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The "truffle syndrome" has evolved multiple times. Ascomycete truffles are "mushrooms" that are produced below or at ground level and do not exhibit active spore dispersal. Tuber is the most species-rich genus of truffles and is widely distributed, with representatives in North America, Central America, Europe, and Asia (Tuber truffles are apparently absent from sub-Saharan Africa and South America, although they have been introduced to Australia). At least 15 independent origins of hypogeous (i.e., subterranean) forms are believed to have occurred within the Pezizales, the fungus order that includes the genus Tuber. (Læssøe and Hans 2007 and references therein; Jeandroz et al. 2008)
Some Tuber truffles are prized as aromatic delicacies. All Tuber species have obligate mycorrhizal associations with the roots of living plants. The truffle obtains food from its host and cannot complete its lifecycle without its plant partner and the host plant reaps nutritional benefits and increased drought tolerance from the relationship. Tuber truffles form ectomycorrhizae with a broad diversity of gymnosperm and angiosperm hosts in a wide range of habitats, including subtropical cloud forests, temperate forests, boreal forests, floodplains, tree nurseries, restoration sites, and Mediterranean woodlands. Tuber species are found in temperate, Mediterranean, and continental climates. They are absent from regions that are tropical, dry (annual rainfall less than 350 mm), or very cold (mean January temperature < 10° C) climates. Hosts include oaks (Quercus), hazels (Corylus), and other broad-leaved trees, as well as conifers and some shrubs (Cistus, Helianthemum) and orchids. In contrast to many mycorrhizal fungi, most Tuber truffles are able to form associations with any host able to form ectomycorrhizae.
Bonito et al. (2010) undertook a molecular phylogenetic analysis of the diversity of the genus Tuber and concluded that there is much unrecognized diversity within the less studied clades that lack economic importance. They also concluded that Tuber species in Europe, Asia, and North America are each restricted to a single continent except where they have been translocated by humans.
Among the most prized edible fungi are the scented truffles T. magnatum (the Piedmont White Truffle) and T. melanosporum (the Périgord Black Truffle). Truffle prices range into the hundreds (or even thousands) of Euros per kilogram. Wild truffle harvests declined dramatically during the 20th century due to an interaction of ecological and social factors. Efforts to cultivate truffles (especially T. melanosporum) have been pursued in various parts of the world with some success, but are not sufficient to meet market demand. Other less valuable species that are also harvested include T. aestivum and T. mesentericum in Europe, T. indicum and T. pseudoexcavatum in Asia, and members of the T. gibbosum complex in North America.
Genetic and morphological analyses by Bonito et al. (2010) revealed unrecognized species diversity within what had formerly been treated as a single species in the Pacific Northwest of the United States and adjacent Canada, T. gibbosum (these truffles are apparently unusual for the genus in the narrowness of their host range, consistently associating with Pseudotsuga). Bonito et al. also provided a broader phylogenetic analysis including numerous representatives of the genus Tuber to resolve relationships among the major lineages.
Circular zones with sparse vegetation around host plants colonized by some ectomycorrhizal fungi have been recognized for centuries. This phenomenon, which is commonly referred to by the French term brûlé ("burned"), results mainly from phytotoxic effects of metabolites emitted by some Tuber species (notably T. melanosporum, T. aestivum, and T. indicum), which affect the roots of host plants. These chemicals apparently have harmful effects on encroaching vegetation, impairing seed germination, altering root morphogenesis and plant hormonal balance, or inhibiting the native rhizospheric microflora regularly associated with the brûlé. The denuded area appears around a range of deciduous woody plants, including oaks (Quercus), hazels (Corylus), poplar (Populus), beech (Fagus), and cistus (Cistus). Streiblova et al. (2012) reviewed the current state of knowledge on the production of volatile secondary metabolites emitted in the course of the truffle life cycle. (Streiblova et al. 2012 and references therein)
The recent detection of the very similar Chinese Black Truffle (T. indicum) in an Italian truffle plantation has raised concerns about the potential impact of this exotic truffle on the native (and more commercially valuable) T. melanosporum (Murat et al. 2008) and, more broadly, highlights the potential ecological and economic risks associated with the inadvertent or intentional introduction of truffles and other ectomycorrhizal fungi from one region to another.
Kües and Martin (2011) provide an overview of what we know about the complex life cycle and reproduction of truffles. Zampieri et al. (2009) reported on efforts to develop simple genetic tools to distinguish Tuber species from other fungi even in soil samples. Wang and Marcone (2011) reviewed the chemical properties of truffles. Mello et al. (2006) reviewed early work using genetic analysis to explore truffle diversity. Trappe and Claridge (2010) provide a popular introduction to truffles.
(Læssøe and Hans 2007; Jeandroz et al. 2008 and references therein; Bonito et al. 2010 and references therein; Kües and Martin 2011 and references therein)
A rich resource for learning about truffles is the North American Truffling Society.