Genetic transformation and live-cell nuclear and actin dynamics during the life cycle of a chytrid

Chytrids are early-diverging fungi that share ancestral features of animals, including cells that crawl and swim. At later stages, chytrid cells resemble fungi with a chitin-based cell wall and hyphal-like structures known as rhizoids. Chytrids are important evolutionary transitional forms, but much remains unknown about their cell biology because we lack genetic tools for the live-cell imaging of their nuclear and cytoskeletal dynamics. Here, we generated stable transgenic lines of the soil chytrid Spizellomyces punctatus, and coupled live-cell microscopy and fluorescent tagging to measure the timing and coordination of growth, the cell cycle, and the actin cytoskeleton. We show that Spizellomyces zoospores rapidly encyst, develop rhizoids, and undergo multiple rounds of synchronous nuclear division in a sporangium, followed by cellularization, to create and release hundreds of zoospores. The life cycle is complete in less than 30 hours. We further demonstrate that crawling zoospores, akin to animal cells, display polymerized actin at the leading edge of amoeboid fronts. After encystment, polymerized actin reorganizes into fungal-like cortical patches and cables that extend into the rhizoid. Actin remains highly dynamic during sporo-genesis with the formation of actin perinuclear shells each cell cycle and the emergence of polygonal territories during cellularization. Spizellomyces is a fast-growing and genetically-tractable organism that should be useful for comparative cell biology and understanding the evolution of fungi and early eukaryotes.