Phylogenetic placements and cultural characteristics of Tuber species isolated from ectomycorrhizas

Three New Species of Tuber Discovered in Alpine Fir Forests in Yunnan, China

Three new species of Tuber, T. albicavum, T. laojunshanense, and T. umbilicicavatum belonging to the Puberulum phylogroup, are described based on specimens collected in alpine Abies forests at 3600-4000 m, Northwest Yunnan, China. T. albicavum is distinguished by its ascomata with a single chamber of 0.5-1.8 cm diameter, with an apical opening of 0.2-0.6 cm in diameter, and light golden-brown alveolate reticulate ascospores up to 30 μm in length; T. laojunshanense is characterized by having ascomata with a slightly tomentose surface, sometimes with a white navel, a relatively thick peridium, up to 280 µm, and yellow-brown spores with alveolate reticulate ornamentation, up to 34 µm in length; T. umbilicicavatum is characterized by smooth ascomata with a distinct white navel, a relatively thin peridium, up to 110 µm, and golden or golden-brown alveolate reticulate ascospores, up to 40 μm in length. The molecular analysis of the internal transcribed spacer region also supports that these three new species differ from previously described Tuber species.

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.

Morphological characteristics of ectomycorrhizas formed by in vitro synthesis between conifer seedlings and Tuber mycelial strains of the Puberulum clade isolated in Japan

Seedlings of Pinus densiflora and Abies sachalinensis were inoculated with Tuber mycelial strains of the Puberulum clade in vitro to examine the morphological characteristics of their ectomycorrhizas. Axenically germinated seedlings were inoculated with the mycelia of five taxa from the Puberulum clade and grown in glass jars for 4 mo in an illuminated incubator. The seedlings were successfully colonized by the inoculated Tuber strains, as confirmed by the nuclear ribosomal internal transcribed spacer barcoding of the synthesized ectomycorrhizas. The ectomycorrhizas were characterized by a pale yellow to brown color, short needle-shaped cystidia, and net-like hyphal arrangement, and epidermoid cells on the mantle surface; notably, these features are similar to the ectomycorrhizas of various Puberulum clade members. As the ectomycorrhizas of different Tuber species are indistinguishable by morphological characters, molecular techniques are necessary to identify ectomycorrhizas formed by Tuber species within the Puberulum clade.