Further evidence for host-specific variants in Zygorhizidium planktonicum

Abstract

Zygorhizidium planktonicum (Chytridiales) an obligate, epibiotic parasite of freshwater diatoms was recently isolated from Lake Lanier, Georgia. Host specificity and ultrastructural morphology of clones infecting Synedra acus were investigated to further evaluate the life history and taxonomic significance of host-specific strains. Host range studies revealed that germination and sporangium development were confined to Synedra sp., S. acus, and S. ulna. On occasion, zoospores loosely adhered to Asterionella formosa; however, subsequent growth did not ensue despite efforts to induce parasitism in mixed cultures and altered environmental conditions. Ultrastructural studies of conjugating gametangia demonstrated the fusion of thalli by means of a narrow, smooth-walled tube and the apparent migration of the nucleus and other organelles from the donor gametangium to the larger, recipient thallus. The resulting resting spore possessed a thick multilayered wall, either one or two nuclei, and electron opaque bodies of unknown composition. Zoospores contained discrete morphological features including a rumposome-associated fibrillar vesicle, dispersed ribosomes, and a dense particulate body. Our findings regarding this disjunct population confirm the occurrence of host-specific variants in Z. planktonicum. Host selection indicates a conserved ontogeny which further suggests that host-selective strains have coevolved with their respective diatom hosts.

Key Words:

• Chytridiales
• diatoms
• host specificity
• parasitism
• resting spores
• ultrastructure
• zoospore
• Zygorhizidium planktonicum