Mosquitoes are much more forceful. Mating occurs for a few seconds in the air. And all it takes to court a male is the sound of a female’s flapping wings. Imagine the researchers’ surprise when a single change completely killed the mosquitoes’ libido.
Now, a UC Santa Barbara study reveals that this is really all there is to it. Researchers in Professor Craig Montell’s lab created deaf mosquitoes and found that the males had no interest in mating. “You could leave them with the females for days and they won’t mate,” Montell said.
The dramatic change was easy to produce. “The absence of a single gene, trpVaproduced this profound effect on the mating behavior of mosquitoes,” explained co-senior author Dhananjay Thakur, a postdoctoral researcher in the Department of Molecular, Cellular and Developmental Biology.
The results, published in the Proceedings of the National Academy of Sciences could have important implications for how we manage disease transmission by better controlling populations of mosquito vectors, such as Aedes aegypti, that infect hundreds of millions of people each year with disease-causing viruses.
Playful Skeeters
“On summer nights, we often see swarms of mosquitoes gathering by the water or under streetlights. These gatherings are essentially mass mating events,” said co-senior author Yijin Wang, a former postdoc at UCSB. Although mosquitoes have an extraordinary ability to reproduce, scientists still have a limited understanding of the molecular and neurological mechanisms involved.
Courtship for Aedes aegypti It usually progresses like this: Females flap their wings at about 500 Hz. When males hear this, they take off, buzzing at about 800 Hz. Males also rapidly modulate this frequency when females are nearby. Then there is a quick meeting in the air and the lovers go their separate ways. Males are always looking for new potential mates, but a female that successfully mates usually will not do so again.
Montell and co-lead authors Yijin Wang, Thakur and Emma Duge suspected that hearing played a role in this behavior, so they investigated the insect’s auditory neurons. These are located at the base of the antennae in a structure called Johnston’s organ. The antennae are magnificent multisensory devices, packed with olfactory, mechanosensory, and even thermal infrared sensilla, as Montell’s lab recently discovered. In the current study, the team focused on a particular sensory channel called TRPVa and the corresponding gene, trpVa — which is the analogue in mosquitoes of a channel necessary for hearing in fruit flies.
The team used CRISPR-Cas9 to disable the gene that encodes TRPVa in Aedes aegypti mosquitoes. The resulting animals showed no reaction to the sound. In fact, they discovered that sound did not cause electrical activity in the neurons of Johnston’s organ. The insects were truly deaf.
And when the authors placed deaf men in rooms with women… nothing happened. “If they can’t hear the female flapping, they’re not interested,” Montell said. Their hearing counterparts, on the other hand, wasted no time and mated many times over the course of a few minutes.
A romantic soundtrack
Hearing is not only necessary for males to mate, but it seems sufficient to awaken their desires. When the authors played the sound of female flapping wings to normal males, the males typically responded with abdominal thrusts. They were primed and ready for action. The deaf men barely moved.
The women, however, were a different story. Deaf women still had some lust left. “The impact on women is minimal, but the impact on men is absolute,” Montell said. The team plans to study these differences in future work.
“I think the reason our main finding is so striking is because, in most organisms, mating behavior depends on a combination of several sensory signals,” said Duge, one of Montell’s doctoral students. “The fact that removing a single sense can completely abolish mating is fascinating.”
And the authors believe their results—the role of sound in mating and the role TRPVa plays in hearing—generalize to other mosquito species.
looking inside
The physiology of a mosquito reveals how important hearing is for these insects. Male mosquitoes have the largest number of auditory neurons of any known insect, Montell explained. Women have half. It’s still a lot, but hearing is much more crucial for men.
Identify which neurons express the trpVa gene, the authors added a gene encoding green fluorescent protein to the mosquito genome. They did it in such a way that the fluorescent protein was indirectly expressed under the control of the trpVa promoter. A promoter is a DNA sequence usually located at the start of a gene where enzymes bind to initiate transcription, which in this case triggers the production of those green fluorescent proteins. Now the mutant mosquitoes would produce green fluorescent protein in all the places they would normally produce TRPVa. The same mosquitoes then provided test subjects for the experiment and a bright green map of TRPVa expression for analysis.
As expected, the team found that trpVa It is expressed in the Johnston’s organ. And they were able to clearly follow the pathways of auditory neurons from there to the brain, as well as see differences in these pathways between male and female mosquitoes.
Abduction of the mosquito courtship
Mosquito-transmitted pathogens. Aedes aegypti They infect about 400 million people a year, of which about 100 million develop diseases such as dengue, Zika and yellow fever. This means that understanding their behavior and life cycle can provide us with tools and knowledge for disease prevention.
One potential method to control insect vectors is the sterile insect technique (SIT), which involves releasing large numbers of sterile males to mate with females. In certain insects, such as mosquitoes, successful mating prevents females from seeking other mates. And, if the female mates with a sterile male, she does not actually produce offspring. In theory, this can suppress the population.
The technique works wonders for certain agricultural pests such as the California Mediterranean Mediterranean fly. “The fact that you haven’t heard of this pest is a testament to the success of SIT, because 30 years ago it was all over the news,” Montell said.
But the success of the TIE in Aedes aegypti is limited by the competitiveness of sterile males; They have to reach the females first for the strategy to work. Currently, the technique does not cause enough suppression in mosquito populations to drop them below the critical threshold and cause the population to plummet. Given the central role of the audience in mosquito courtship, trpVa could provide a target to increase the effectiveness of SIT. Montell’s lab is working on several ways to produce sterile males that can outcompete their natural counterparts. Hopefully the trick will be as simple as these simple lovers.