Zoospore ultrastructure in the genus Rhizophydium (Chytridiales)

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Abstract
The zoospore ultrastructure of 12 species of Rhizophydium is described. Species include the following: R. chlorogonii (Serbinow) Jaczewski; R. constantineani Saccardo; R. haynaldii (Schaarschmidt) Fischer; R. capillaceum Barr; two morphologically and cytologically different species, each previously identified as R. sphaerotheca Zopf; R. patellarium Scholz; R. biporosum (Couch) Barr; R. subangulosum (Braun) Rabenhorst; R. laterale (Braun) Rabenhorst; R. sphaerocarpum (Zopf) Fischer var. spirogyrae Barr; and two isolates of R. pollinis-pini (Braun) Zopf. The Rhizophydium zoospore is basically similar to the Chytridium zoospore having (1) the nucleus, a compact cluster of ribosomes, one or more mitochondria, and a microbody – lipid globule complex compartmentalized into the core of the zoospore by a double membrane system and (2) two to five microtubules connecting one side of the kinetosome to the rumposome on the lipid globule surface and thus anchoring the lipid globule in a lateral–posterior position in the zoospore. Rhizophydium patellarium does not have kinetosome-associated microtubules or a rumposome but does have the membrane-bound core area. In all species, a microbody and mitochondrion are associated with the lipid globule. The number of mitochondria varies from 1 in some species to several or to over 30 in other species. In one isolate of R. pollinis-pini, there is 1 large mitochondrion and in the other there were 30–35 small mitochondria. The peripheral cytoplasm of all species contains clusters of vesicles or endoplasmic reticulum which bud from the double membrane system, vesicles of moderate electron density, and vacuoles of various sizes; R. capillaceum, R. patellarium, and R. subangulosum have in addition vesicles which contain very electron-dense material. Rhizophydium capillaceum and R. sphaerocarpum zoospores have virus-like particles and the R. biporosum zoospore contains a paracrystalline body. The taxonomic significance of the observations and the relationship of Rhizophydium to other chytrids are stressed in the Discussion.