Diverse organizations of actin and nuclei underpin the evolution of indeterminate growth in Chytridiomycota and Dikarya

Abstract

Indeterminate growth, as in the hyphae of the “Humongous Fungus” of Michigan, requires sustained nuclear migration and cell wall remodeling. In this study, we compare actin organization and patterns of nuclear positioning among four distantly related, indeterminate species of phylum Chytridiomycota: Cladochytrium replicatum Karling (Cladochytriaceae), Physocladia obscura Sparrow (Chytriomycetaceae, Chytridiales), Nowakowskiella sp. J. Schröt. (Nowakowskiellaceae), and Polychytrium aggregatum Ajello (Polychytriales). We combined light microscopy, nuclear staining with 4′,6-diamidino-2-phenylindole, and actin staining with rhodamine phalloidin to analyze actin distribution and nuclear migration during somatic growth in the four Chytridiomycota species. Actin formed plaques, filaments, cables, and perinuclear shells in patterns that varied across the four species. All four species initiated indeterminate growth by extending branching, anucleate rhizomycelium, <1 µm in diameter. Nuclei, some elongated as if migrating, first appear in intercalary segments that widened to diameters >1 µm. After mitosis, an intercalary swelling in C. replicatum became septate and a single, distal nucleus migrated tipwards to a new swelling. In Physocladia obscura, swellings were aseptate and multinucleate, and several nuclei migrated tipwards into a new swelling. Nuclei migrated tipwards from irregularly cylindrical filaments in Nowakowskiella sp., and in Polychytrium aggregatum, from regular, hypha-like filaments. Thus, distantly related lineages of zoosporic fungi deploy ancestral morphogenetic machinery in differing patterns that result in convergent, indeterminate growth.