Functional necessity of the cytoskeleton during cleavage membrane development and zoosporogenesis in Allomyces macrogynus

Cleavage membrane development and cytokinesis were examined in zoosporangia of Allomyces macrogynus treated with cytoskeletal inhibitors and compared to zoosporogenesis under control conditions. Developing membranes were visualized in living zoosporangia with laser-scanning confocal microscopy using the lipophilic membrane dye FM4-64. Under control conditions, cleavage membranes developed in four discrete stages, ultimately interconnecting to delimit the cytoplasm into polygonal uninucleate domains of near uniform size. Disruption of microtubules did not impede the normal four-stage development of cleavage membranes, and cytokinesis occurred with only minor detectable anomalies, although zoospores lacked flagella. Disruption of actin microfilaments did not inhibit membrane formation but blocked nuclear migration and significantly disrupted membrane alignment and cytoplasmic delimitation. This resulted in masses of membrane that remained primarily in cortical regions of the zoosporangia, as did nuclei, throughout zoosporogenesis. Zoospores formed in the absence of microtubules had only a slightly larger mean diameter than control zoospores, although nearly 50% of spores contained two or more nuclei. Microfilament inhibitor treatments produced spores with substantially larger mean diameters and correspondingly larger numbers of nuclei per spore, with greater than 85% containing three or more nuclei. These results showed that a functional actin microfilament cytoskeleton was required for proper alignment of cleavage elements and cytokinesis in Allomyces zoosporangia while microtubules played a less significant role.