Nagoya University clarifies that male mosquitoes have more complex sound reception maps than females

A research group led by Professor Azusa Kamikouchi and Designated Assistant Professor Matthew Su from Nagoya University's Graduate School of Science, along with Postdoctoral Researcher Takuro Ohashi (at the time of the research, currently at the University of Washington in the U.S.), announced that they have successfully visualized auditory information processing in mosquito brains for the first time in the world. Using this method, they revealed that male auditory sensory nerve cells have more complex structures than females, and that males have complex sound reception maps in their brains. This achievement is expected to lead to the development of more effective insect traps and other applications. Their results were published in the June 4 issue of Science Advances.

As carriers of diseases, mosquitoes are one of the deadliest organisms in the world. Every year, approximately 720,000 people die because of them. The existence of individuals resistant to the "pyrethroid insecticides" made from the components of pyrethrum used to kill mosquitos has been confirmed, meaning that a more effective control method is needed.

The sounds produced by the wings of male and female mosquitoes differ. Males form swarms and locate females based on these sounds when a small number of females enter the swarm. The antennae, which perform functions as mosquito 'ears', are significantly larger in males than in females, and are known to be more complex.

The research group had previously revealed that males in swarms recognize females through distortion generated within their auditory organs as a result of mixed male and female wing sounds. However, how the brain responds to these sounds internally remained unclear. Therefore, the group aimed to elucidate the auditory mechanisms in Aedes aegypti mosquito brains using knowledge and experimental methods developed from Drosophila fruit flies.

First, they investigated which brain regions the antennal auditory sensory nerve cells transmit sound to by labeling auditory sensory nerve cells with neural tracers (dyes) and observing nerve cell morphology.

They found that, while these nerve cells extend to one side of the brain in many animals, in both male and female mosquitoes they are connected to the antennal mechanosensory and motor center (AMMC) in both hemispheres. Furthermore, there were groups of nerve cells found only in males, revealing that the structure of auditory nerve cells in males is more complex than in females.

Next, they investigated what sound information mosquito brains receive using calcium imaging, visualizing their responses to sound. They observed the responses of the AMMC to sounds of various frequencies (100-500 Hz).

This revealed that the frequencies that elicited strong responses were completely different between males and females. Males show strong responses to frequencies corresponding to distortion products and broader frequency responses than females.

The primary auditory cortex in human brains, which first processes sound, has an auditory reception map that responds to different frequency sounds depending on location. To verify whether mosquitoes have a similar mechanism, the group classified different response patterns in mosquito auditory regions using hierarchical clustering methods.

The results revealed that while female response patterns were mostly uniform, males had complex sound reception maps. Furthermore, to investigate the molecular basis of these sex differences in auditory abilities, they comprehensively examined genes expressed at the antennal base and compared them between males and females.

This showed that genes related to cilia were highly expressed in males. This difference suggests possible involvement in the differences in sound reception.

Journal Information
Publication: Science Advances
Title: Diversity and complexity of auditory representation in the hearing systems of Aedes aegypti mosquitoes
DOI: 10.1126/sciadv.ads2689