycetes shows similarities in F-actin associated with wall synthesis, zoospore morphogenesis, and cytokinesis but differences in nuclear-associated F-actin, suggesting that an F-actin role in the former processes is a widespread and ancient phenomenon.