Active amplification in insect ears: mechanics, models and molecules

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.

Active oscillations in microscale navigation

Living organisms routinely navigate their surroundings in search of better conditions, more food, or to avoid predators. Typically, animals do so by integrating sensory cues from the environment with their locomotor apparatuses. For single cells or small organisms that possess motility, fundamental physical constraints imposed by their small size have led to alternative navigation strategies that are specific to the microscopic world. Intriguingly, underlying these myriad exploratory behaviours or sensory functions is the onset of periodic activity at multiple scales, such as the undulations of cilia and flagella, the vibrations of hair cells, or the oscillatory shape modes of migrating neutrophils. Here, I explore oscillatory dynamics in basal microeukaryotes and hypothesize that these active oscillations play a critical role in enhancing the fidelity of adaptive sensorimotor integration.

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.

Vibration detection in arthropods: Signal transfer, biomechanics and sensory adaptations

In arthropods, the detection of vibrational signals and stimuli is essential in several behaviours, including mate recognition and pair formation, prey detection, and predator evasion. These behaviours have been studied in several species of insects, arachnids, and crustaceans for vibration production and propagation in the environment. Vibration stimuli are transferred over the animals' appendages and the body to vibrosensory organs. Ultimately, the stimuli are transferred to act on the dendrites of the mechanosensitive sensilla. We refer to these two different levels of transfer as macromechanics and micromechanics, respectively. These biomechanical processes have important roles in filtering and pre-processing of stimuli, which are not carried out by neuronal components of sensory organs. Also, the macromechanical transfer is posture-dependent and enables behavioural control of vibration detection. Diverse sensory organs respond to vibrations, including cuticular sensilla (slit sensilla, campaniform sensilla) and internal chordotonal organs. These organs provide various adaptations, as they occur at diverse body positions with different mechanical couplings as input pathways. Macromechanics likely facilitated evolution of vibrosensory organs at specific body locations. Thus, vibration detection is a highly complex sensory capacity, which employs body and sensory mechanics for signal filtering, amplification, and analysis of frequency, intensity and directionality.

The Antenna Base Plays a Crucial Role in Mosquito Courtship Behavior

Mosquitoes are vectors of pathogens that cause diseases like malaria, dengue fever, yellow fever, chikungunya and Zika. For mosquito control it is crucial to understand their hearing system, as mosquitoes’ courting behavior is mostly auditory. Many nonlinear characteristics of the mosquito hearing organ have been observed through behavioral studies and neural measurements. These enable mosquitoes to detect and synchronize to other mosquitoes. Many hypotheses concerning the role of the flagellum and the fibrillae of the antenna in mosquito hearing have been made, and neural processes have been considered as the origin of the nonlinearities. In this study we introduce a geometric model based on the morphology of the mosquito antenna base. The model produces many of the observed nonlinear characteristics, providing evidence that the base of the antenna plays a crucial role in mosquito hearing. Even without neural processing, the antenna response to sound produces behaviorally relevant cues that can inform about the presence, location, and sex of other mosquitoes.

Bridging the Gap Between Mammal and Insect Ears – A Comparative and Evolutionary View of Sound-Reception

Insects must wonder why mammals have ears only in their head and why they evolved only one common principle of ear design—the cochlea. Ears independently evolved at least 19 times in different insect groups and therefore can be found in completely different body parts. The morphologies and functional characteristics of insect ears are as wildly diverse as the ecological niches they exploit. In both, insects and mammals, hearing organs are constrained by the same biophysical principles and their respective molecular processes for mechanotransduction are thought to share a common evolutionary origin. Due to this, comparative knowledge of hearing across animal phyla provides crucial insight into fundamental processes of auditory transduction, especially at the biomechanical and molecular level. This review will start by comparing hearing between insects and mammals in an evolutionary context. It will then discuss current findings about sound reception will help to bridge the gap between both research fields.

