Comparative ultrastructure and biochemistry of chytridiomycetous fungi and the future of the Harpochytriales

Abstract

The ultrastructure of the zoospores of Oedogoniomyces lymnaeae and Monoblepharella sp. are reexamined using serial sections. The relative molecular weights of the ribosomal RNA of various chytridiomycetous fungi are also determined and compared. The fine structure of each of these zoospores is very similar. The nucleus and "nuclear cap" are centrally located in the zoospores. The anterior region contains lipid globules, a few large, empty vacuoles and numerous, small vesicles. The posterior region is filled with an electron-translucent material throughout which are scattered spherical mitochondria and large vacuoles with electron-opaque inclusions. The basal body complex is located at the center of the posterior end, and the rumposomal complex is adjacent to the plasmalemma in the posterior region of these zoospores. A system of cisternae that are associated with microbodies appears to connect the lipid globules in the anterior region with the rumposomal complex in the posterior region of these zoospores. Small differences in the structure and distribution of certain organelles are considered minor compared with the overall similarities between these two zoospores. It is suggested that Oedogoniomyces be transferred to the Monoblepharidales. The fine structure of the Harpochytrium hedinii zoospore is not only similar to that of the zoospores studied herein, but also resembles the zoospores of chytrids. The possible relationship of Harpochytrium to the Chytridiales is supported by zoospore ultrastructure and the relative molecular weights of ribosomal RNA. The data available at the present time suggest that the order Harpochytriales be abandoned and that Harpochytrium be considered either a genus of nematosporangial, operculate, eucarpic chytrids or a monoblepharidalean genus with Oedogoniomyces. It is recommended that speculation concerning the taxonimic relationship between Harpochytrium and other Chytridiomycetes be delayed until more data concerning other species of Harpochytrium and monoblepharidalean fungi are available.