Population genetics and dynamics of the black truffle in a man-made truffle field

Abstract

The colonization dynamics of the black true in an arti®cial ®eld were assessed through analyses of microsatellite and RAPD markers. The true ®eld was composed of three tree species and mycelial inoculum of three dierent origins, and was monitored for the ®rst three years of true production. We found very low levels of genetic diversity. Isolation by distance was detected only at the betweentree level. This could be interpreted as local colonization around each tree facilitated by the presence of the tree root system. At the larger spatial scale of the European range, the absence of isolation by distance corroborates the hypothesis of an impact of glaciation on genetic variation, followed by rapid postglaciation demographic expansion. In addition, genetic variation of harvested trues was explained by neither inoculation origin, nor tree species. Our study questions the real impact of manmade inoculation of tree root systems with fungal mycelia.Keywords: biogeography, microsatellite, population genetics, RAPD, true, Tuber melanosporum. IntroductionThe black true (Tuber melanosporum Vitt.) is a hypogeous ectomycorrhizal ascomycete mainly found in France, Italy and Spain (Callot, 1999), and highly appreciated for its remarkable organoleptic properties (taste and odour). Haploid mycelium can be found in the soil, usually in contact with or near to tree root systems. Fruiting bodies (i.e. ascocarps) result from the fusion of two haploid mycelia, and are therefore supposedly diploid (Moore-Landecker, 1996). They are composed of conjunctive tissue (the gleba) that surrounds haploid spores. These spores are produced by meiosis after karyogamy.The black true naturally occurs on calcareous soils, in association with the root systems of trees, most often oaks (Quercus spp.) and hazelnuts (Corylus avellana). Since the 1960s, attempts have been made to create favourable arti®cial habitats to grow and monitor trues, resulting in managed ®elds of planted trees.Traditionally, a true ®eld is created by sowing oak acorns and relying on the presence of true mycelium in the soil to interact with the developing tree root system. Under such conditions the ®rst trues are usually harvested about 6±8 years after sowing. Alternatively, it is now possible to inoculate the roots of young trees with speci®c mycelia before planting these trees directly in the soil (Chevalier & Grente, 1979). Trues are harvested earlier (3±4 years) under these conditions (Chevalier & Dupre , 1988) and can even be grown in areas where the mycelium did not previously occur (e.g. New Zealand and Tasmania; Callot, 1999). However, when T. melanosporum mycelia or spores already exist in the soil (mainly in France, Italy and Spain), the usefulness of planting inoculated trees is unclear. In particular, it has not been established whether the resulting trues originated from the arti®cially generated mycorrhiza or from formerly present local (and potentially competitive) mycelia. In other words, when a black true is harvested around an arti®cially inoculated tree, t...

Black Truffle, a Hermaphrodite with Forced Unisexual Behaviour

“…First, segregated patches of maternal individuals with identical mating types (i.e., exclusively MAT1-1 or MAT 1-2) were found in populations. Second, microsatellite analyses revealed a strong isolation by distance among maternal individuals, that is, spatially close individuals are genetically close [2,7]. The later feature may relate to the local deposition by animal dispersers of spores issuing from a single or from few fruitbodies, so that immigrant spores are genetically related.…”

“…The DNA extraction protocols used in the first instance revealed a single partner producing the outer skin and internal tissues (the so-called gleba), and because of this, truffles were long considered autogamous [2]. More recently, harsher methods allowed the extraction of spore DNA and identified a second partner that contributes to spore genomes, together with the previous partner [3].…”

Genetic and Phylogeographic Structures of the Symbiotic Fungus Tuber magnatum

“…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.…”

“…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).…”

“…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).…”