French rulers have long believed in the link between sex and truffles. King Henri IV offered them to his mistresses as an aphrodisiac. King Louis XV fed them to his wife, who gave him ten children. And when Napoleon Bonaparte had a son, he credited the event to his diet of them. So it is appropriate that the discovery which could save the most exquisite delicacy in French cuisine from extinction concerns its sexuality.

Italian researchers have found that the black, bulbous fungus that grows by oak and other trees in parts of Europe is almost certainly born of the meeting of male and female spores. This revelation could help to bring about a revival of production after 100 years of decline - to the relief of gastronomes who describe the pungent, earthy flavours given off by the black Périgord truffle as the greatest experience that a palate can enjoy.

It comes amid a flurry of other scientific projects - including test-tube cloning - that attempt to explain the mysterious, symbiotic relationship between the subterranean fungi and the tree roots they colonise.

At stake is the future of a food that probably provokes more passion than any other.

“I love truffles like I love women,” says Bruno Clément, 61, who is widely recognised as one of France's greatest truffle connoisseurs. He is the founder of Terre de Truffes, an oak-panelled truffle restaurant in Central Paris which counts President Sarkozy among its admirers and sells a 60g jar of truffles for €90 in an adjacent boutique.

On the menu are summer, Burgundy and brumale truffles. But these pale into insignificance beside the most revered and expensive variety of all, tuber melanosporum, the Périgord truffle also known as the black diamond.

“A lot of truffle-lovers say it's melanosporum or nothing,” says Rose Métivier, the restaurant director. “They say that the other truffles are just for decoration.”

With the black truffle season getting under way in France this month, Terre de Truffes is now serving them with pigeon pie and pancetta and foie gras stuffing for €55; with Rossini beef filet, foie gras and fried mushrooms for €70; or with confit of lamb and gratin dauphinois for €66.

But Clement, who has left Paris to run a restaurant in Lorgues, in Provence, dismisses these dishes as conneries, which can be translated as bullshit. He says that they are on the menu only because “people are prepared to pay for them and you need to make money ... truffles require simplicity. They are a princess who wants to marry a peasant.

“Foie gras is enough in itself. You don't need a truffle with it. If you want truffles, have them with eggs, potatoes, pasta, toast or salad. If you find a beautiful truffle in the morning, cut a slice off it and have that on a little toast with olive oil and rock salt - then you have an imperial dish.”

Many truffle connoisseurs would agree with him, although some say that truffle omelette is even better than truffle toast. One method involves grating the black diamond over the omelette, but purists insist on putting the unbroken eggs and a slice of truffle in a sealed jar in the fridge for several days to ensure a full flavour.

Their cherished delicacy, however, is in the throes of an historic crisis in which total annual European production has fallen from 2,000 tonnes at the end of the 19th century to just 60 tonnes now. France's 20,000 or so trufficulteurs - mostly farmers who cultivate a few truffle oaks for a little extra winter revenue - contribute about 25 tonnes, which is only half of what their compatriots consume.

The slump is blamed on the rural exodus, which left woods unkempt and unsuitable for truffles - they need a light, airy environment - but also on global warming, which has upset the delicate balance of rainfall and sun. With specialists warning that French truffles could disappear altogether if the trend continues, growers have responded by seeking to upgrade ancestral methods.

They have abandoned pigs, which have a fine sense of smell but an unfortunate habit of eating the truffles that they find. Instead, most modern producers train dogs to do the job. They also plant mychorised trees, the roots of which have been rubbed in a truffle paste to help the formation of mycelium, from which truffles develop.

But the results are poor, according to Jean-Charles Savignac, chairman of the French Federation of Truffle Growers. He says that only about 20 per cent of mychorised trees actually produce truffles, and that these tend to be small and disappointing.

The result is an increasingly frantic hunt for truffles. In Jarnac, southwest France, this month a truffle that weighed 1.6 kg when it was dug up, and 1kg once the earth was brushed off, was auctioned for €400 - but prices can reportedly reach €1,000 a kilo, although this is hard to verify as most transactions are carried out in side-roads with the merchandise hidden in car boots and payment in cash to avoid alerting the taxman.

Given the rewards, it is perhaps not surprising that black diamonds should draw the sort of profiteers who exasperate Michel Tournayre, a producer on his family farm in the hills above Uzès in southern France. They include the importers of Chinese truffles, which can be bought for €40 a kilo and served with artificial truffle flavouring to give them the taste that they otherwise lack.

But they also include the thieves who have stolen his truffles, his truffle trees and even Julie, his seven-year-old truffle-hunting mongrel. “We get regular thefts of truffles around here,” he says, surveying his 100 hectares of trees from his stone-walled house in the middle of his land. “My neighbour lost two only recently.”

He is banking on French justice to make an example of the thieves. But most of all he is counting on science to end the French truffle shortage, which has made the sector so attractive to criminals, by increasing the quantity, quality and size of the fungi gathered.

Although truffles have been eaten for 4,000 years, producers say that they still don't know what makes them develop under one tree and not another. “If you grow corn, you see when something's wrong,” says Tournayre, “but a truffle is under the ground. It's hard to know what it needs.”

A breakthrough may not be far away, though, according to Jean-Marc Olivier, France's most celebrated truffle specialist, who says that scientists are making huge strides in unravelling the mysteries of la truffe.

One project in Corrèze, Central France, involves putting the culture of cloned truffles on to baby trees in test tubes. Researchers hope to gain an insight into the development of the precious fungi and improve tree selection techniques.

A second, at the Plant Genetic Institute in Perugia, Italy, has unlocked the DNA structure of the truffle and discovered that it almost certainly has the fungal equivalent of a sex life. Andrea Rubini, a researcher at the institute, told The Times that his team had found truffles to be a cross-fertilising species and not a self-fertilising organism, as previously believed. The truffle is developed from a cross between male and female strains of mycelia - and that is of huge significance to truffle growers and eaters.

If mychorised trees lack the key male-female mix, it might explain why so few of them produce truffles.

“We should get much better results by checking that both sexes are present,” says Olivier. “There won't be a miracle recipe, but there's a lot of work going on and I'm confident that it will lead to an increase in production in France, maybe to about 60 tonnes a year.”

That's good news for truffle lovers - and maybe for other lovers, if all those French monarchs were right.

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