Ultrastructure of Synchytrium macrosporum zoospores

Abstract

The ultrastructure of the posteriorly uniflagellate zoospores of Synchytrium macrosporum Karling, an obligate plant parasitic Chytridiomycete with a wide host range, is described. The zoospore possesses characteristic and intricate internal organellar associations involving the nucleus, mitochondria, microbody, lipid globule, endoplasmic reticulum, rumposome, and Golgi apparatus. The nucleus is in the anterior portion of the cell, and the lipid globule is in the posterior region. A complex microbody is between the nucleus and the lipid globule and branches out to partly encircle the nucleus, the lipid globule, and the mitochondria. A well-developed rumposome is in the posterior half of the cell, but a nuclear cap is absent. The rumposome is continuous with the rough endoplasmic reticulum, and is associated with a single dictyosome near the kinetosome. The rumposome encapsulates most of the exposed surface of the posterior portion of the lipid globule. Vacuoles have various shapes and contents, including some with the appearance of gamma bodies seen in other chytrids. A system of branched knobbed props links the kinetosome to the plasma membrane of the zoospore. The kinetosome and the secondary centriole are transversely striated, but no terminal plate is present. This study shows that the zoospore of S. macrosporum has a distinctive cell organization as compared with Synchytrium endobioticum and other chytrid zoospores.