Ultrastructure of Harpochytrium Hedinii

SUMMARYAn ultrastructural study of growth and reproduction of Harpochytrium hedinii Wille is presented. The fine structure of the zoospore and its subsequent development to a mature thallus reveals a remarkable reorganization of cellular organelles as the fungus transforms from flagellated spore to vegetative thallus. The zoospore displays a complex assemblage of two centrioles, numerous microtubules, a nucleus, and a rumposome, all apparently integrated by striated rootlet material. The “rumposomal complex” is particularly obvious in this organism and mito- chondrial-flagellum associations are lacking. The vegetative phase discloses a close association between nucleus, centrioles, cytoplasmic microtubules, and dictyosomes. Zoosporogenesis is first indicated by initiation of cleavage membranes. This process is followed by axoneme synthesis, formation of the rumposomal complex, and aggregation of ribosomes into a membrane-bounded nuclear cap. The ultrastructure of Harpochytrium indicates Chytridiomycete affinities for this problematical protist. The general zoospore plan is similar to that found in the Monoblepharidales, with noticeable differences in the structure of the rumposomal complex and arrangement of the nuclear cap.