The ultrastructure of the zoospore of Hyphochytrium catenoides

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Abstract
The ultrastructure of the zoospore of Hyphochytrium catenoides Karling is described. The zoospore has a single, anterior, tinsel flagellum. The nucleus is elongate and convoluted with an indentation at the anterior end in which the Golgi cisternae are located. There are large lipid globules in the posterior end of the cell. The ribosomes are loosely enclosed by endoplasmic reticulum, the nuclear envelope, and mitochondria. The mitochondria have tubular cristae in a dense matrix. Microbodies are found appressed to the nuclear envelope and also free in the ribosomal region. Endoplasmic reticulum sheets traverse the ribosome region. The vesiculate cytoplasm has several distinct types of membrane-bound inclusions: (i) multivesicular bodies, (ii) vesicles containing presumptive mastigonemes, (iii) vesicles having an electron-dense cortex with an electron-transparent center, and (iv) electron-opaque vesicles whose contents seem condensed and only partially fill the vesicles. The transition zone from the flagellum to the kinetosome has three segments: a distal set of struts extending from the axonemal doublets into the axoneme core, a midsection of electron-opaque rings, and a distinctive "disclike" terminal plate with a thickened portion between the doublets and the flagellar membrane. The three-part rootlet system has (i) a "ribbed" pair of microtubules on one side of the kinetosome, (ii) a curved "ribbed," single microtubule with electron-opaque backing which originates near the nonfunctional centriole, and (iii) a straight doublet of microtubules without ribs extending from the nonfunctional centriole posteriorly to the midregion of the zoospore.