Taxonomic characterisation of the Regianum clade (genus Tuber) and the trait evolution of spore size among true truffles

Abstract

The genus Tuber comprises ectomycorrhizal fungal species producing belowground ascomata, including the gastronomically most prominent hypogeous fungi. Since the discovery and description of new species are ongoing, the proportion of undescribed species can be considerable and the taxonomy of the genus goes often through changes. The taxonomy of the genus Tuber would therefore benefit from a periodic review. Tuber species described in Europe in recent decades include Tuber regianum, T. bernardinii and T. magentipunctatum. The common characteristics of these three species are the relatively small-sized spores with alveolate-reticulate ornamentation, a high number of spores per ascus (most frequently 6–8 spores) and small-sized ascomata with a verrucose-papillate or smooth surface. The present study aimed at examining the morphology and ecology of the three species, and providing a detailed taxonomic description of the Regianum clade using a multilocus phylogenetic analysis. In addition to this, we examined whether the apparently plesiomorphic morphological character states of this phylogenetically basal clade are the result of the conservation of ancestral character states. Our results show that in the case of certain morphological traits of the Regianum clade, the apparently plesiomorphic character states are indeed retained ancestral states, while in others, they are convergently reappeared ones. Furthermore, taking an overlook at all Tuber clades, we found that some morphological characters, such as spore length, number of spores per ascus and ascus length, changed together in the same evolutionary patterns, while others transformed in different ways within the genus.