ScienceDaily (Oct. 25, 2010) — They are one of the most highly prized delicacies in the culinary world, and now scientists have discovered that black truffles -- unlike many other fungi that reproduce themselves -- rely on sexual reproduction between opposite mating types. The research, published in New Phytologist as the truffle season begins, represents a breakthrough in the understanding of truffle cultivation and distribution.

The teams, led by Dr Francesco Paolocci and Dr Andrea Rubini from the CNR Plant Genetics Institute in Perugia and by Dr Francis Martin from INRA in Nancy, carried out their research on the reproduction strategy of the highly prized black truffle, Tuber melanosporum, which is grown across southern Europe. During the truffle season, between late autumn and winter, fruiting truffles can grow up to 7 cm in diameter, weighing up to 100 g with a value often measured in hundreds of Euros.

'Fruiting' is the crucial part of the truffle life cycle, occurring when the fungi interacts with and colonises host plants, usually at the roots. However, the process which causes this transition from vegetative to reproductive state remains unknown.

"It is commonly believed that truffles, like other fungi, are homothallic, meaning that they reproduce themselves," said Paolocci. "Because fungi that reproduce this way do not need a sexual partner, it was believed that truffle cultivation relied only on the environment and nutrition; now we know that is wrong."

The Périgord black truffle (Tuber melanosporum Vittad.) and the Piedmont white truffle dominate today’s truffle market1, 2. The hypogeous fruiting body of T. melanosporum is a gastronomic delicacy produced by an ectomycorrhizal symbiont3 endemic to calcareous soils in southern Europe. The worldwide demand for this truffle has fuelled intense efforts at cultivation. Identification of processes that condition and trigger fruit body and symbiosis formation, ultimately leading to efficient crop production, will be facilitated by a thorough analysis of truffle genomic traits. In the ectomycorrhizal Laccaria bicolor, the expansion of gene families may have acted as a ‘symbiosis toolbox’4. This feature may however reflect evolution of this particular taxon and not a general trait shared by all ectomycorrhizal species5. To get a better understanding of the biology and evolution of the ectomycorrhizal symbiosis, we report here the sequence of the haploid genome of T. melanosporum, which at ~125 megabases is the largest and most complex fungal genome sequenced so far. This expansion results from a proliferation of transposable elements accounting for ~58% of the genome. In contrast, this genome only contains ~7,500 protein-coding genes with very rare multigene families. It lacks large sets of carbohydrate cleaving enzymes, but a few of them involved in degradation of plant cell walls are induced in symbiotic tissues. The latter feature and the upregulation of genes encoding for lipases and multicopper oxidases suggest that T. melanosporum degrades its host cell walls during colonization. Symbiosis induces an increased expression of carbohydrate and amino acid transporters in both L. bicolor and T. melanosporum, but the comparison of genomic traits in the two ectomycorrhizal fungi showed that genetic predispositions for symbiosis—‘the symbiosis toolbox’—evolved along different ways in ascomycetes and basidiomycetes.

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.