How the truffle got its mate: insights from genetic structure in spontaneous and planted Mediterranean populations of Tuber melanosporum

Abstract

The life cycles and dispersal of edible fungi are still poorly known, thus limiting our understanding of their evolution and domestication. The prized Tuber melanosporum produces fruitbodies (fleshy organs where meiospores mature) gathered in natural, spontaneously inoculated forests or harvested in plantations of nursery-inoculated trees. Yet, how fruitbodies are formed remains unclear, thus limiting yields, and how current domestication attempts affect population genetic structure is overlooked. Fruitbodies result from mating between two haploid individuals: the maternal parent forms the flesh and the meiospores, while the paternal parent only contributes to the meiospores. We analyzed the genetic diversity of T. melanosporum comparatively in spontaneous forests vs. plantations, using SSR polymorphism of 950 samples from South-East France. All populations displayed strong genetic isolation by distance at the metric scale, possibly due to animal dispersal, meiospore persistence in soil, and/or exclusion of unrelated individuals by vegetative incompatibility. High inbreeding was consistently found, suggesting that parents often develop from meiospores produced by the same fruitbody. Unlike maternal genotypes, paternal mycelia contributed to few fruitbodies each, did not persist over years, and were undetectable on tree mycorrhizae. Thus, we postulate that germlings from the soil spore bank act as paternal partners. Paternal genetic diversity and outbreeding were higher in plantations than in spontaneous truffle-grounds, perhaps because truffle growers disperse fruitbodies to maintain inoculation in plantations. However, planted and spontaneous populations were not genetically isolated, so that T. melanosporum illustrates an early step of domestication where genetic structure remains little affected.

Black Truffle, a Hermaphrodite with Forced Unisexual Behaviour

“…By contrast, paternal individuals are very rarely found over 2 successive years and/or in multiple fruitbodies, suggesting much smaller size and shorter lifespan; moreover, they are never found in surrounding mycorrhizae [6,9,12]. On the one hand, the latter observation is consistent with the spatial aggregation of mating types on mycorrhizae, with local patches of individuals with a single mating type.…”

“…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. However, isolation by distance was observed both in a naturally established truffle field and in plantations of trees inoculated in the nursery [9] where inoculation methods differ from natural conditions by often involving mixes of several fruitbodies. Moreover, immigration of spores from a few fruitbodies does not explain the spatial aggregation of mating types, since spores of a fruitbody are MAT1-1 and MAT 1-2, as the result of meiosis of the MAT1-1/MAT 1-2 zygote.…”

“…Several maternal individuals survive from one year to another; they sometimes produce several neighbouring fruitbodies during several years [7,9], suggesting that their mycelia can reach metric size in soil; and, as stated above, they colonize surrounding tree mycorrhizae [9,12] (Figure 1). By contrast, paternal individuals are very rarely found over 2 successive years and/or in multiple fruitbodies, suggesting much smaller size and shorter lifespan; moreover, they are never found in surrounding mycorrhizae [6,9,12].…”

“…Paternal genotypes revealed two striking features. First, in most fruitbodies, they turned out to be genetically very similar to maternal individuals [3,9,12], significantly more than if mating occurred at random within the population. The load of consanguinity is mainly acting on diploids (due to the accumulation of rare deleterious recessive mutations, hidden by diploidy), but is limited on fully haploid organisms such as the black truffle.…”

“…Paternal individuals only recently attracted detailed investigations [6,9,12]. Their genotype can be deduced from the zygotic genotype obtained by bulk extraction of spore DNA, by subtracting the maternal genotype as obtained by TIMI 1467 1-4 simple gleba DNA extraction (Figure 1).…”