What are Spizellomycetalean Chytrids?

In 1980 Donald Barr recognized that zoospore subcellular architectures of chytrids classified in the order Chytridiales were fundamentally different. Because of the conservative nature of zoospore features, he revised the order Chytridiales, splitting out from it the Spizellomycetales.

A constellation of zoospore ultrastructural features distinguish the Spizellomycetales from the revised Chytridiales. In the Spizellomycetales the nucleus is spatially or structurally associated with the kinetosome, a non-flagellated centriole lies at an angle to the kinetosome, organelles of MLC are loosely associated, and ribosomes are not aggregated near or around the nucleus. At the light microscopic level, spizellomycetalean zoospores are recognizable because they can be highly amoeboid while actively swimming and flagellar insertion may move to a lateral position.

Current systematic synthesis of the order Spizellomycetales:

The Order Spizellomycetales includes ten genera:

• Kochiomyces (1 species)
• Spizellomyces (8 species)
• Triparticalcar (1 species)
• Gaertneriomyces (3 species)
• Powellomyces (2 species)
• Catenomyces (1 species) (based on recent molecular sequence analyses)
• Rhizophlyctis (33 species)
• Olpidium (98 species), (questionable members based on recent molecular sequence analyses)
• Rozella (29 species), questionable members based on recent molecular sequence analyses)
• Caulochytrium (2 species)

Spizellomycetalean Chytrids are a priority to study:

Spizellomycetalean chytrids are common in the soil and are also parasites of a range of soil organisms and plants. They are readily recovered from crop soils and disturbance appears to increase their populations. Certain soil chytrids can withstand drying and high temperatures. Researchers are just now including chytrids in studies of fertility and nutrient dynamics of soils. Greater molecular taxon sampling of these soil chytrids is needed to compare with gene clones from environmental samples to elucidate their ecological role in soils.

Some spizellomycetalean chytrids are important pathogens that can have beneficial as well as detrimental impacts. Although most spizellomycetalean chytrids are terrestrial, Olpidium and Rozella parasitize aquatic organisms. Species of Spizellomyces and Gaertneriomyces parasitize oospores of downy mildews as well as soil nematodes, with resulting improved plant growth. As parasites of vesicular-arbuscular mycorrhizal (VAM) fungi, however, Spizellomyces species may adversely affect VAM-dependent plant health. Olpidium sps. parasitize plants, but their greatest damage is in transmitting plant disease virus, such as big vein of lettuce. Caulochytrium parasitizes hyphal fungi growing on leaves. This is an intriguing genus because in addition to the thallus produced on the host fungal hyphae, Caulochytrium also produces a stalked sporangium that resembles a protostelid.

Studying spizellomycetalean chytrids is a priority in understanding the evolutionary adaptations of fungi. In molecular phylogenetic analyses of chytrids, the spizellomycetalean chytrids are derived from more aquatic taxa. Especially interesting is the early divergence of the core chytrids from the lineage leading to higher fungi. Studying these "twigs and leaves" on the fungal tree of life helps elucidate transitional adaptations from water to land and how these adaptations parallel those found in the lineage that includes the Blastocladiales and leads to higher fungi.

References:

• Barr, D.J.S. 1980. An outline for the reclassification of the Chytridiales, and for a new order, the Spizellomycetales. Can. J. Bot. 58: 2380-2394.
• Barr, D.J.S. 1981b. Ultrastructure of the Gaertneriomyces zoospore (Spizellomycetales,
• Chytridiomycetes). Can. J. Bot. 59: 83-90.
• Barr, D.J.S. 1984a. Cytological variation in zoospores of Spizellomyces (Chytridiomycetes). Can. J. Bot. 62: 1202-1208.
• Barr, D.J.S. 1984b. The classification of Spizellomyces, Gaertneriomyces, Triparticalcar, and Kochiomyces (Spizellomycetales, Chytridiomycetes). Can. J. Bot. 62: 1171 -1201.
• Barr, D.J.S. and P.M.E. Allan. 1981. Ultrastructure of Kochiomyces and Triparticalcar zoospores (Spizellomycetales, Chytridiomycetes). Can. J. Bot. 59: 649-661.
• Barr, D.J.S. and N.L. Desaulniers. 1986. Four zoospore subtypes in the Rhizophlyctis-Karlingia complex (Chytridiomycetes). Can. J. Bot. 64: 561-572.
• Koch, W.J. 1958. Studies of the motile cells of chytrids. 11. Internal structure of the body observed with light microscopy. Amer. J. Bot. 45: 59-72.
• Longcore, J.E., D.J.S. Barr and N. Desaulniers. 1995. Powellomyces, a new genus in the Spizellomycetales. Can. J. Bot. 73: 1385-1390.
• Lozupone , C.A. and D.A. Klein. 2002. Molecular and cultural assessment of chytrid and Spizellomyces populations in grassland soils. Mycologia 94: 411-420.
• Powell, M.J. 1974. Fine structure of plasmodesmata in a chytrid. Mycologia 66: 606-614.
• Powell, M.J. 1975. Ultrastructural. changes in nuclear membrane and organelle associations during mitosis of the aquatic fungus Entophlyctis sp. Canad. J. Bot. 53: 627-646.
• Powell, M.J. 1976a. Development of the discharge apparatus in the fungus Entophlyctis. Arch. Microbiol. 111: 59-71.
• Powell, M.J. 1976b. Ultrastructure and isolation of glyoxysomes (microbodies) and zoospores of the fungus Entophlyctis. Protoplasma 89: 1-27.
• Powell, M.J. 1978. Phylogenetic implications of the microbody lipid globule complex in zoosporic fungi. BioSystems 10: 167-180.
• Powell, M.J. 1979. The structure of microbodies and their associations with other organelles in zoosporangia of Entophlyctis variabilis. Protoplasma 98: 177-198.
• Powell, M.J. 1981b. Zoospore structure of the mycoparasitic chytrid Caulochytriumprotostelioides Olive. Amer. J. Bot. 68: 1074-1089.
• Powell, M.J. and W.J. Koch. 1977a. Morphological variations in a new species of Entophlyctis. I. The species concept. Canad. J. Bot. 55: 1668-1685.
• Powell, M. J. and W.J. Koch. 1977b. Morphological variations in a new species of Entophlyctis. Il. Influence of growth conditions on morphology. Canad. J. Bot. 55: 1686-1695.