3/26/10 - truffles

In today's excerpt - truffles, the revered culinary delicacies are symbiotic with trees and evolved their rich aromas as an enticement to foraging animals to aid in their spore disbursement:

"Throughout history, truffles have appeared on the menu and in folklore. The Pharaoh Khufu served them at his royal table. Bedouins, Kalahari Bushmen and Australian Aborigines have hunted them for countless generations in deserts. The Romans savored them and thought they were produced by thunder. Modern epicures prize truffles for their earthy aroma and flavor, and are willing to pay steep prices at the market—recently more than $3,000 per kilogram for the Italian white variety. Yet despite humanity's abiding interest in the fungi, much about their biology has remained veiled in mystery. ...

"Truffles like mushrooms are the fruit of fungi. These fleshy organs are temporary reproductive structures that produce spores which eventually germinate and give rise to new offspring. What sets truffles apart from mushrooms is that their spore-laden fruit forms below ground rather than above. ...

"All truffles and mushrooms produce networks of filaments or hyphae that grow between plant rootlets to form a shared absorptive organ known as a mycorrhiza. Thus joined, the fungus provides the plant with precious nutrients and water, its tiny hyphae able to reach into pockets of soil inaccessible to the plant's much larger roots. The plant in turn furnishes its consort with sugars and other nutrients that it generates through photosynthesis—products that the fungus needs but cannot produce on its own because it does not photosynthesize. So beneficial is this partnership that nearly all trees and other woody plants require it for survival as do the associated fungi. Most herbaceous plants (those that do not have a permanent woody stem aboveground) form mycorrhizae too albeit with different fungi. ...

"Given that truffles require aboveground dispersal of their spores to propagate why would natural selection favor the evolution of species that hide underground? Consider the reproductive tactic of mushrooms. Mushrooms ... all have fruiting bodies that can discharge spores directly into the air. ... It is a highly effective approach.

"The mushroom strategy is not foolproof, however. Most mushrooms have little defense against environmental hazards such as heat, drying winds, frost and grazing animals. Every day a few spores mature and are discharged. But if inclement weather dries or freezes a mushroom spore production usually grinds to a halt.

"Where such hazards are commonplace new evolutionary adaptations have arisen. The most successful alternative has been for the fungus to fruit underground. Once the soil is wet enough for the subterranean fruiting body to form it is insulated from vagaries of weather. The truffle develops with relative impunity, continuing to produce and nurture its spores even when aboveground conditions become intolerable to mushrooms. At first glance, the truffle's solution might seem facile. The form of a truffle is visibly less complex than that of a mushroom. No longer does the fungus need to expend the energy required to push its spore-bearing tissues aboveground. The truffle is but a lump of spore-bearing tissue usually enclosed by a protective skin.

"The problem is that the truffles cannot themselves liberate their spores trapped as they are in their underground realm. That feat demands an alternative dispersal system. And therein lies the complexity of the truffle's scheme. Over millions of years, as truffles retreated underground mutations eventually led to the formation of aromatic compounds attractive to animals. Each truffle species has its own array of aromatics that are largely absent in immature specimens, but intensify and emerge as the spores mature. ... When an animal [is attracted by the aroma and] eats a truffle most of the flesh is digested, but the spores pass through unharmed and are defecated on the ground where they can germinate if the conditions are right."


James M. Trappe and Andrew W. Claridge


"The Hidden Life of Truffles"


Scientific American




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