European black truffles are all of the same species, but genetic analysis shows that Chinese truffles are something quite different.
THE BLACK DIAMOND! An immense amount of ink has been spilled in singing its praises. No food writer fails to mention its appearance on a menu, and no chef neglects to feature it when he aims for stars. For centuries the merits of the various black truffles that grow in Western Europe have been debated. The black truffle of Périgord is recognized have a quite different taste from the one found in Burgundy, and naturally the truffles found in France are claimed by the French to be far superior to those of Spain and Italy. Can science provide an objective basis for these opinions?
In Europe there are ten sorts of truffles, which is to say mushrooms of the Tuber genus. The black truffle, also called a Périgord truffle (Tuber melanosporum), is harvested principally in Spain, France, and Italy, but its gastronomic qualities vary from region to region. Michel Raymond and his colleagues at the Centre National de la Recherche Scientifique, the Institut National de la Recherche Agronomique (INRA), and the Institut pour la Recherche et le Développement in Montpellier sought to determine whether these differences have a genetic basis.
More than 200 samples from various regions in France and Italy were analyzed. The Montpellier biologists studied satellite DNA sequences, which differ substantially between species of the same genus, and observed no genetic variability in the samples of black truffles. They also compared black truffles with summer truffles (Tuber aestivum) and Burgundy truffles (Tuber unciatum). Differences were found between the black and summer truffles, as expected, but the species boundaries between summer and Burgundy truffles turned out to be fuzzy. Subsequent studies confirmed that the latter two types must be considered distinct varieties—what biologists call a species complex.
How is the genetic homogeneity of the Tuber melanosporum species to be explained? It is believed that the last Ice Age trapped a small population of black truffles next to the Mediterranean, along with the trees on which they develop (the mushroom forms a subterranean root network, of which the truffle is only the reproductive organ). Because the black truffle matures during the winter, from November to February, its propagation was confined to the most southerly zones. During the later climatic warming, the black truffle is thought to have recolonized the regions where its favored trees first developed, as weather conditions permitted. Ten thousand years would have been enough time for the species to reestablish itself in southern Europe but not enough time for it to evolve.
During the same period, by contrast, summer and Burgundy truffles, which mature in spring and fall, respectively, are thought to have been confined to a more northerly area. Certainly the fact that they are found in countries further to the north and east of France proves that they are able to tolerate colder climates. The current genetic diversity of species therefore results from the fact that present-day truffles are descended from a numerous and varied population.
A Himalayan Truffle
Genetic studies completed these results by illuminating the problem of Chinese truffles, which every year swindlers try to pass off as French truffles. In 1996, Marie-Claude Janex-Favre and her colleagues at the University of Paris–VI studied the Chinese truffle, which initially had been assigned to the species Tuber himalayense, Tuber indicum, and Tuber sinense. These truffles come from the foothills of the Himalayas, where they are harvested at an altitude of about 2,000 meters (6,500 feet), at least a dozen centimeters (four or five inches) below ground. They are easily confused with the French truffle, and the cost of harvesting them is far lower.
The Chinese truffles have a very irregular, bumpy surface. Reaching as much as 7 centimeters (almost 3 inches) in diameter, they are covered with low scales displaying an inverted pyramid form with a square base. This general appearance is almost identical to that of a particular kind of Périgord truffle that is covered with large flat scales and devoid of marked protrusions. The spores of the two types of truffles nonetheless look different under the microscope. Does this difference in appearance result from a difference in developmental conditions or from speciation? Genetic studies carried out by Delphine Graneboeuf and her colleagues at the INRA station in Clermont-Ferrand have established that they belong to different species. The mild taste of Chinese truffles therefore is a consequence of genetic rather than environmental factors.
In the case of black truffles, although environmental factors are now known to be responsible for differences in gastronomic quality, science has not yet explained how the soil and climate exert their influence. Biologists are busy trying to find out.