In the rain forests of Costa Rica lives Anelosimus octavius
, a species of spider that sometimes displays a strange and ghoulish habit
.
When infected by thorny-headed worms (the orange
spot), gammarids swim toward light. At the water's surface they become
easy prey for birds, the next creature the worm needs to infect to
complete its life cycle.
From time to time these spiders abandon their own webs and build
radically different ones, a home not for the spider but for a parasitic
wasp that has been living inside it. Then the spider dies — a zombie
architect, its brain hijacked by its parasitic invader — and out of its
body crawls the wasp’s larva, which has been growing inside it all this
time.
The
current issue
of the prestigious Journal of Experimental Biology is entirely
dedicated to such examples of zombies in nature. They are far from rare.
Viruses, fungi, protozoans, wasps, tapeworms and a vast number of other
parasites can control the brains of their hosts and get them to do
their bidding. But only recently have scientists started to work out the
sophisticated biochemistry that the parasites use
“The knowledge that parasites can manipulate their hosts is old. The new part is how they do it,” said
Shelley Adamo
of Dalhousie University in Nova Scotia, a co-editor of the new issue.
“The last 5 to 10 years have really been exciting.”
In the case of the Costa Rican spider, the new web is splendidly suited
to its wasp invader. Unlike the spider’s normal web, mostly a tangle of
threads, this one has a platform topped by a thick sheet that protects
it from the rain. The wasp larva crawls to the edge of the platform and
spins a cocoon that hangs down through an opening that the spider has
kindly provided for the parasite.
To manipulate the spiders, the wasp must have genes that produce
proteins that alter spider behavior, and in some species, scientists are
now pinpointing this type of gene. Such is the case with the
baculovirus, a virus sprinkled liberally on leaves in forests and
gardens. (The cabbage in a serving of coleslaw carries 100 million
baculoviruses.)
Human diners need not worry, because the virus is harmful only to
caterpillars of insect species, like gypsy moths. When a caterpillar
bites a baculovirus-laden leaf, the parasite invades its cells and
begins to replicate, sending the command “climb high.” The hosts end up
high in trees, which has earned this infection the name treetop disease.
The bodies of the caterpillars then dissolve, releasing a rain of
viruses on unsuspecting hosts below.
David P. Hughes of Penn State University and his colleagues
have found that a single gene, known as egt, is responsible
for
driving the caterpillars up trees. The gene encodes an enzyme. When the
enzyme is released inside the caterpillar, it destroys a hormone that
signals a caterpillar to stop feeding and molt.
Dr. Hughes suspects that the virus goads the caterpillar into a feeding
frenzy. Normally, gypsy moth caterpillars come out at night to feed and
then return to crevices near the bottom of trees to hide from predators.
The zombie caterpillars, on the other hand, cannot stop searching for
food.
“The infected individuals are out there, just eating and eating,” Dr. Hughes said. “They’re stuck in a loop.”
Other parasites manipulate their hosts by altering the neurotransmitters
in their brains. This kind of psychopharmacology is how thorny-headed
worms send their hosts to their doom.
Their host is a shrimplike crustacean called a gammarid. Gammarids,
which live in ponds, typically respond to disturbances by diving down
into the mud. An infected gammarid, by contrast, races up to the surface
of the pond. It then scoots across the water until it finds a stem, a
rock or some other object it can cling to.
The gammarid’s odd swimming behavior allows the parasite to take the
next step in its life cycle. Unlike baculoviruses, which go from
caterpillar to caterpillar, thorny-headed worms need to live in two
species: a gammarid and then a bird. Hiding in the pond mud keeps a
gammarid safe from predators. By forcing it to swim to the surface, the
thorny-headed worm makes it an easy target.
The invader provokes a strong response from the gammarid’s immune cells,
which unleash chemicals to kill the parasite. But the parasite fends
off these attacks, and the host’s immune system instead produces an
inflammation that infiltrates its own brain. There, it disrupts the
brain’s chemistry — in particular, causing it to produce copious amounts
of the neurotransmitter serotonin.
