The Flagellated Fungi

Flagellated fungi as we are discussing them here include several divisions that are not closely related and is used here only as a matter of convenience. Also, while these divisions do have flagellated stages, they are not necessarily restricted to an aquatic environment. Many species, including the well known Phytophthora infestans, which causes the Late Blight of Potato, have become adapted to the terrestrial environment. We will discuss two divisions of aquatic fungi, in detail, the Chytridiomycota and Oomycota, and briefly describe a third, the Hyphochytridiomycota.   Flagellated fungi reproduce asexually by means of flagellated spores called zoospores that are produced in zoosporangia. These divisions were once regarded as the most primitive fungi because of their aquatic habitat, when mycelium is produced, it is usually coenocytic (nonseptate), asexual spores are produced in a sporangium, and they lack a complex spore producing body (=sporocarp)

Students of flagellated fungi have long regarded the morphology of the zoospore as an important morphological feature in the systematics of these organisms and have long believed that the different zoospore morphologies (Fig. 1), of the different taxa, to represent taxa that are not closely related.

Chytrid_zoospore.jpg (6994 bytes) Primary_zoospore.jpg (10620 bytes) Secondary_zoospore2.jpg (8605 bytes) Hypho_zoospore.jpg (10479 bytes)
Figure 1A: Zoospore with a single, posterior, whiplash flagellum of Chytridiomycota. 1B: Biflagellated, primary zoospore of Oomycota, with posterior whiplash and tinsel type flagella. 1C: Biflagellated, secondary  zoospore of Oomycota, with lateral whiplash and tinsel type flagella. 1D: Anterior, uniflagellate, tinsel type flagellum of Hyphochytridiomycota. 

The two types of flagella illustrated, above, are whiplash and tinsel type flagella. The whiplash flagellum is unbranched with an acute bend at the end. As flagellar movement propels the cell, the bent end whips back and forth. Thus, the name whiplash. The tinsel type is branched with many mastigonemes, along its axis and resembles the tinsels of an aluminum christmas tree.

Results of phylogenetic studies utilizing more recently developed methods, e.g. cell wall biochemistry, ultrastructure and sequencing of the 18S Ribosome, have largely verified the traditional practice of utilizing zoospore morphology as the basis for separating the different taxa of flagellated fungi was a sound one.

For many years, fungi, in the strict sense, have been defined according to Margulis and Schwartz (1988): "Heterotrophic, eukaryotic organisms that derive nutrition by absorption, produce aplanospores (=nonmotile spores) on mycelium and/or yeast thalli and have cell walls in their assimilative stage that are composed mostly of chitin". Thus, since flagellated fungi have motile spores, they have all been classified in the kingdom Protista rather than in Myceteae (=Fungi). However, some rather significant changes in our concepts of these fungi have come about since the advent of molecular studies. The following tree, which include organisms that have traditionally been classified as fungi, has been generated based primarily on small subunit robosomal deoxyribonucleic acid (rDNA) sequence analysis: 

Fungi_tree.jpg (35440 bytes)
The above tree is based on Patterson and Sogin (1992), Sogin (1992), Barr (1992) and Bruns et al., (1991, 1993).

Note that in the above tree, despite the presence of flagellate stages in their life cycles, the division Chytridiomycota is now included with the true fungi. Based on cell wall biochemistry, the Chytridiomycota was first classified with the true fungi, according to Bartnicki-Garcia (1970). However, because of the lack of morpohological similarities to the terrestrial fungi, there was a great deal of hesitation in classifying them as fungi and they continued to be maintained in the kingdom Protista since the onset of the five kingdom system of classification. However, since molecular evidence now supports Bartnicki-Garcia's cell wall classification scheme, the Chytridiomycota have been included with the true fungi. The Oomycota and Hyphochytridiomycota, however, now appear to be unrelated to the fungi and some mycologists now include them in a separate Kingdom, Stramenopila, which also include the algal divisions, Phaeophyta and Chrysophyta. The kingdom Stramenopila include organisms that have common cell wall biochemistry, zoospore morphology and food storage material. More will be said on these characteristic when we discuss the Oomycota.

Literature Cited

Barr, D.J.S. 1992. Evolution and Kingdoms of Organisms from the Perspective of a Mycologist. Mycologia 84:1-11.

Bartnicki-Garcia, S. 1970. Cell Wall Composition and Other Biochemical Markers in Fungal Phylogeny. Pp. 81-103. In: Phytochemical Phylogeny. Ed. J.B. Harborne. Academic, London.

Bruns, T.D., R. Vilgalys, S.M. Barns, D. Gonzalez, D.S. Hibbett, D.J. Lane, L. Simon, S. Stickel, T.M. Szaro, W.g. Weisburg, and M.L. Sogin. 1993. Evolutionary Relationship within the Fungi: Analyses of Nuclear Small Subunit rRNA Sequences. Mol. Phylogenet. Evol. 1:231-241.

Margulis, L., and K. V. Schwartz. 1988. Five Kingdoms, An Illustrated Guide to the Phyla of Life on Earth . W.H. Freeman and Company, New York.

Patterson, D.J., and M.L. Sogin. 1992. Eukaryote Origins and Protistan Diversity. Pp. 13-46. In: The Origin and Evolution of Prokaryotic and Eukaryotic Cells. Eds. H. Hartman and K. Matsuno. World Scientific, Singapore.

Sogin, M.L. 1992. Comments on Genome Sequencing. Pp. 387-389. In: The Origin and Evolution of Prokaryotic and Eukaryotic Cells. Eds. H. Hartman and K. Matsuno. World Scientific, Singapore.

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