Trifling variation in truffles

Abstract

Of the ten species of European truffle (fungi of the genus Tuber, phylum Ascomycota), some have economic value because of their organoleptic properties (taste and perfume), in particular the black truffle (Tuber melanosporum Vitt.) and the summer and burgundy truffles 1,2 . The black truffle is mainly found in Spain, France and Italy (Fig. 1a), and it shows variation in several traits, including in its famous organoleptic properties, across this geographical range. Here we show that this variation probably results from environmental, rather than genetic, influences.In an attempt to explain the variation in T. melanosporum across its geographical range and to study the distribution of genetic variability within and among populations, we analysed fruiting bodies (ascocarps) from different populations in France and Italy for random amplified polymorphic DNA (RAPD) and microsatellite polymorphism. We found an extremely low level of polymorphism over the whole study area for both types of marker (Fig. 1b). This pattern contrasts dramatically with that of the sympatric summer truffle (T. aestivum Vitt., which forms a species complex with the burgundy truffle, T. uncinatum Chatin), which has distinct internal transcribed spacer alleles 3 and highly variable RAPD patterns 4 .The absence of heterozygotes in T. melanosporum, shown by study of microsatellites, is consistent with a very closed mating system, such as homothallism or even exclusive selfing. As the two putative haploid genomes from an ascocarp are very similar, we considered each variable RAPD band to be a distinct locus exhibiting two alleles (presence or absence of a band). The genetic models used subsequently showed no increase in genetic differentiation with geographic distance (Fig. 1c).The low level of genetic diversity of the black truffle probably cannot be explained by its present large population size. In France, 1-3 ǂ10 4 kilograms of black truffles are now officially sold each year, and more than 10 6 kilograms per year were traded in the last century (the average weight of an ascocarp is about 30-40 grams).A population bottleneck probably occurred during the last, and coldest, glaciation, when the broadleaved forest of Europe was considerably reduced and restricted mainly to the Mediterranean coastal zone 5 . The black truffle ripens in winter (November-February), which probably contributed to its drastic reduction in population size and restriction to its southernmost limits during the glaciation, as ascocarps of contemporary T. melanosporum are susceptible to frost. The present low level of genetic variability in black truffle populations is consistent with such a bottleneck occurring 10,000 years ago, followed by a rapid colonization of southwestern Europe, which would also explain the absence of phylogeographic signals in the few polymorphic markers found. The 'glaciation hypothesis' would be strengthened if more southerly populations (in Spain or Italy) were found to show greater genetic diversity.The seasonal behaviour of the summer and burgu...

Genetic and Phylogeographic Structures of the Symbiotic Fungus Tuber magnatum

“…In the related species T. melanosporum, there is little or no genetic differentiation between the sampled populations, and the species is regarded as one that self-fertilizes for sexual reproduction and that has an ascocarp consisting primarily of diploid (dikaryotic) hyphae (3,4,21). The few studies of genetic structure and mode of reproduction of T. magnatum found very limited intraspecific allozyme variation and a complete absence of heterozygous individuals (8).…”

“…Some species are widely distributed and have pronounced morphological and molecular variability (9,11,22,25,27,32), while others have a more restricted distribution and little intraspecific polymorphism in either morphological or genetic traits (3,4,8,9,26). Tuber magnatum Pico, which produces white ascomata, is harvested only in Italy and some countries on the Balkan Peninsula.In the related species T. melanosporum, there is little or no genetic differentiation between the sampled populations, and the species is regarded as one that self-fertilizes for sexual reproduction and that has an ascocarp consisting primarily of diploid (dikaryotic) hyphae (3,4,21). The few studies of genetic structure and mode of reproduction of T. magnatum found very limited intraspecific allozyme variation and a complete absence of heterozygous individuals (8).…”

“…The spatial distribution and ecological requirements of these symbiotic fungi vary by species. Some species are widely distributed and have pronounced morphological and molecular variability (9,11,22,25,27,32), while others have a more restricted distribution and little intraspecific polymorphism in either morphological or genetic traits (3,4,8,9,26). Tuber magnatum Pico, which produces white ascomata, is harvested only in Italy and some countries on the Balkan Peninsula.…”

Diversity, population genetics, and evolution of macrofungi associated with animals

“…In the most productive areas of Italy and Istria peninsula, T. magnatum showed a significant positive correlation between genetic and geographical distances and that the southernmost and northwesternmost populations were significantly differentiated from other populations (Rubini et al 2005) (Table 1). T. melanosporum had also low-level genetic diversity across its geographical range of France and Italy (Bertault et al 1998) (Table 1) but presented significant geographic differentiation among populations (Murat et al 2004; Riccioni et al 2008) (Table 1). The southernmost populations of T. melanosporum also showed higher allelic richness than the northern populations (Riccioni et al 2008).…”

“…The southernmost populations of T. melanosporum also showed higher allelic richness than the northern populations (Riccioni et al 2008). These similar population genetic characteristics were explained by the “glaciation hypothesis.” In the hypothesis, the truffle populations in southern areas in Italy and Spanish acted as epibiotic species during glaciation and they spread northward along with their host trees as the ice receded (Bertault et al 1998). The research results of García-Cunchillos et al (2014) also supported this hypothesis.…”