Insights into the cryptic mating behaviour of Chaga (Inonotus obliquus) (Hymenochaetaceae)

Abstract

Inonotus obliquus(Ach. ex Pers.) Pilàt is a parasitic white heart rot fungus of circumboreal distribution belonging to the family Hymenochaetaceae. It is primarily hosted on birch, but can occur on other hardwoods. Typically, this fungus is recognized by a charcoal-like sterile conk with a yellowish-brown interior that forms on the host tree. Colloquially, this is known as Chaga and is hailed for its health benefits, cultural significance, and economic value. As the name suggests, the sterile conk does not produce any basidiospores. Once in an infection cycle of approximately 10 - 80 years, a spore-producing resupinate and poroid basidioma will form beneath the bark of a recently dead host. This sporadic occurrence makes them difficult to locate and study. The mating system of this fungus has been speculated to be amphithallic (primarily pseudohomothallic + tetrapolar heterothallic). A clarification of general taxonomy, characteristics of decay, life cycle, medicinal properties, cultivation, conservation, agaricomyceteous mating systems, and the classical/genomic characteristics of I. obliquus are provided. Wild strains of I. obliquus were isolated from paper birch in North-Western Ontario and examined for in vitro growth and fruiting characteristics. A protocol was developed to form fertile fruit bodies reliably under laboratory conditions. The nuclei within basidiospores were examined using Hoechst 33342 and fluorescence microscopy throughout the maturation of basidiomata. It was discovered that the basidiospores of I. obliquus have highly variable nuclear characteristics, as they can contain between 1 and 6 nuclei in different ratios at different points in development. Comparisons were made between similar work on Chinese I. obliquus, which showed similarity in some cases and major differences in others. Speculation on implications of basidiospore nuclear behaviour are made. As this fungus can reliably fruit in vitro, it may grant us a greater understanding of mating in other members of the Hymenochaetaceae as a model organism.