Zoospore ultrastructure of Zygorhizidium affluens and Z. planktonicum, two chytrids parasitizing the diatom Asterionella formosa

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Abstract
The fine structure of zoospores of two Zygorhizidium species pathogenic on the freshwater planktonic diatom Asterionella formosa Hassall, are described. Zygorhizidium affluens Canter and Z. planktonicum Canter have monocentric operculate sporangia and zoospores which have single lipid globules to which the cell nucleus and single mitochondrion are closely adpressed. Several features place Z. affluens in the Chytridiales sensu Barr. These include having its kinetosome and nonfunctional centrioles not closely associated with the nucleus and having a ribosome-rich core region delimited by endoplasmic reticulum. However, there are no cytoplasmic microtubules or membranous rumposome complex in this species. In contrast, zoospores of Z. planktonicum have dispersed or loosely aggregated ribosomes and a rumposome complex. The zoospores of this species appear to belong to the same morphological subtype as the pathogenic genus Synchytrium, which has also been placed in the Chytridiales. Zygorhizidium planktonicum zoospores possess a number of vesicle types apparently unique to this species and have a nonfunctional centriole which is constructed of a ring of singlet microtubules and is often completely disassociated from the kinetosome. Both species contain "peripheral vesicle" systems which are probably involved in regulating internal water potential. From this study it seems unlikely that the two species are closely related, although they have been placed in the same genus.