Assignment of the rumen anaerobe Neocallimastix frontalis to the Spizellomycetales (Chytridiomycetes) on the basis of its polyflagellate zoospore ultrastructure

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Abstract
Neocallimastix frontalis is an anaerobic herbivore gut inhabitant which occupies an uncertain taxonomic position, probably among zoospore fungi. It produces uninucleate zoospores which are bipartite with an organelle-free anterior portion and a posterior half bearing about 10 flagella and all organelles. The flagella are arranged primarily in two rows and beat as a unit. The flagellar roots are composed of two arrays of single microtubules, all of which emanate, in groups of about 14, from single kinetosome-associated spurs. The main array forms a divergent cone with which the microbodies (hydrogenosomes?) are intimately associated. The second group (the posterior fan) forms a fan-shaped array lying close to the posterior plasmalemma. Zoospores encyst, develop rhizoids, and can produce new zoospores at any stage in their subsequent development from a uninucleate to a multinucleate eucarpic thallus. Zoospore production involves intracytoplasmic flagellar vesicles, organelle clustering, and loss of the sporangium plasmalemma. Zoospore release follows nonlocalized dissolution of the entire sporangium wall. On these data, we assign the genus to a new family, the Neocallimasticaceae, in the Spizellomycetales of the Chytridiomycetes and thereby redefine these taxa to accommodate organisms producing polyflagellate zoospores.