LITMIR.BIZ

      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.

      Why should anyone care about such apparently esoteric information? After all, most people can happily pass their lives without seeing a truffle of any kind, and who but an outstanding eccentric would spend hours carefully digging around in the litter under trees to find false truffles of the inedible kind? But then, who would guess that truffle evolution was crucial to the survival of several charming Australian marsupials? For the Australian group of truffles, including some placed in the genus Hydnangium, were also independently evolved in close association with Eucalyptus trees. These false truffles provide a prime foodstuff for bettongs and potoroos, which are delightful, nocturnal cat-sized animals that are now the focus of intensive conservation efforts. The more that is known of their requirements the more likely they are to survive in the twenty-first century. False truffles are as important to their continued existence as keeping them from the depredations of feral cats. So what might at first seem extraordinarily specialized information has links to those ‘pretty furry things’ after all; nature is seamless, its connections multifarious.

      The truffle example also links back to where we started – the questions of taxonomy. Every time a truffle under examination turns out to be related to an entirely different mushroom, we can imagine a curator cursing quietly under his breath and moving the relevant preserved specimens to a different drawer. This is an extreme case of ‘revision’ – revisiting taxonomy. The point is that we expect classification systems, genera, families and so on in ascending order, to reflect fundamental resemblances between the species included in them. The species themselves are the units of this classification – at least they are if we have recognized them correctly – and they are the real things that get shifted around from one drawer to another. The genus or family whose name might be written on the drawer or cupboard is a theoretical concept, subject to change as science advances. As with the truffles, species may be added or taken away or moved around. The up-to-date taxonomist wants his classification concepts to square with modern views. For most such scientists this means that the species included in a genus, for example, should have descended from a common ancestor – that is, constitute what is known as a clade. The characters shared by the species in a genus – and nowadays these can be molecular characters as much as the traditional ‘hairs on legs’ – are what defines it, makes it a natural entity. Discover new characters and the concept of the genus may well change, and so will the species included within it. This results in changes in generic names for a given species that irritate many people, and particularly knowledgeable amateur scientists. ‘Why do they have to keep changing the names?’ is a common complaint. However, the contemporary investigator is obliged to seek out genera, or families, that are clades; the scientific method used in recognizing these groups is known as cladistics; and the whole business of examining relationships between organisms in this way is usually termed phylogenetic analysis, or simply phylogenetics. If names have to change as a result of careful reconsideration of species, well, that’s the price of progress. Much modern taxonomy is based upon computer analysis of relationships, where all the characters possessed by a group of organisms under study are allowed to fight it out until the ‘best’ arrangement of species is discovered, resulting in a diagram – a cladogram – showing how species relate to one another. The eventual classification is then drawn up directly from the cladogram.

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