The Périgord black truffle (Tuber melanosporum Vittad.) and the Piedmont white truffle dominate today's truffle market. The hypogeous fruiting body of T. melanosporum is a gastronomic delicacy produced by an ectomycorrhizal symbiont 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'. This feature may however reflect evolution of this particular taxon and not a general trait shared by all ectomycorrhizal species. 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 approximately 125 megabases is the largest and most complex fungal genome sequenced so far. This expansion results from a proliferation of transposable elements accounting for approximately 58% of the genome. In contrast, this genome only contains approximately 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.

Summary• Glaciations and postglacial migrations are major factors responsible for the present patterns of genetic variation we see in natural populations in Europe. For ectomycorrhizal fungi, escape from refugia can only follow range expansion by their specific hosts.• To infer phylogeographic relationships within Tuber melanosporum , sequences of internal transcribed spacers (ITS) and the 5.8S coding region of the ribosomal DNA repeat were obtained for 188 individuals sampled over the entire distribution of this species in France, and in north-western Italy and north-eastern Spain.• Ten distinct ITS haplotypes were distinguished, mapped and treated using F -and N ST -statistics and nested clade (NCA) analyses. They showed a significant genetic differentiation between regional populations. NCA revealed a geographical association of ITS haplotypes, an old fragmentation into two major groups of populations, which likely colonized regions on different sides of the French Central Massif.• This re-colonization pattern is reminiscent of the one observed for host trees of the Perigord truffle, such as oaks and hazelnut trees. This suggests that host postglacial expansion was one of the major factors that shaped the mycobiont population structure.

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Mycorrhizal fungi of the genus Tuber are classified by morphological characters that allow differentiation of most species. However, some economically important species are difficult to differentiate on morphological grounds. When morphological traits are not sufficient to discriminate between taxa, other markers are needed. Genetic variation of fruit bodies of 12 Tuber taxa was studied by the random amplified polymorphic DNA (RAPD) technique. High interspecific variability was observed between most species. Moreover, important infraspecific variation occurred in all species, except Tuber brumale s.L, Tuber melanosporum, and Tuber magnatum. Southern hybridization patterns of RAPD products of the various species were used to confirm the data. Relationships among Tuber species were determined by cluster analyses. UPGMA analyses revealed several main clusters and a low genetic similarity between taxa. These results indicate that RAPD and polymerase chain reaction are useful for analysing genetic variation within Tuber species. Most species can be identified by differences in their amplified DNA profiles. However, the two pairs of closely related taxa Tuber aestivum – Tuber uncinatum and Tuber brumale var. brumale – Tuber brumale var. moschatum did not appear to differ genotypically. Key words: Tuber, RAPD, Southern, UPGMA, inter- and infra-specific variability.

Three species of edible desert truffles, Terfezia arenaria, Terfezia claveryi, and Tirmania pinoyi associated with Helianthemum guttatum in Algeria were examined in this study. The mycorrhizal synthesis between the three species of desert truffles and Helianthemum guttatum was carried out with two different methods: (i) under axenic conditions on perlite permeated by a nutritive solution, by inoculating the seedlings with mycelial fragments; (ii) under gnotoxenic conditions on natural substrates, by inoculating plants with spore suspensions. The method of synthesis did not have any influence on the type of mycorrhizae. The desert truffles formed two types of mycorrhizae according to the fertility of the substrate: ectomycorrhizae without sheath, in the phosphorus rich substrates, or ectendomycorrhizae, also without sheath, in the phosphorus deficient substrates. Primordial ascomata were obtained in the syntheses involving polysporous inocula. Key words: ectomycorrhizae, ectendomycorrhizae, Helianthemum guttatum, Terfezia, Tirmania.