Daily News Archive
on Mosquito Resistance to Insecticides
(Beyond Pesticides, August 6, 2004) Researchers
have identified that a specific point in the genetic code appears not
only to control an organism's resistance to a class of pesticides but
also to significantly influence the ability of an organism to evolve
such resistance at all. Identifying such specific and strong constraints
on short-term evolutionary change are likely to help ecologists and
public-health experts understand, and potentially predict, the ability
of particular species to quickly develop resistance to substances such
as insecticides. The new work also illuminates the kind of genetic technicality
that can shape evolution.
The work, performed by an international team led by Mylene Weill of
the Université Montpellier II (France), concerned the ability
of mosquito species to develop resistance to two major classes of insecticides,
carbamates and organophosphates (OPs). Previous work had shown that
a single base-pair alteration, G119S, within the mosquito's version
of the AchE1 gene conferred high levels of resistance to these insecticides.
Not all mosquito species exposed to high levels of carbamates and OPs
develop resistance, however. For example, Anopheles gambiae, the mosquito
vector for malaria, is able to develop resistance in this way, whereas
Aedes aegytpi, the vector for yellow fever and dengue fever, has never
developed high levels of resistance.
The new study, "Insecticide resistance: a silent base prediction,"
published in the July 27, 2004 edition of Current
Biology reveals the reason for this striking discrepancy in
adaptation. First, the researchers determined that the G119S version
of the Ae. aegypti AchE1 protein was indeed resistant to insecticide
action in the test tube, suggesting that the mutation would confer resistance
to the mosquito in principle but that for some reason the mutation does
not appear in Ae. aegypti populations. Looking more closely at the Ae.
aegypti gene sequence for AchE1 revealed the answer. The researchers
found that in this species, the three-letter DNA code at glycine position
119 is different from that found in the other mosquitoes studied thus
far. The difference is "silent," that is, the gene still codes
for the same amino acid at the 119 position. But it means - critically,
as it turns out - that a single mutation of the site cannot result in
the G119S change needed for resistance. In A. gambiae, it only takes
one base mutation to alter the code in the right way; in Ae. aegypti,
it takes two adjacent base mutations - a far less likely event.
The researchers went on to sequence the 119 position in 26 natural populations
of Ae. aegypti in 12 countries and found that in all cases the three-letter
codon at this position was the same, fitting with the lack of AchE1-based
resistance in this species observed worldwide. They also showed that
the "constrained" codon is present in 31 of 44 additional
mosquito species, almost all of which indeed appear to have failed to
develop resistance. Among those species with the codon version that
easily mutates to confer resistance, about 50% have already developed
high AchE1-based resistance. Most of the others are not insecticide-controlled.
Insecticide resistance does occur in mosquitoes that carry West Nile
virus. A 2003 study published in Nature, "Comparative
Genomics: Insecticide Resistance in mosquito vectors" found
that mosquitoes carrying West Nile virus and malaria developed resistance
to organophosphate and carbamate insecticides as a result of a single
genetic mutation. Such resistance renders the broadcast spraying of
mosquito adulticides an inefficient form of control that puts public
health and the environment at risk to the chemical’s adverse effects.
TAKE ACTION: Fight to prevent unnecessary
adulticiding in your community and promote effective, intelligent mosquito
management. For help see Beyond Pesticides Tools
for Activists page. For more information on West Nile Virus and
mosquito management see a new factsheet by Beyond Pesticides: The
Truth About Mosquitoes, Pesticides, and West Nile Virus.