Romance can be quite intricate among humans, involving various factors like personality, attractiveness, and social interactions. In contrast, mosquitoes are straightforward in their approach to mating, which happens swiftly in midair and relies mainly on the sound of a female’s wingbeats to attract a male. So, it was quite astonishing for researchers to discover that a single genetic alteration could entirely diminish the mating instincts of these insects.
In contrast, mosquitoes have a more straightforward approach to romance, mating quickly in midair, relying on the sound of female wingbeats to attract males. Researchers were surprised to find that a single genetic change could completely eliminate the mating desire in these insects.
A recent study from UC Santa Barbara has shed light on this simplicity. In Professor Craig Montell’s lab, scientists produced deaf mosquitoes and observed that the males showed no interest in mating whatsoever. As Montell elaborated, “You could leave them with the females for days, and they won’t mate.”
The surprising change was easy to create. Co-lead author Dhananjay Thakur, a postdoctoral scholar in Molecular, Cellular, and Developmental Biology, explained, “Losing just one gene, trpVa, led to this significant alteration in mosquito mating behavior.”
These findings, published in the Proceedings of the National Academy of Sciences, could greatly affect how we manage the spread of diseases by improving the control of mosquito populations, such as Aedes aegypti, which infects hundreds of millions with various viruses annually.
Excitable mosquitoes
“On warm summer nights, we often encounter swarms of mosquitoes near water or under streetlights, which are essentially just mass mating events,” noted co-lead author Yijin Wang, a former postdoc at UCSB. Although mosquitoes are prolific breeders, our understanding of the molecular and neurological processes underlying their behavior remains limited.
The mating ritual for Aedes aegypti typically unfolds as follows: females flap their wings around 500 Hz, prompting males to respond with flight and buzzing at about 800 Hz. Males even adjust this frequency rapidly in the presence of females. This encounter leads to a quick mating session before the insects part ways, with males constantly searching for new mates, while mated females seldom engage again.
Montell and his colleagues suspected that hearing played a crucial role in these behaviors, prompting them to explore the auditory neurons situated at the base of the antennae within a structure known as Johnston’s organ. Antennae are remarkable multi-sensory tools, brimming with olfactory, mechanosensory, and even thermal infrared receptors, which Montell’s lab recently identified. In their latest research, the team focused on a specific sensory channel called TRPVa, linked to hearing in fruit flies, and its corresponding gene, trpVa.
Utilizing CRISPR-Cas9 technology, the researchers disabled the gene responsible for TRPVa in Aedes aegypti mosquitoes. The resulting mutants exhibited no response to sound, revealing no electrical activity in the neurons of Johnston’s organ. Essentially, these mosquitoes were completely deaf.
When placed in chambers with females, the deaf males showed no reaction. As Montell stated, “If they cannot hear the wingbeat of the female, they lose interest.” In contrast, the hearing males were eager and quick to mate multiple times within mere minutes.
A romantic soundtrack
A male mosquito’s hearing appears not just necessary but also sufficient to ignite their mating interest. When the researchers played recordings of female wingbeats to the hearing males, they typically responded with abdominal thrusts, ready to mate. The deaf males, however, hardly moved.
Interestingly, the deaf females still retained some level of desire, unlike the males. Montell remarked, “The effect on the female was minimal, while it was total for the male.” Further investigations are planned to explore these differences.
Duge, a doctoral student in Montell’s lab, noted, “The reason our key finding is so surprising is that in many species, mating behavior relies on various sensory cues. The fact that removing a single sense can entirely eradicate mating is captivating.”
The authors believe that their discoveries regarding the role of sound in mating and the significance of TRPVa may apply to other mosquito species as well.
Examining the inner workings
The physiological structure of mosquitoes highlights the essential role of hearing. Male mosquitoes possess the highest number of auditory neurons among known insects, explained Montell. Females have about half that number, which is still considerable, but hearing is critical for males.
To determine which neurons express the trpVa gene, the researchers introduced a gene for a green fluorescent protein into the mosquito genome. They did this in such a manner that the fluorescent protein was expressed under the control of the trpVa promoter, which triggers enzyme binding to initiate transcription. This meant that mutant mosquitoes would generate green fluorescent protein where TRPVa would typically be produced, providing a vibrant indicator of TRPVa expression for analysis.
The team surprisingly confirmed that trpVa is expressed in Johnston’s organ. They traced the auditory neuron pathways from there to the brain and observed differences in these pathways between male and female mosquitoes.
Disrupting mosquito mating
The Aedes aegypti mosquito is responsible for spreading diseases that affect approximately 400 million people annually, with around 100 million experiencing illnesses such as dengue, Zika, and yellow fever. Gaining insights into its behavior and life cycle can aid in disease prevention efforts.
One method to control insect populations is known as the sterile insect technique (SIT), where a significant number of sterile males are released to mate with females. For many insects, including mosquitoes, successful mating inhibits females from seeking additional partners. Should a female mate with a sterile male, she will not produce offspring, potentially leading to population suppression.
This technique has been remarkably effective against certain agricultural pests, such as the California medfly. “If you haven’t heard of this pest, it’s because SIT has been so successful that it was once widely publicized over 30 years ago,” Montell observed.
However, the effectiveness of SIT in Aedes aegypti is hampered by the need for sterile males to compete effectively for mates. Currently, the technique does not achieve sufficient population reduction to bring them below critical levels. Given the importance of hearing in mosquito courtship, the researchers consider trpVa as a possible target for enhancing SIT’s effectiveness. Montell’s lab is investigating various strategies to create competitive sterile males, hoping that achieving this will be as straightforward as the mating rituals of these uncomplicated insects.
