Dry Store Room No. 1: The Secret Life of the Natural History Museum. Richard Fortey

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      The author handles an edible black true truffle (Tuber) from Sardinia.

      Edible black truffle. Photo © Jackie Fortey.

      When the truffles were first recognized as fungi rather than some spontaneously generated freak of nature, it was thought that such curious productions comprised a single group of organisms – a reasonable assumption, one might think. They deserved one of Linnaeus’ high-level classification tags – an Order. But when microscopes came to be focussed on the tissues inside the truffle, where the spores were developing, an interesting discovery was made. Not all truffles were alike. Those that graced the tables of the rich and hedonistic showed features at the microscopic level like those of another gourmet treat, the morel (Morchella esculenta). In other, and more technical, words they were ascomycetes. These fungi bear their spores inside minute sacs or asci of the order of a tenth of a millimetre long – there are usually eight such spores, so the asci have a very typical microscopic appearance, rather like eggs wrapped in a sausage. However, some other truffles, for example a genus called Hysterangium, showed evidence that they were related instead to the gasteromycetes – the group of fungi that includes puffballs and stinkhorns. These are basidiomycetes, which carry their spores in an entirely different way from the ascomycetes; they are typically borne atop a special cell called a basidium, usually four spores in a loose cluster. The white mushrooms that fill vats in supermarkets are distantly related basidiomycetes, as are the majority of fungi that troop through the woods in autumn. The ascomycetes separated from the basidiomycetes very early in earth history, and certainly more than a billion years ago. It is preposterous to classify truffles together that have such different evolutionary origins – and so the ascus-bearing truffles were separated from the basidium-bearing truffles: so far, so sensible, and resulting in two Orders. For common names we now had ‘truffles’ and ‘false truffles’.

      However, the story did not end there. From other microscopic hints there were suspicions that there were several origins for truffles in both the ascomycete group and the basidiomycete group. Truffles might have arisen repeatedly, on separate evolutionary trees, for all their superficial similarity. The closest relatives of a truffle might prove to be one of several different kinds of more normal-looking mushrooms and other fungi. The truffle shape, including its subterranean growth, is a specific adaptation – a mode of life, if you like. It was not so difficult to imagine a ‘truffle habit’ originating several times, because most fungi do indeed develop underground, and only later erupt at the surface. If development were somehow ‘arrested’ at the early stage – well, then you might have something like a truffle. The trouble is how could you pair the truffle with its closest-related mushroom, since there is so little general resemblance between them? This is where the molecular evidence should come into its own. The appropriate mushroom partner should, in principle, show more similar sequence patterns at the molecular level to its truffle relatives than it does to other truffles or indeed other mushrooms. So it has proved. Using the appropriate genomic tool, especially one known as ribosomal ITS (Internal Transcribed Spacer), the complexity of the origin of truffles has been demonstrated. It turns out that at least six different kinds of mushrooms – that is, the basidium-bearing kind – have become ‘truffleized’, to coin a term. To add to this there are several more origins of truffles of the ascus-bearing kind, of which the true truffle, Tuber, is one. Far from being a natural group of organisms, the truffles originated from numerous different fungi on several different occasions, and it all probably happened millions of years ago.

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