21/28 December 2019 | New Scientist | 73Taschen 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,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“ Mother truffles seem to hold all
the power. Fathers are nowhere”
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 hit-
and-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