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The secret life of truffles
The secret life of truffles Powerful family clans. Mysterious sex lives. Constant warfare. Alison George unearths the extraordinary hidden world of truffles
RENAUD VIGOURT
D
EEP in the forests of the north, a vicious battle is raging. For generations, rival families have fought to protect their territory at all costs. Death and destruction are widespread, sex is used ruthlessly for personal gain and rumours abound about the fighters’ true lineage. It is a little like something out of HBO’s Game of Thrones, only even more gruesome. While the winners in the fictional continent of Westeros get to sit on the Iron Throne, in the forests of France and Italy, the victors are plucked, peeled and delicately grated over home-made tagliatelle. Being a truffle, it turns out, isn’t easy. Truffles are best known for their distinctive flavour and extravagant price, but there is more to them than their gastronomic appeal. Recent investigations have shown that their underground existences are far more complex than we ever imagined. “Truffle reproduction is very bizarre,” says Marc-André Selosse at the French National Museum of Natural History in Paris. For one thing, we still don’t know exactly how they pull it off. Truffles live in powerful family clans that exclude their rivals, where the mothers seem to hold all the power and the fathers are nowhere to be found. Working out why they live as they do won’t just shed light on the subterranean life of this valuable fungus, it could finally solve a problem that has long eluded truffle-growers: how to reliably produce a crop. Like all fungi, truffles send spores out into the world that grow to form a new generation of organisms. The knobbly lump we eat is the fruiting body – the part that produces the spores. “They are like a black, crusty potato, with black diamonds in the crust,” says Elisa 72 | New Scientist | 21/28 December 2019
“Mother truffles seem to hold all the power. Fathers are nowhere” Taschen at the French National Institute for Agricultural Research in Montpellier, describing the black truffles that she works on. Inside, the white flesh is marbled with black dots, which are the spores. But the edible truffle is just a small part of a much bigger organism, made up of a network of filaments known as a mycelium that forms a symbiotic relationship with trees by colonising their roots. The fungus provides the tree with valuable nutrients in exchange for carbohydrates that the tree generates by photosynthesis. It is a cushy arrangement,
but one that poses a problem when it comes to spore dispersal. Whereas mushrooms and toadstools grow fruiting bodies above ground to spread spores on the wind, the underground truffle generates a powerful scent to attract animals to eat its fruiting bodies and then disperse the spores via their faeces. Truffles can be collected from forests using dogs or pigs to sniff them out, but these days, many are cultivated in orchards. This is a hitand-miss process because farmers can’t yet control their reproduction reliably. Those who study them are none the wiser about truffle reproduction either. “It really is a mystery,” says Francesco Paolocci at the Institute of Biosciences and Bioresources in Perugia, Italy. Until recently, the assumption was that truffles self-fertilised. Then, in 2008, Paolocci and his colleagues discovered that the genome inside the spores contained DNA sequences not present in that of the flesh. This means a truffle, in fact, has two parents. “It needs a sexual partner,” he says. One parent, dubbed the mother, supplies all nutrients and tissue to support the developing truffle. “The male partner solely provides genes,” says Paolocci.
Rumble in the fungal More surprises were thrown up in 2010 with the sequencing of the black truffle genome. This showed that, like other fungi, each truffle parent has a distinct mating type, dubbed MAT1 and MAT2. In order to produce truffle offspring, the parents must be opposite mating types, so a maternal MAT2 type, for example, has to find and mate with a paternal MAT1 truffle, while a maternal MAT1 has to mate with a paternal MAT2. When Paolocci and his colleagues went to sites in central Italy to observe how this happened in the wild, they were in for more shocks. They found that each tree was dominated by just one mating type, a situation that Paolocci describes as “a mating type war”. “It’s roughly like one type induces necrosis in the other,” says Paolocci. “It seems to be counter-intuitive. They need each other but they compete for the host trees.” Similar patterns of mating types have also been found in truffle orchards in France, Australia and China. It is a bizarre situation, says Selosse. “They don’t have their sexual partners around. It’s like
having cities of only men or only women.” It is possible that these exclusion zones are a way of boosting groups of closely related kin, which fuse to form super-mycelia that outcompete their genetically distant rivals. But under these seemingly deadly conditions, how do potential truffle parents ever meet up to have sex? To find out, researchers needed to find the elusive fathers. To do this, they have devised a sort of truffle paternity test. The maternal genome can be discovered from the flesh of the truffle, and the genome from both parents is obtained from the spores. “We subtract the maternal genotype from that of the spores and, by deduction, can find the genetic identity of the father,” says Taschen. Her study of French forests only ever found the maternal truffles interacting with the trees. “We never see the paternal genotype on the trees,” she says. This study also showed that each father produced only one, possibly two fruiting bodies, then disappeared. “We never see it again, not in another truffle or another year. Not on neighbouring trees.” Work carried out by Selosse, Taschen and others has ruled out the possibility that the fathers are living in the roots of nearby plants, such as thyme or grasses. So could the fathers be coming from far away to mate with the mothers? It seems not. “The fathers are genetically very close to the mothers, and we know that the more physically close they are, the more genetically close,” says Selosse. The latest hypothesis is that the fathers are hiding out as spores in the soil, where they germinate at some point after lying dormant and have sex with an established mother. Experiments to prove this idea are under way, in which spores from crushed fruiting bodies are introduced to a tree and tracked to see whether they develop into truffles. If true, this method could help growers improve domesticated yields. Either way, it is a transient existence for the truffle males. “They survive for a bit, then most vanish. Sex then death,” says Selosse. When you play the game of truffles, you win or you die. ❚
