Ultrastructure of chlamydospore development in the plant pathogenic fungus Thielaviopsis basicola

Abstract

Chlamydospores of Thielaviopsis basicola developed in short chains consisting of three to eight spores subtended by one to three thin-walled basal cells. A chlamydospore chain developed in an acropetal fashion from a specialized sporogenous cell which often arose immediately below a phialide. This cell developed in an enteroblastic fashion. It ruptured the outer wall layer of its parent hyphae while the inner wall layer covered the developing cell. As a sporogenous cell elongated its nucleus underwent mitosis and the formation of a transverse septum created two uninucleate cells. The basal cell ceased development while the apical cell continued to grow and divide until a chain of young chlamydospores was formed. A thick, multilayered wall then formed around each spore. This wall was distinct from the original wall of the sporogenous cell which formed an outer covering or envelope over the entire spore chain. A septal pore initially was present between adjacent spores but became plugged and covered with wall material as spores matured. At maturity chlamydospore chains tended to fragment into individual spores. Separation of adjacent spores began near the septal pore plug and progressed outward. Separation was accomplished when the chain envelope broke and pore plugs either split transversely or pulled out of the wall of one of the adjacent spores.

Key Words:

• aleuriospores
• Chalara elegans
• conidium development