Tsetse fly region
Genome Yields Clues to Tsetse Fly’s
A tsetse fly bite feels like a hammer blow.
Smaller than horse flies but more aggressive,
these African insects also carry trypanosome
parasites responsible for sleeping sickness
and the cattle disease nagana, two scourges of
sub-Saharan Africa. Chemical traps and aer-
ial spray programs targeting tsetse flies have
replaced cruder control methods—inspectors
at car checkpoints used to swat any flies they
saw on dashboards. But 70 million Africans
are still at risk for sleeping sickness, which
causes extreme lethargy and can be fatal, and
an estimated 3 million farm animals die each
year from nagana.
Now, the first sequenced genome of a
tsetse fly species, Glossina morsitans, may
prime the pump for better control efforts.
On page 380, and in 11 satellite papers in
Public Library of Science (PLOS) journals,
a collaboration called the International
Glossina Genome Initiative (IGGI) reveals
hints of what makes tsetse flies so successful
and how the disease-causing trypanosomes
interact with the flies that carry them.
“Understanding the biology of the tsetse fly
is an essential step to fight the disease and
limit its diffusion,” says Philippe Bastin, a
parasitologist at the Institut Pasteur in Paris.
Most people think of mosquitoes as
bloodsuckers, but they pale in comparison
with tsetse flies, arguably the vampires of the
insect world. Whereas mosquitoes also sip
nectar and only the females suck blood, one
tsetse fly can drink almost its weight in blood
each meal, and both sexes consume nothing
else. The fly needs a special repertoire
of proteins for procuring, filtering, and
packaging blood, as well as resident bacteria
to provide nutrients not found in blood.
Researchers are also eager to understand the
genes and proteins underlying another tsetse
fly oddity: They bear live young—one at a
time—and nourish the developing larva with
“milk” secreted from special glands.
The new genome is still in many pieces,
but Serap Aksoy and Geoffrey Attardo of
Yale University and their colleagues have
already discerned 12,300 protein-coding
genes. The research team also surveyed
gene activity in various tissues, such as the
salivary glands, which trypanosomes use
as a launching point for infecting their next
hosts, and milk glands, to see what proteins
With its narrow blood diet, the tsetse fly
doesn’t need the full range of taste and odor
receptors found, for example, in fruit flies,
says Daniel Masiga, a molecular biologist at
the International Centre of Insect Physiology
and Ecology in Nairobi. He and his colleagues
uncovered what appear to be 46 tsetse proteins
sensitive to odors (the fruit fly has 58) and just
14 proteins for taste (the fruit fly has 73), with
none sensitive to sugar. The fly also has fewer
genes for proteins that recognize pathogens—
possibly because its restricted diet exposes it
to fewer infectious threats. The tsetse fly has
more proteins for detecting carbon dioxide,
however, likely useful for finding mammalian
quarry through their exhalations.
Once a tsetse fly bites a victim, 250
proteins in its saliva work to keep blood
from coagulating and to protect against
the bitten host’s immune system, among
other things. But the team found that a
trypanosome interferes with this efficient
feeding machine—likely to its own benefit.
With the parasite in its salivary glands, the
fly produces fewer of these proteins, making
feeding less efficient and forcing it to bite
more often. That “will favor transmission” of
the parasite, Bastin says.
To cope with its diet, the fly has an
unusually large number of genes for proteins
called aquaporins, which move water between
cells. When those genes are knocked out,
the tsetse fly has trouble excreting the bolus
of water in ingested blood, has lower heat
tolerance, and spends a longer time pregnant,
the initiative reports in PLOS Neglected
Tropical Diseases. Some aquaporin genes
are particularly active in the milk gland and
may be important in formulating milk, the
researchers note. The genome also contains
eight newly discovered milk protein genes,
and a paper in PLOS Genetics reports that
when a female is lactating, those genes and
other associated milk gland genes rev up their
activity 40-fold and account for half of the
fly’s total gene activity.
Risky territory. People and livestock across Central
Africa are at risk for diseases carried by the tsetse fly.
Ouch! With each brutal bite, a tsetse fly can take in
almost its weight in blood.
Strange and Deadly Ways