Serotonin modulation in the male Aedes aegypti ear influences hearing

Perspectives on the manipulation of mosquito hearing

Vector control is essential for preventing mosquito-borne diseases. However, different challenges associated with the development of insecticide resistance and behavioural adaptations across mosquito populations means novel control strategies are urgently needed. In recent years, disrupting mosquito mating has emerged as an alternative target of control tools because of its potential to reduce mosquito population numbers. Mosquito mating relies on sophisticated auditory processing for mate finding in many medically important species. Manipulating this key process could provide novel methods for mosquito control.

Mosquitoes as a model for understanding the neural basis of natural behaviors

Mosquito behaviors have been the subject of extensive research for over a century due to their role in the spread of human disease. However, these behaviors are also beginning to be appreciated as excellent models for neurobiological research in their own right. Many of the same behaviors and sensory abilities that help mosquitoes survive and reproduce alongside humans represent striking examples of generalizable phenomena of longstanding neurobiological interest. In this review, we highlight four prominent examples that promise new insight into (1) precise circadian tuning of sensory systems, (2) processing of complex natural odors, (3) multisensory integration, and (4) modulation of behavior by internal states.

Differences in male Aedes aegypti and Aedes albopictus hearing systems facilitate recognition of conspecific female flight tones

When Aedes albopictus mosquitoes invade regions predominated by Aedes aegypti, either the latter can be displaced or the species can coexist, with potential consequences on disease transmission. Males from both species identify females by listening for her flight sounds. Comparing male hearing systems may provide insight into how hearing could prevent interspecific mating. Here, we show that species-specific differences in female wing beat frequencies are reflected in differences in male ear mechanical tuning frequencies and sound response profiles. Though Aedes albopictus males are attracted to sound, they do not readily display abdominal bending, unlike Aedes aegypti. We observed interspecific differences in male ear mechanical, but not electrical, tuning, suggesting a conserved primary auditory processing pathway. Our work suggests a potential role for hearing in the premating isolation of Aedes aegypti and Aedes albopictus, with implications for predicting future dynamics in their sympatric relationships and our understanding of mosquito acoustic communication.

An update and review of arthropod vector sensory systems: Potential targets for behavioural manipulation by parasites and other disease agents

For over a century, vector ecology has been a mainstay of vector-borne disease control. Much of this research has focused on the sensory ecology of blood-feeding arthropods (black flies, mosquitoes, ticks, etc.) with terrestrial vertebrate hosts. Of particular interest are the cues and sensory systems that drive host seeking and host feeding behaviours as they are critical for a vector to locate and feed from a host. An important yet overlooked component of arthropod vector ecology are the phenotypic changes observed in infected vectors that increase disease transmission. While our fundamental understanding of sensory mechanisms in disease vectors has drastically increased due to recent advances in genome engineering, for example, the advent of CRISPR-Cas9, and high-throughput "big data" approaches (genomics, proteomics, transcriptomics, etc.), we still do not know if and how parasites manipulate vector behaviour. Here, we review the latest research on arthropod vector sensory systems and propose key mechanisms that disease agents may alter to increase transmission.

Characterization of a novel RNAi yeast insecticide that silences mosquito 5-HT1 receptor genes

G protein-coupled receptors (GPCRs), which regulate numerous intracellular signaling cascades that mediate many essential physiological processes, are attractive yet underexploited insecticide targets. RNA interference (RNAi) technology could facilitate the custom design of environmentally safe pesticides that target GPCRs in select target pests yet are not toxic to non-target species. This study investigates the hypothesis that an RNAi yeast insecticide designed to silence mosquito serotonin receptor 1 (5-HTR1) genes can kill mosquitoes without harming non-target arthropods. 5-HTR.426, a Saccharomyces cerevisiae strain that expresses an shRNA targeting a site specifically conserved in mosquito 5-HTR1 genes, was generated. The yeast can be heat-inactivated and delivered to mosquito larvae as ready-to-use tablets or to adult mosquitoes using attractive targeted sugar baits (ATSBs). The results of laboratory and outdoor semi-field trials demonstrated that consumption of 5-HTR.426 yeast results in highly significant mortality rates in Aedes, Anopheles, and Culex mosquito larvae and adults. Yeast consumption resulted in significant 5-HTR1 silencing and severe neural defects in the mosquito brain but was not found to be toxic to non-target arthropods. These results indicate that RNAi insecticide technology can facilitate selective targeting of GPCRs in intended pests without impacting GPCR activity in non-targeted organisms. In future studies, scaled production of yeast expressing the 5-HTR.426 RNAi insecticide could facilitate field trials to further evaluate this promising new mosquito control intervention.

Hearing of malaria mosquitoes is modulated by a beta-adrenergic-like octopamine receptor which serves as insecticide target

Malaria mosquitoes acoustically detect their mating partners within large swarms that form transiently at dusk. Indeed, male malaria mosquitoes preferably respond to female flight tones during swarm time. This phenomenon implies a sophisticated context- and time-dependent modulation of mosquito audition, the mechanisms of which are largely unknown. Using transcriptomics, we identify a complex network of candidate neuromodulators regulating mosquito hearing in the species Anopheles gambiae. Among them, octopamine stands out as an auditory modulator during swarm time. In-depth analysis of octopamine auditory function shows that it affects the mosquito ear on multiple levels: it modulates the tuning and stiffness of the flagellar sound receiver and controls the erection of antennal fibrillae. We show that two α- and β-adrenergic-like octopamine receptors drive octopamine's auditory roles and demonstrate that the octopaminergic auditory control system can be targeted by insecticides. Our findings highlight octopamine as key for mosquito hearing and mating partner detection and as a potential novel target for mosquito control.

The auditory efferent system in mosquitoes

Whilst acoustic communication forms an integral component of the mating behavior of many insect species, it is particularly crucial for disease-transmitting mosquitoes; swarming males rely on hearing the faint sounds of flying females for courtship initiation. That males can hear females within the din of a swarm is testament to their fabulous auditory systems. Mosquito hearing is highly frequency-selective, remarkably sensitive and, most strikingly, supported by an elaborate system of auditory efferent neurons that modulate the auditory function - the only documented example amongst insects. Peripheral release of octopamine, serotonin and GABA appears to differentially modulate hearing across major disease-carrying mosquito species, with receptors from other neurotransmitter families also identified in their ears. Because mosquito mating relies on hearing the flight tones of mating partners, the auditory efferent system offers new potential targets for mosquito control. It also represents a unique insect model for studying auditory efferent networks. Here we review current knowledge of the mosquito auditory efferent system, briefly compare it with its counterparts in other species and highlight future research directions to unravel its contribution to mosquito auditory perception.