A Contrast Between Oomycetes and Other Taxa of Mycelial Fungi in Regard to Metachromatic Granule Formation

Summary Forty‐nine cultures of mycelial fungi, representing five different taxonomic classes and diverse ecological groupings, were screened for the presence of metachromatic granules by staining hyphae with toluidine blue. Regardless of the ecological roles of the fungi, granules were detected in all representatives of the following classes: Zygomycetes, Ascomycetes, Deuteromycetes and Basidiomycetes. When selected mycelia were starved of phosphorus during growth, they lacked granules. However, these same mycelia would subsequently form granules readily if they were incubated for several hours in a phosphate‐containing solution, which supports the belief that these met achromatic granules represent a store of polyphosphate. No met achromatic granules could be detected in any of the fungi from the class Tomcats, which provides further evidence of the unique nature of this particular group of fungi. If their inability to form met achromatic granules is indicative of a limited capacity to store phosphorus, it could explain why no reliable examples have been discovered of Oomycete fungi forming symbiotic mycorrhizal associations with higher plants.