Ultrastructural description of a new chytrid genus of caecum anaerobe, Caecomyces equi gen. nov., sp. nov., assigned to the Neocallimasticaceae.

Vegetative and reproductive stages of Caecomyces equi gen. nov., sp. nov. isolated from the horse caecum were examined by light and electron microscopy. This organism, which is similar to isolates known as Sphaeromonas communis, produces uniflagellate, uninucleate zoospores whose perikinetosomal structures, i.e. circumflagellar ring, spur, struts and scoop, are similar in many respects to those described in species of Neocallimastix. Microtubular roots extend basally from the spur and associate with hydrogenosomes and the nucleus. Another group of microtubules radiates laterally in a fan-shaped array close to the plasmalemma. Zoospores encyst, shedding their flagella with basal bodies, and germinate to diglobular thalli. Either coralloid or bulbous rhizoids form in plant material, but only the latter in axenic culture. Incipient zoospores are produced from a multinucleate eucarpic thallus and devlop within cleavage vacuoles containing flagella. An isolate from the cow rumen was found to be similar to C. equi in morphology and zoospore ultrastructure. On the basis of zoospore ultrastructure, we assign the new genus to the Neocallimasticaceae of the order Spizellomycetales. Organisms previously described as Sphaeromonas communis and Piromonas communis are renamed Caecomyces communis and Piromyces communis and assigned to the same family.