The development and zoospore ultrastructure of a polycentric chytridiomycete gut fungus, Orpinomyces joyonii comb.nov.

Abstract

Orpinomyces bovis, a polycentric gut fungus isolated from a steer, was examined with both light and electron microscopy and renamed Orpinomyces joyonii comb.nov. on the basis of its general morphology and zoospore ultrastructure. The multinucleate rhizomycelium is extensively branched, and sporangia form exogenously on branched or unbranched sporangiophores. The organelles in the zoospores have a distribution pattern typical of other gut fungi, i.e., anterior ribosomal aggregates, central nuclei, and posterior presumptive hydrogenosomes. The perikinetosomal apparatus in O. joyonii is comparable to that in monocentric gut fungi but with minor variations. New details of the posterior dome are described. It contains highly ordered specialized lamellae, peripheral granules, and megatubules. Microtubules intersect the dome predominantly at approximately right angles to its surface; this differs from monocentric gut fungi, in which microtubules form a posterior fan running parallel to the dome. We suggest that both monocentric and polycentric gut fungi are monophyletic, since both have a similar, distinctive perikinetosomal apparatus, posterior dome, and organelle distribution pattern. Key words: Orpinomyces joyonii, gut fungi, ultrastructure, posterior dome, perikinetosomal apparatus.