Summary• In light of the recent finding that Tuber melanosporum, the ectomycorrhizal ascomycete that produces the most highly prized black truffles, is a heterothallic species, we monitored the spatial distribution of strains with opposite mating types (MAT) in a natural truffle ground and followed strain dynamics in artificially inoculated host plants grown under controlled conditions.• In a natural truffle ground, ectomycorrhizas (ECMs), soil samples and fruit bodies were sampled and genotyped to determine mating types. Simple sequence repeat (SSR) markers were also used to fingerprint ECMs and fruit bodies. The ECMs from nursery-inoculated host plants were analysed for mating type at 6 months and 19 months post-inoculation.• In open-field conditions, all ECMs from the same sampling site showed an identical mating type and an identical haploid genotype, based on SSR analysis. Interestingly, the gleba of fruit bodies always demonstrated the same genotype as the surrounding ECMs. Although root tips from nursery-grown plants initially developed ECMs of both mating types, a dominance of ECMs of the same MAT were found after several months.• The present study deepens our understanding of the vegetative and sexual propagation modes of T. melanosporum. These results are highly relevant for truffle cultivation.
Summary The genome of Tuber melanosporum has recently been sequenced. Here, we used this information to identify genes involved in the reproductive processes of this edible fungus. The sequenced strain (Mel28) possesses only one of the two master genes required for mating, that is, the gene that codes for the high mobility group (HMG) transcription factor (MAT1‐2‐1), whereas it lacks the gene that codes for the protein containing the α‐box‐ domain (MAT1‐1‐1), suggesting that this fungus is heterothallic. A PCR‐based approach was initially employed to screen truffles for the presence of the MAT1‐2‐1 gene and amplify the conserved regions flanking the mating type (MAT) locus. The MAT1‐1‐1 gene was finally identified using primers designed from the conserved regions of strains that lack the MAT1‐2‐1 gene. Mating type‐specific primer pairs were developed to screen asci and gleba from truffles of different origins and to genotype single ascospores within the asci. These analyses provided definitive evidence that T. melanosporum is a heterothallic species with a MAT locus that is organized similarly to those of ancient fungal lineages. A greater understanding of the reproductive mechanisms that exist in Tuber spp. allows for optimization of truffle plantation management strategies.
SummaryThe genetic structure of ectomycorrhizal (ECM) fungal populations results from both vegetative and sexual propagation. In this study, we have analysed the spatial genetic structure of Tuber melanosporum populations, a heterothallic ascomycete that produces edible fruit bodies.Ectomycorrhizas from oaks and hazels from two orchards were mapped and genotyped using simple sequence repeat markers and the mating type locus. The distribution of the two T. melanosporum mating types was also monitored in the soil. In one orchard, the genetic profiles of the ascocarps were compared with those of the underlying mycorrhizas.A pronounced spatial genetic structure was found. The maximum genet sizes were 2.35 and 4.70 m in the two orchards, with most manifesting a size < 1 m. Few genets persisted throughout two seasons. A nonrandom distribution pattern of the T. melanosporum was observed, resulting in field patches colonized by genets that shared the same mating types.Our findings suggest that competition occurs between genets and provide basic information on T. melanosporum propagation patterns that are relevant for the management of productive truffle orchards.
Summary• Tuber melanosporum is an ectomycorrhizal ascomycete producing edible ascocarps. The prevalent view is that this species strictly selfs, since genetic analyses have never detected heterozygotic profiles in its putatively diploid/dikaryotic gleba. The selfing model has also forged the experimental approaches to assess the population genetic variability. Here, the hypothesis that T. melanosporum outcrosses was tested.• To this end, SSR (simple sequence repeats) and ITS (internal transcribed spacer) markers were employed to fingerprint asci and the surrounding gleba within single ascocarps. The distribution of genetic variability was also investigated at different geographical levels using single (SSR and ITS) and multilocus (AFLP, amplified fragment length polymorphism) markers.• It is shown that T. melanosporum outcrosses since asci display additional alleles besides those present in the surrounding, uniparental, gleba. Furthermore, SSR and AFLP data reveal a high rate of intrapopulation diversity within samples from the same ground and root apparatus and the highest rate of genetic variability within the southernmost populations of the distributional range.• These data call for a profound re-examination of T. melanosporum mating system, life cycle and strategies for managing man-made plantations. They also strongly support the idea that the last glaciation restricted the species distribution to the Italian and Spanish peninsulas.
ruffle fungi differentiate into subterranean fruiting bodies bearing spores sequestered in an inconspicuous globouslike mass of hyphae. Truffle-forming species have evolved in nearly every major group of fleshy fungi over 100 times independently within the Ascomycota and Basidiomycota and the majority of evolutionary transitions to a truffle morphology have occurred in lineages establishing ectomycorrhizal mutualistic symbiosis with plants 1. This pattern suggests that symbiosis has been a major driver in the evolution of truffle diversity. The evolution of the hypogeous lifestyle across a diversity of truffle lineages also suggests that the transition from epigeous to hypogeous fruiting is driven by strong selection for traits, for example pungent odours, that promote
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