Axonemal Dynein DNAH5 is Required for Sound Sensation in Drosophila Larvae

Chordotonal neurons are responsible for sound sensation in Drosophila. However, little is known about how they respond to sound with high sensitivity. Using genetic labeling, we found one of the Drosophila axonemal dynein heavy chains, CG9492 (DNAH5), was specifically expressed in larval chordotonal neurons and showed a distribution restricted to proximal cilia. While DNAH5 mutation did not affect the cilium morphology or the trafficking of Inactive, a candidate auditory transduction channel, larvae with DNAH5 mutation had reduced startle responses to sound at low and medium intensities. Calcium imaging confirmed that DNAH5 functioned autonomously in chordotonal neurons for larval sound sensation. Furthermore, disrupting DNAH5 resulted in a decrease of spike firing responses to low-level sound in chordotonal neurons. Intriguingly, DNAH5 mutant larvae displayed an altered frequency tuning curve of the auditory organs. All together, our findings support a critical role of DNAH5 in tuning the frequency selectivity and the sound sensitivity of larval auditory neurons.

Coevolution of cognitive abilities and identity signals in individual recognition systems

Social interactions are mediated by recognition systems, meaning that the cognitive abilities or phenotypic diversity that facilitate recognition may be common targets of social selection. Recognition occurs when a receiver compares the phenotypes produced by a sender with a template. Coevolution between sender and receiver traits has been empirically reported in multiple species and sensory modalities, though the dynamics and relative exaggeration of traits from senders versus receivers have received little attention. Here, we present a coevolutionary dynamic model that examines the conditions under which senders and receivers should invest effort in facilitating individual recognition. The model predicts coevolution of sender and receiver traits, with the equilibrium investment dependent on the relative costs of signal production versus cognition. In order for recognition to evolve, initial sender and receiver trait values must be above a threshold, suggesting that recognition requires some degree of pre-existing diversity and cognitive abilities. The analysis of selection gradients demonstrates that the strength of selection on sender signals and receiver cognition is strongest when the trait values are furthest from the optima. The model provides new insights into the expected strength and dynamics of selection during the origin and elaboration of individual recognition, an important feature of social cognition in many taxa. This article is part of the theme issue 'Signal detection theory in recognition systems: from evolving models to experimental tests'.

The Cognitive Ecology of Stimulus Ambiguity: A Predator-Prey Perspective

Organisms face the cognitive challenge of making decisions based on imperfect information. Predators and prey, in particular, are confronted with ambiguous stimuli when foraging and avoiding attacks. These challenges are accentuated by variation imposed by environmental, physiological, and cognitive factors. While the cognitive factors influencing perceived ambiguity are often assumed to be fixed, contemporary findings reveal that perceived ambiguity is instead the dynamic outcome of interactive cognitive processes. Here, we present a framework that integrates recent advances in neurophysiology and sensory ecology with a classic decision-making model, signal detection theory (SDT), to understand the cognitive mechanisms that shape perceived stimulus ambiguity in predators and prey. Since stimulus ambiguity is pervasive, the framework discussed here provides insights that extend into nonforaging contexts.

Material stiffness variation in mosquito antennae

The antennae of mosquitoes are model systems for acoustic sensation, in that they obey general principles for sound detection, using both active feedback mechanisms and passive structural adaptations. However, the biomechanical aspect of the antennal structure is much less understood than the mechano-electrical transduction. Using confocal laser scanning microscopy, we measured the fluorescent properties of the antennae of two species of mosquito- Toxorhynchites brevipalpis and Anopheles arabiensis-and, noting that fluorescence is correlated with material stiffness, we found that the structure of the antenna is not a simple beam of homogeneous material, but is in fact a rather more complex structure with spatially distributed discrete changes in material properties. These present as bands or rings of different material in each subunit of the antenna, which repeat along its length. While these structures may simply be required for structural robustness of the antennae, we found that in FEM simulation, these banded structures can strongly affect the resonant frequencies of cantilever-beam systems, and therefore taken together our results suggest that modulating the material properties along the length of the antenna could constitute an additional mechanism for resonant tuning in these species.

Travelling waves and tonotopicity in the inner ear: a historical and comparative perspective

In the 1940s, Georg von Békésy discovered that in the inner ear of cadavers of various vertebrates, structures responded to sound with a displacement wave that travels in a basal-to-apical direction. This historical review examines this concept and sketches its rôle and significance in the development of the research field of cochlear mechanics. It also illustrates that this concept and that of tonotopicity necessarily correlate, in that travelling waves are consequences of the existence of an ordered, longitudinal array of receptor cells tuned to systematically changing frequencies along the auditory organ.

