Findings present potential strategy to control spread of malaria.
Tinkering with male mosquitoes so that they cannot produce sperm is a
promising way to control the spread of the malaria-carrying insects in
the wild.
Researchers had been concerned that female Anopheles gambiae
mosquitoes might not be fooled into mating with the spermless males,
but lab tests show that they are just as attracted to sterile males as
to normal ones1.
Releasing such males into the wild offers a way to control malaria
that does not rely on insecticides, which many species are increasingly
developing resistance to.
"Any strategy that targets sperm is safe," says Flaminia
Catteruccia, a molecular entomologis at Imperial College London, UK, and
lead author of the study1.
Pest control
The creation of sterile males is a strategy that has long been used to
control insect pests, such as the Mediterranean fruitfly, or medfly, in
countries including the United States. But researchers have struggled to
apply the technique to mosquitoes. Sterility is traditionally induced
by irradiating males, but in the mosquito's case this affects fitness
and ability to complete with untreated males for mates.
Moreover, there was a question mark over how the females would
respond to the males because some insects are not fooled by spermless
males. In the fruitfly Drosophila melanogaster, for instance, it is the presence of sperm after mating that stops females from remating and stimulates them to lay eggs.
"It is critical that the males behave normally," says Catteruccia, "but nothing was known about how mosquitoes reproduce."
Catteruccia and her team created 96 spermless male mosquitoes to test how the deficiency would affect female mating behaviour.
They used the technique of RNA interference (RNAi), injecting
mosquito embryos with fragments of RNA that disrupted a crucial gene
involved in the development of the testes, preventing males from
producing sperm. This method of sterilization did not affect any other
physiological function.
"Spermless males behave exactly like those with sperm," Catteruccia says. "We saw no difference in their ability to compete."
In addition, mating with sterile males did not affect female
egg-laying, nor did it stimulate females to mate a second time. Some 74%
of females that mated with spermless males produced eggs, laying an
average of 58. By contrast, 83% of females mated with males with sperm
produced eggs, and averaged 49 eggs.
"It is quite remarkable that females don't use sperm as a measure of
mating success," says Catteruccia. She says more work is needed to
figure out how the females know that mating has been successful, but
suggests that the presence of seminal fluid could be one clue.
"This is a crucial scientific demonstration and a huge step
forward," comments Elena Levashina, who studies malaria carrying
mosquitoes at the Institute of Molecular and Cellular Biology in
Strasbourg, part of the French National Centre for Scientific Research
(CNRS).
Scaling up
The next
challenge is finding a way to produce sterile males in sufficient
quantity, says Levashina. The method the team used to sterilize the
males is too time-consuming for large-scale use in the field.
Catteruccia hopes that someone will develop a more efficient way to
introduce the elements required for RNAi into embryos. Levashina
suggests a genetic modification could be introduced to render males
infertile.
Such mosquitoes would differ from existing genetically modified ones
created by Oxitec, a company in Oxford, UK, that specializes in insect
control. Oxitec uses a genetic tweak that makes the offspring of Aedes aegypti
mosquitoes dependent on the drug tetracycline for survival. The drug is
fed to the mosquitoes in the lab, but is not available in the wild.
When the modified mosquitoes mate with the local A. aegypti mosquitoes, they spread the lethal gene through the population, in theory causing it to crash.
The tweaked mosquitoes have been tested in field trials, but have yet to be released on a large scale.
References
- Thailayil, J., Magnusson, K., Godfray, H. C. J., Crisanti, A. & Catteruccia, F. Proc. Natl Acad. Sci. USA doi:10.1073/pnas.1104738108 (2011).