Sexuality in the life cycle of truffles: importance and repercussions of being compatible

The spontaneous production of the most valuable truffles has been facing with a severe decline all over the distributional range of these species. Further to this, cultivation of these fungi is possible only for some species and, in many cases, with controversial results. About two decades ago, we started studies with the aim to assist truffle growers and nurseries providing them with innovative tools to boost the production of these fungi. The initial goal was to prevent economic frauds and ecological damages linked to the paucity of reliable morphological traits to identify with certainty the truffle species present on the roots of host plants and/or their ascocarps. Several molecular markers (i.e. simple sequence repeat, SSR loci) and methods have then been developed so that it is now possible to type not only the species but also the strains that give rise to fruit bodies and mycorrhizas of the most valuable Tuber species. As a follow up of these studies, we demonstrated that it is possible to differentiate by molecular markers natural populations of both Tuber melanosporum and Tuber magnatum according to their different geographic provenance (Rubini et al., AEM 2005; Riccioni et al., New Phytol 2008). Even more intriguingly, we found that, within any given truffle population, strains not only share but also exchange alleles with each other. This last finding has propelled basic studies to obtain an advancement in our understanding of life cycle and sexual reproduction of these symbiotic species (Paolocci et al., AEM 2006; Rubini et al., New Phytol 2007). The hypothesis to be tested was whether truffles were strictly selfing species, at that time the most accredited thesis (Bertault et al., Nature, 1998), or, instead, they could outcross as hinted at by our preliminary data. The sequencing of T. melanosporum genome has enabled us to look at the structure and organization of the MAT locus that is the genomic region controlling the sexual reproduction strategy of fungi (Martin et al., Nature 2010). As the two alternative genes, known as mating type genes, at this locus are harbored by different strains it has been concluded that T. melanosporum is a heterothallic, thus an obligatory outcrossing fungus (Rubini et al., New Phytol 2011a). In turn, this finding has promoted efforts to monitor the spatial and temporal distributional patterns of strains of different mating type in the soil and on host plants, either from natural or man-made truffle plantations (Rubini et al., New Phytol 2011b). Further to this, the mining of the MAT genes from T. melanosporum genome has set the ground for deciphering the genetic control of the sexual reproduction in other Tuber spp. Here we report on our recent advances on the life cycle and propagation patterns of Tuber spp. These findings call for a profound reconsideration of the strategies to inoculate and grow host plants and manage productive truffle plantations. Additionally, approaches to unveil the interplay between environmental and genetic determinants controlling the switch from vegetative to the sexual phase in these fungi are discussed. Understanding the dynamics that govern this transition is crucial to promote truffle fructification.