Stay tuned: active amplification tunes tree cricket ears to track temperature-dependent song frequency

Tree cricket males produce tonal songs, used for mate attraction and male-male interactions. Active mechanics tunes hearing to conspecific song frequency. However, tree cricket song frequency increases with temperature, presenting a problem for tuned listeners. We show that the actively amplified frequency increases with temperature, thus shifting mechanical and neuronal auditory tuning to maintain a match with conspecific song frequency. Active auditory processes are known from several taxa, but their adaptive function has rarely been demonstrated. We show that tree crickets harness active processes to ensure that auditory tuning remains matched to conspecific song frequency, despite changing environmental conditions and signal characteristics. Adaptive tuning allows tree crickets to selectively detect potential mates or rivals over large distances and is likely to bestow a strong selective advantage by reducing mate-finding effort and facilitating intermale interactions.

Concept of an Active Amplification Mechanism in the Infrared Organ of Pyrophilous Melanophila Beetles

Jewel beetles of the genus Melanophila possess a pair of metathoracic infrared (IR) organs. These organs are used for forest fire detection because Melanophila larvae can only develop in fire killed trees. Several reports in the literature and a modeling of a historic oil tank fire suggest that beetles may be able to detect large fires by means of their IR organs from distances of more than 100 km. In contrast, the highest sensitivity of the IR organs, so far determined by behavioral and physiological experiments, allows a detection of large fires from distances up to 12 km only. Sensitivity thresholds, however, have always been determined in non-flying beetles. Therefore, the complete micromechanical environment of the IR organs in flying beetles has not been taken into consideration. Because the so-called photomechanic sensilla housed in the IR organs respond bimodally to mechanical as well as to IR stimuli, it is proposed that flying beetles make use of muscular energy coupled out of the flight motor to considerably increase the sensitivity of their IR sensilla during intermittent search flight sequences. In a search flight the beetle performs signal scanning with wing beat frequency while the inputs of the IR organs on both body sides are compared. By this procedure the detection of weak IR signals could be possible even if the signals are hidden in the thermal noise. If this proposed mechanism really exists in Melanophila beetles, their IR organs could even compete with cooled IR quantum detectors. The theoretical concept of an active amplification mechanism in a photon receptor innervated by highly sensitive mechanoreceptors is presented in this article.

Hearing in Insects

Insect hearing has independently evolved multiple times in the context of intraspecific communication and predator detection by transforming proprioceptive organs into ears. Research over the past decade, ranging from the biophysics of sound reception to molecular aspects of auditory transduction to the neuronal mechanisms of auditory signal processing, has greatly advanced our understanding of how insects hear. Apart from evolutionary innovations that seem unique to insect hearing, parallels between insect and vertebrate auditory systems have been uncovered, and the auditory sensory cells of insects and vertebrates turned out to be evolutionarily related. This review summarizes our current understanding of insect hearing. It also discusses recent advances in insect auditory research, which have put forward insect auditory systems for studying biological aspects that extend beyond hearing, such as cilium function, neuronal signal computation, and sensory system evolution.

Diverse Roles of Axonemal Dyneins in Drosophila Auditory Neuron Function and Mechanical Amplification in Hearing

Much like vertebrate hair cells, the chordotonal sensory neurons that mediate hearing in Drosophila are motile and amplify the mechanical input of the ear. Because the neurons bear mechanosensory primary cilia whose microtubule axonemes display dynein arms, we hypothesized that their motility is powered by dyneins. Here, we describe two axonemal dynein proteins that are required for Drosophila auditory neuron function, localize to their primary cilia, and differently contribute to mechanical amplification in hearing. Promoter fusions revealed that the two axonemal dynein genes Dmdnah3 (=CG17150) and Dmdnai2 (=CG6053) are expressed in chordotonal neurons, including the auditory ones in the fly’s ear. Null alleles of both dyneins equally abolished electrical auditory neuron responses, yet whereas mutations in Dmdnah3 facilitated mechanical amplification, amplification was abolished by mutations in Dmdnai2. Epistasis analysis revealed that Dmdnah3 acts downstream of Nan-Iav channels in controlling the amplificatory gain. Dmdnai2, in addition to being required for amplification, was essential for outer dynein arms in auditory neuron cilia. This establishes diverse roles of axonemal dyneins in Drosophila auditory neuron function and links auditory neuron motility to primary cilia and axonemal dyneins. Mutant defects in sperm competition suggest that both dyneins also function in sperm motility.