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Abstract
The fungal parasite Zygorhizidium planktonicum (Chytridiales) exhibits a haplontic life cycle characterized by an asexual infection phase and the formation of sexual resting spores following gametangial conjugation. Uninucleate gametangia fuse via an epibiotic conjugation tube formed by the smaller, donor thallus. Plasmogamy entails allogamous and autogamous fusions; however, mechanisms for sexual expression remain unresolved. Karyogamy occurs within the recipient gametangium as documented by a single nucleus following deposition of a thick, multi-layered wall. The zygotic resting spore remains undifferentiated throughout dormancy at 4 C. Upon activation, a single germination pore is formed, and a thin-walled, operculate meiosporangium emerges as an extension of the inner spore wall. The nucleus migrates into the meiosporangium during premeiotic interphase or prophase of meiosis I. Meiotic nuclei are evidenced by synaptonemal complex formation 24–48 h after germination. Nuclear division and meiosis II proceed in rapid succession followed by at least one mitotic division prior to zoosporogenesis. Confirmation of zygotic meiosis in Z. planktonicum further substantiates the prevalence of haplontic life cycles in the Chytridiomycetes. The taxonomic implications of sexual reproduction are reevaluated with particular attention to plasmogamous mechanisms and nuclear cycles.