A chemical that the male Aedes aegypti mosquito transfers during sex plays a key role in shaping the female’s sexual proclivities, research shows.
The work, from the lab of Leslie B. Vosshall at Rockefeller University, could yield new strategies for keeping the pests—and the Zika, dengue, and yellow fever they bring—in check.
The female Ae. aegypti mates but once, in seconds and on the wing, with one male; spurns all further advances from other potential suitors; and stores enough sperm from that single encounter to lay more than 500 eggs, which she nourishes with the blood of human hosts.
Many of the mechanisms governing her mating habits remain a mystery.
Postdoctoral associate Laura Duvall set out to learn more about the biology behind female mosquitoes’ host-seeking behaviors. In particular, she was curious about the role of a small protein called HP-I. Previous studies had shown that this molecule is produced primarily by male Ae. aegypti, and transferred during sex to females, in whom it lasts for only two hours—findings that Duvall and her colleagues confirmed.
The team disproved findings from other studies, however, which had suggested that HP-I suppresses the female’s urge to seek out human hosts. The researchers paired males and females, including both mutant mosquitoes that were genetically modified to produce virtually no HP-I, and normal (or “wild-type”) ones. After letting their subjects mate with one another in various combinations, the team looked for changes in the females’ host-seeking behavior. But no matter with whom they had mated, the females remained just as keen on finding people to bite.
Their attraction to male mosquitoes, however, was a different story.
Scientists have long known that females mate only once, a phenomenon known as monandry; but they didn’t know why. Given that HP-I is transferred to the female along with the male’s semen, Duvall and her colleagues wondered if it might play a role in switching off her sex drive.
“Whenever you see mixed fluorescent and non-fluorescent larvae, you know the female received sperm from more than one male.”
To test that hypothesis, the team once again exposed females to males that produced HP-I, and males that didn’t. But this time, they added a third group of suitors: males that produced HP-I, but were genetically modified so that their offspring would glow bright blue when viewed through a fluorescent microscope.
By presenting the females with different combinations of fluorescent and non-fluorescent males (i.e., fluorescent males together with non-fluorescent males that produced HP-I, versus fluorescent males together with non-fluorescent males that lacked HP-I), the researchers were able to determine when the females were willing to accept only one mate—and when they were willing to play the field.
“Whenever you see mixed fluorescent and non-fluorescent larvae, you know the female received sperm from more than one male,” Duvall explains.
The results of this mosquito paternity test were definitive: females that got a dose of HP-I during sex and then were offered another mate within an hour remained loyal to their initial partners, while females that got no HP-I did not. (Nevertheless, after 24 hours even females that mated with males lacking HP-I rejected additional partners, suggesting that other chemicals transmitted by the male are responsible for influencing female behavior over the long term.)
Subsequent experiments showed that simply injecting HP-I directly into females was enough to trick the insects into thinking they had already mated, leading them to reject genuine warm-blooded males.
Mosquito ‘love potion’?
In a final series of experiments, Duvall and her colleagues investigated inter-species sex.
In the southern United States, Ae. aegypti is being displaced by its cousin Ae. albopictus, which can transmit the same diseases but thrives over a much larger range. While Ae. albopictus males can mate with Ae. aegypti females, those pairings don’t produce viable eggs; and the Ae. aegypti females will subsequently refuse to mate with males of their own species, making it impossible for them to reproduce. The reverse, however, does not hold true: male Ae. aegypti do not effectively sterilize Ae. albopictus females.
Duvall and her team discovered that HP-I may help explain this curious reproductive pattern, as well: while the Ae. albopictus version of HP-I activates a receptor in Ae. aegypti females, the Ae. aegypti version of this protein does not activate the corresponding receptor in Ae. albopictus.