Cell wall-forming chitin synthases in a chytrid fungus.

Chitin is a critical structural component of fungal cell walls, yet our understanding of its synthesis across the kingdom Fungi remains limited. Here, we investigate chitin synthase diversity, transcription and localisation in the aquatic saprotrophic chytrid Rhizoclosmatium globosum, expanding insights into fungal cell wall biology beyond Dikaryan models. We identified 20 chitin synthase genes in the R. globosum genome, mostly canonical Division I and II types with conserved functional motifs. Transcriptomic analysis through zoospore, germling and immature thallus developmental stages revealed stage-specific expression patterns, with active gene diversity correlating with increasing morphological complexity. Using electroporation-based transformation and fluorescent fusion constructs, we indicate expression and localisation of two chitin synthases during cell development. Localisation patterns suggest dynamic redistribution from cytoplasmic dispersion in early encysted cells to concentrated signals at the sporangium wall. Expression in and around the apophysis indicates the importance of these structures in cell maintenance. Our findings highlight functional specialisation among chitin synthases and underscore the importance of cell wall integrity in chytrid development. This work establishes R. globosum as a genetically tractable model for studying chytrid cell biology and contributes to a broader understanding of fungal evolution and cell wall dynamics.