ADVANCES
16 Scientific American, May 2020 Illustration by Thomas Fuchs
NATHALIE FEINERLund UniversityPA R A S I T O L O G Y
Parasites
on the Brain
Lizard embryos host invaders
When Nathalie Feiner spotted a tiny
nematode worm wriggling in an embry-
onic lizard’s brain from the French Pyre-
nees, she thought it was a freak accident.
She was dissecting hundreds of common
wall lizard embryos for a study and had
never encountered this invader before—
but soon she started finding them in more
of the still unhatched reptiles’ brains.
Intrigued, Feiner, then with the Univer-
sity of Oxford, and a colleague examined
the embryos’ parents. They found nema-
todes only in the ovaries of mothers that had
produced infected embryos, suggesting the
parasites were migrating to their offspring in
a way researchers had thought impossible.
Parasites such as nematodes, which do
not multiply in their hosts, often pass from
mother to children through mammals’ pla-
centas or milk. But scientists had assumed
that in birds and reptiles, the eggshell that
forms around the developing animal acts
as a barrier to such invasions. Parasite infec-
tion through a reptile egg had never been
observed before, Feiner says: “It seems like
we have hit on an entirely new lifestyle that
these nematodes have evolved.”
For a paper accepted by the American
Naturalist last December, Feiner and her
colleagues examined 720 eggs laid by 85
female common wall lizards from six loca-
tions. The researchers found the nema-
todes in lizards from only that first Pyre-
nees population. Infected females trans-
mitted the parasite to between 50 and
76.9 percent of their embryos.
DNA analysis showed these nematodes
are similar to, though much smaller than, aspecies found in the lizards’ gut; researchers
say they may have evolved from that species.
Feiner says scientists could have missed
the possibility of egg transmission because
they have mainly looked at parasites in birds
and turtles, whose eggshells form shortly
after fertilization when the embryo is just
a clump of cells—too small to act as a host.
But in lizards and snakes, the shells form
when the embryo is bigger, making parasite
transmission more plausible. James Harris
of the Research Center in Biodiversity and
Genetic Resources in Portugal, who was not
involved with the work, says this form of
transmission could be widespread if the
team’s hypothesis is correct.
Feiner suspects the nematode could
change its host’s behavior—a technique
brain parasites often use to infect an animal’s
predators. For instance, mice infected with
Toxoplasma drop their tendency to avoid cat
urine. This makes them more easily eaten,
transmitting the parasite to the next part of
its life cycle. “Identifying the presence of ‘our’
nematode in a predator of the European
wall lizard would make [this strategy] more
likely,” Feiner says. — Sandrine CeurstemontB I O L O G Y
Ant-agonism
Ants do not attack if they cannot
smell enemies’ precise scents
Accurately distinguishing friend from foe
is a matter of life and death for ants: mis-
taking an invader for a nest mate—or the
reverse—can lead to fatal chaos.
Scientists have long observed ants
deftly navigating through crowds, attack-
ing only individuals that might be hostile.
New research confirms how smell recep-
tors on the insects’ antennae hold the key
to this selective violence: without them,
ants are socially blind and will not attack.
“The current consensus was aggression
between ants follows a simple rule: if [an
ant] smells something different from the
home colony, attack,” says Laurence
Zwiebel, a co-author on the new study and
a biologist at Vanderbilt University. But the
new research shows it is not that simple.
Ants hold off on attacking if they cannot
smell anything—or even if they do not rec-
ognize a scent. “Rather a precise signal
present on the non-nest mate must be
correctly decoded for aggression to occur,”
Zwiebel says.
He and his colleagues built on previous
studies that identified a mix of odors on
ants’ exoskeletons, as well as odorant
receptors that pick up these scents from
others. The new study found that if the
receptors were compromised, ants could
no longer differentiate nest mates from
intruders they would normally fight; in
stead they became docile. The researchers
reported their findings in January in the
Journal of Experimental Biology.
After designing a miniature dueling
arena—a plate with plastic dividers—the
scientists chemically manipulated the odor-
ant receptors of Florida carpenter ants from
the same and neighboring colonies, either
blocking or overexciting the receptors.
When the ants were placed into the arena
and the dividers lifted, ants with disrupted
receptors were meek even when faced with
a stranger. “Our study clearly demonstrates
that neither the lack of any odor nor the
presence of a confusing odor was sufficient
to elicit ant aggression,” Zwiebel says.
Ants have more than 400 odorant re -ceptors, and Zwiebel says a next step is
to determine which of them must function
correctly to decode an enemy’s smell.
(For this study, researchers dampened or
excited all of them.)
Volker Nehring, a biologist at the Uni-
versity of Freiburg in Germany, who was
not involved with the study, says this re -
search could also pave the way for other
studies of how animals recognize one
another. “We hardly understand how the
ants know their own nest odor in the first
place,” he says, “and temporarily interfer-
ing with the receptors might be a good
way to address that.” — Jillian KramerWall lizard embryo