Amylostereum is the single genus in the fungal family Amylostereaceae. The genus currently comprises four saprotrophic and parasitic species, which live off living or dead wood. The Amylostereaceae cause white rot in the wood by disintegrating the tissue component lignin. They produce crust-like, partially wavy fruit bodies on the surface of infested trees, which are similar to those produced by Stereum species.
There are four described species in the Amylostereaceae: A. chailletii (the type), A. areolatum, A. ferreum and A. laevigatum. The species were initially considered part of Stereum until mycologist Jacques Boidin found atypical microscopic differences between them. Forty years after his extensive researches from 1958, Boidin reclassified Amylostereum into its own family.
Three Amylostereum species are symbionts of wood wasps in the genera Sirex, Urocerus, and Xoanon, which infest conifers. The female wood wasps deposit their eggs together with fungal spores and mucus in trees, and the fungus is eaten by the wasp's larva as food. The fungus propagates vegetatively through the formation of asexual spores in newly emerged females that are stored in special structures adapted for the transport of symbiotic fungi. The A. areolatum–Sirex woodwasp (S. noctilio) symbiont complex has been studied extensively because of its potential to cause substantial economic losses in the forestry industry, particularly in non-native regions.
Taxonomy and history of research
Amylostereaceae species were for a long time classified in the genus Stereum, based primary on the layered structure of the fruit body and the similar physiological activity. Mycologist Jacques Boidin separated Amylosterum from Stereum in 1958, justifying this decision by explaining that microscopic differences such as amyloid spores and encrusted cystidia were sufficiently distinct to warrant recognition as a new genus. Although the type species of the genus (today called A. chailletii) was initially named Trichocarpus ambiguus, the name Trichocarpus had already been used for a genus in the flowering plant family Malvaceae. Boidin thus chose the genus name Amylostereum, referring to the amyloid spores.
Based on DNA analysis, Boidin in 1998 moved Amylostereum into a new, monotypic family, the Amylostereaceae, which he attributed to the Hericiales order. Later studies, however, supported the initial classification in the Russulales.
Classification
The classification of the Amylostereaceae is not completely resolved. The next closest relatives might be – depending on the research – either Echinodontium tinctorium and most other species of the genus Echinodontium, or Artomyces pyxidatus. Most of the previous DNA analysis results suggest a narrow relation to Echinodontium, but several results of studies partially contradict this conclusion. Only the classification to the Russulales is regarded as correct. Some authors have suggested that Amylostereum should be placed in the family Echinodontiaceae.
The similarities between A. chailletii and A. areolatum have caused some confusion regarding their placement in the genus. As only the size of their fruit bodies differ from each other in appearance, researcher German Josef Krieglsteiner assumed that both are the same species in different age stages. Experiments with pure cultures of the fungi, however, showed that the mycelia of A. chailletii, A. laevigatum and A. ferreum were partially compatible to each other, but the mycelium of A. areolatum was incompatible to other species. Boidin believes that the common ancestor of all Amylostereum fungi used yellowwoods as a host. This genus of conifers was native in Europe until the Paleogene and Neogene Periods (66 million to 2.6 million years ago), but became extinct there, so the Amylostereum fungi specialized on other conifers and differentiated into several species. Only A. ferreum specialized on yellowwoods in South America.