Mitospore formation on pure cultures of Tuber japonicum (Tuberaceae, Pezizales) in vitro.

The members of the genus Tuber are Ascomycota that form ectomycorrhizal associations with various coniferous and broadleaf tree species. In the teleomorphic stage, the species of the genus produce fruit bodies known as true truffles. Recent studies have discovered mitosporic structures, including spore mats, of several Tuber species on forest soils, indicating the presence of a cryptic anamorphic stage or an unknown reproductive strategy. Here, we report in vitro mitospore formation on the mycelium of T. japonicum, which belongs to the Japonicum clade, collected in several regions in Japan. Twenty of the 25 strains formed mitospores on modified Melin-Norkrans agar medium, indicating that mitospore formation is likely a common trait among strains of T. japonicum. The fungus forms repeatedly branched conidiophores on aerial hyphae on colonies and generates holoblastic mitospores sympodially on the terminal and near apical parts and/or occasionally on the middle and basal parts of the conidiogenous cells. Mitospores are hyaline and elliptical, obovate, oblong, or occasionally bacilliform, with a vacuole and often distinct hilar appendices. Formation of mitospores by T. japonicum in vitro is useful in understanding the functions of mitospores in the genus Tuber under controlled environmental conditions.