Alison George is a features editor at New Scientist. She is dreaming of a white truffle
21/28 December 2019 | New Scientist | 73
RENAUD VIGOURT
D
EEP in the forests of the north, a vicious battle is raging. For generations, rival families have fought to protect their territory at all costs. Death and destruction are widespread, sex is used ruthlessly for personal gain and rumours abound about the fighters’ true lineage. It is a little like something out of HBO’s Game of Thrones, only even more gruesome. While the winners in the fictional continent of Westeros get to sit on the Iron Throne, in the forests of France and Italy, the victors are plucked, peeled and delicately grated over home-made tagliatelle. Being a truffle, it turns out, isn’t easy. Truffles are best known for their distinctive flavour and extravagant price, but there is more to them than their gastronomic appeal. Recent investigations have shown that their underground existences are far more complex than we ever imagined. “Truffle reproduction is very bizarre,” says Marc-André Selosse at the French National Museum of Natural History in Paris. For one thing, we still don’t know exactly how they pull it off. Truffles live in powerful family clans that exclude their rivals, where the mothers seem to hold all the power and the fathers are nowhere to be found. Working out why they live as they do won’t just shed light on the subterranean life of this valuable fungus, it could finally solve a problem that has long eluded truffle-growers: how to reliably produce a crop. Like all fungi, truffles send spores out into the world that grow to form a new generation of organisms. The knobbly lump we eat is the fruiting body – the part that produces the spores. “They are like a black, crusty potato, with black diamonds in the crust,” says Elisa 72 | New Scientist | 21/28 December 2019
“Mother truffles seem to hold all the power. Fathers are nowhere” Taschen at the French National Institute for Agricultural Research in Montpellier, describing the black truffles that she works on. Inside, the white flesh is marbled with black dots, which are the spores. But the edible truffle is just a small part of a much bigger organism, made up of a network of filaments known as a mycelium that forms a symbiotic relationship with trees by colonising their roots. The fungus provides the tree with valuable nutrients in exchange for carbohydrates that the tree generates by photosynthesis. It is a cushy arrangement,
but one that poses a problem when it comes to spore dispersal. Whereas mushrooms and toadstools grow fruiting bodies above ground to spread spores on the wind, the underground truffle generates a powerful scent to attract animals to eat its fruiting bodies and then disperse the spores via their faeces. Truffles can be collected from forests using dogs or pigs to sniff them out, but these days, many are cultivated in orchards. This is a hitand-miss process because farmers can’t yet control their reproduction reliably. Those who study them are none the wiser about truffle reproduction either. “It really is a mystery,” says Francesco Paolocci at the Institute of Biosciences and Bioresources in Perugia, Italy. Until recently, the assumption was that truffles self-fertilised. Then, in 2008, Paolocci and his colleagues discovered that the genome inside the spores contained DNA sequences not present in that of the flesh. This means a truffle, in fact, has two parents. “It needs a sexual partner,” he says. One parent, dubbed the mother, supplies all nutrients and tissue to support the developing truffle. “The male partner solely provides genes,” says Paolocci.
Rumble in the fungal More surprises were thrown up in 2010 with the sequencing of the black truffle genome. This showed that, like other fungi, each truffle parent has a distinct mating type, dubbed MAT1 and MAT2. In order to produce truffle offspring, the parents must be opposite mating types, so a maternal MAT2 type, for example, has to find and mate with a paternal MAT1 truffle, while a maternal MAT1 has to mate with a paternal MAT2. When Paolocci and his colleagues went to sites in central Italy to observe how this happened in the wild, they were in for more shocks. They found that each tree was dominated by just one mating type, a situation that Paolocci describes as “a mating type war”. “It’s roughly like one type induces necrosis in the other,” says Paolocci. “It seems to be counter-intuitive. They need each other but they compete for the host trees.” Similar patterns of mating types have also been found in truffle orchards in France, Australia and China. It is a bizarre situation, says Selosse. “They don’t have their sexual partners around. It’s like
having cities of only men or only women.” It is possible that these exclusion zones are a way of boosting groups of closely related kin, which fuse to form super-mycelia that outcompete their genetically distant rivals. But under these seemingly deadly conditions, how do potential truffle parents ever meet up to have sex? To find out, researchers needed to find the elusive fathers. To do this, they have devised a sort of truffle paternity test. The maternal genome can be discovered from the flesh of the truffle, and the genome from both parents is obtained from the spores. “We subtract the maternal genotype from that of the spores and, by deduction, can find the genetic identity of the father,” says Taschen. Her study of French forests only ever found the maternal truffles interacting with the trees. “We never see the paternal genotype on the trees,” she says. This study also showed that each father produced only one, possibly two fruiting bodies, then disappeared. “We never see it again, not in another truffle or another year. Not on neighbouring trees.” Work carried out by Selosse, Taschen and others has ruled out the possibility that the fathers are living in the roots of nearby plants, such as thyme or grasses. So could the fathers be coming from far away to mate with the mothers? It seems not. “The fathers are genetically very close to the mothers, and we know that the more physically close they are, the more genetically close,” says Selosse. The latest hypothesis is that the fathers are hiding out as spores in the soil, where they germinate at some point after lying dormant and have sex with an established mother. Experiments to prove this idea are under way, in which spores from crushed fruiting bodies are introduced to a tree and tracked to see whether they develop into truffles. If true, this method could help growers improve domesticated yields. Either way, it is a transient existence for the truffle males. “They survive for a bit, then most vanish. Sex then death,” says Selosse. When you play the game of truffles, you win or you die. ❚
Alison George is a features editor at New Scientist. She is dreaming of a white truffle
21/28 December 2019 | New Scientist | 73