What are Spizellomycetalean Chytrids?
In 1980 Donald Barr recognized that the zoosporic 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.
Current systematic synthesis of the order Spizellomycetales:
The thallus (=body) of these chytrids consist of a sperical, multinucleate sporangium bearing branched rhizoids with rounded tips. In all genera except Powellomyces, there typically is a subsporangial swelling, the apophysis, from which the rhizoids originate. When the sporangium is mature, it releases large numbers of uniflagellate zoospores from one to several discharge pores on the sporangium. 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.
A constellation of zoospore ultrastructural features distinguishes 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. The Order Spizellomycetales includes five genera (Wakefield et al. 2010), and each has an ultrastructurally distinctive zoospore:
- Fimicolochytrium (2 species)
- Gaertneriomyces (3 species)
- Geranomyces (4 species)
- Kochiomyces (1 species)
- Powellomyces (1 species)
- Spizellomyces (8 species)
- Thoreauomyces (1 species)
- Triparticalcar (1 species)
Several genera earlier classified in the Spizellomycetales have either been moved to the new order Rhizophlyctidales (Rhizophyctis with 33 species) or are considered questionable members based on recent molecular sequence analyses (Rozella with 29 species, Olpidium with 98 species, and Caulochytrium with 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. 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.
In a multigene phylogenetic analysis of Spizellomycetales that included the type cultures on which the taxa are based, Wakefield et al (2010) found that the genera Kochiomyces, Powellomyces and Triparticalcar were all monophyletic. However, Gaertneriomyces and Spizellomyces were polyphyletic. Some of the culture isolates studied were distinct from described species, indicating that species diversity is greater than currently recognized. As a part of the NSF PEET project, these taxa are being revised and new taxa discovered and described.
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.
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