Auditory sensitivity, spatial dynamics, and amplitude of courtship song in Drosophila melanogaster

Body size and sequence of host colonisation predict the presence of acoustic signalling in beetles

Acoustic communication is widespread in beetles, is often sexually dimorphic, and plays a significant role in behaviours such as premating recognition, courtship, and copulation. However, the factors that determine the presence or absence of acoustic signalling in a given species remain unclear. We examined acoustic communication in bark beetles (Scolytinae) and pinhole borers (Platypodinae), which are two speciose groups with widespread sound production capabilities. We show that body size along with the sequence of host colonisation predict the presence of acoustic communication, and report, for the first time in the animal kingdom, a size limit-1.9 mm-below which acoustic signalling ceases to be present.

Neural network organization for courtship-song feature detection in Drosophila

Animals communicate using sounds in a wide range of contexts, and auditory systems must encode behaviorally relevant acoustic features to drive appropriate reactions. How feature detection emerges along auditory pathways has been difficult to solve due to challenges in mapping the underlying circuits and characterizing responses to behaviorally relevant features. Here, we study auditory activity in the Drosophila melanogaster brain and investigate feature selectivity for the two main modes of fly courtship song, sinusoids and pulse trains. We identify 24 new cell types of the intermediate layers of the auditory pathway, and using a new connectomic resource, FlyWire, we map all synaptic connections between these cell types, in addition to connections to known early and higher-order auditory neurons-this represents the first circuit-level map of the auditory pathway. We additionally determine the sign (excitatory or inhibitory) of most synapses in this auditory connectome. We find that auditory neurons display a continuum of preferences for courtship song modes and that neurons with different song-mode preferences and response timescales are highly interconnected in a network that lacks hierarchical structure. Nonetheless, we find that the response properties of individual cell types within the connectome are predictable from their inputs. Our study thus provides new insights into the organization of auditory coding within the Drosophila brain.

Non-visual cues and indirect strategies that enable discrimination of asymmetric mates

The postulates of developmental instability-sexual selection hypothesis is intensely debated among evolutionary biologists, wherein despite a large amount of empirical data, evidence for or against it has been largely inconclusive. A key assumption of this hypothesis is that animals assess symmetry in potential mates as an indicator of genetic quality (developmental stability), and consequently use this information to discriminate against those with higher asymmetries while choosing mates. However, the perceptional basis that must underlie such discriminatory behavior (is symmetry a signal or is symmetry signaled) is not clearly defined. It is also argued that since asymmetry levels in natural populations are very low, the low signal-to-noise ratio would make accurate assessment of symmetry both difficult and costly. Rather than attempting to validate this hypothesis or even as to whether animals assess mate symmetry, this review simply aims to examine the plausibility that animals perceive symmetry (directly or indirectly) and consequently discriminate against asymmetric mates in response to perceived irregularities during courtship. For this, we review mate choice and courtship literature to identify potential sensory cues that might advertise asymmetry or lead to discrimination of asymmetric individuals. Although signaling associated with mate choice is commonly multimodal, previous studies on asymmetry have mainly focused on visual perception. In the light of a recent study (Vijendravarma et al., 2022, Proceedings of the National Academy of Sciences of the United States of America, 119, e2116136119), this review attempts to balance this bias by emphasizing on non-visual perception of asymmetry. In conclusion, we discuss the methodological challenges associated with testing the role of multimodal cues in detecting mate asymmetry, and highlight the importance of considering ecological, behavioral, and evolutionary aspects of animals while interpreting empirical data that test such hypothesis.

Drosophila females have an acoustic preference for symmetric males

Theoretically, symmetry in bilateral animals is subject to sexual selection, since it can serve as a proxy for genetic quality of competing mates during mate choice. Here, we report female preference for symmetric males in Drosophila, using a mate-choice paradigm where males with environmentally or genetically induced wing asymmetry were competed. Analysis of courtship songs revealed that males with asymmetric wings produced songs with asymmetric features that served as acoustic cues, facilitating this female preference. Females experimentally evolved in the absence of mate choice lost this preference for symmetry, suggesting that it is maintained by sexual selection.

In many species, including humans and Drosophila, symmetric individuals secure more matings, suggesting that bilateral symmetry signals the quality of potential mates and is subject to sexual selection. However, this idea remains controversial, largely because obtaining conclusive experimental evidence has been hindered by confounding effects arising from the methods used to increase asymmetry in test subjects. Here, we show that altering gravity during development increases asymmetry in Drosophila melanogaster without a detrimental effect on survival, growth, and behavior. Testing males with altered-gravity–induced asymmetry in female mate-choice assays revealed symmetry-based discrimination of males via auditory cues. Females similarly discriminated against males with genetically induced asymmetry, suggesting that their preference for symmetry is not specific to altered gravity. By segmenting the male courtship song into left and right wing-generated song-bouts, we detected asymmetry in the courtship song of altered-gravity males with asymmetric wings that experienced rejection. Females experimentally evolved in the absence of mate choice lacked this preference for symmetry, suggesting that symmetry is maintained by sexual selection. Our data provide evidence for the role of symmetry in sexual selection and reveal how nonvisual cues can flag mate asymmetry during courtship.

Fast and accurate annotation of acoustic signals with deep neural networks

Acoustic signals serve communication within and across species throughout the animal kingdom. Studying the genetics, evolution, and neurobiology of acoustic communication requires annotating acoustic signals: segmenting and identifying individual acoustic elements like syllables or sound pulses. To be useful, annotations need to be accurate, robust to noise, and fast.

We here introduce DeepAudioSegmenter (DAS), a method that annotates acoustic signals across species based on a deep-learning derived hierarchical presentation of sound. We demonstrate the accuracy, robustness, and speed of DAS using acoustic signals with diverse characteristics from insects, birds, and mammals. DAS comes with a graphical user interface for annotating song, training the network, and for generating and proofreading annotations. The method can be trained to annotate signals from new species with little manual annotation and can be combined with unsupervised methods to discover novel signal types. DAS annotates song with high throughput and low latency for experimental interventions in realtime. Overall, DAS is a universal, versatile, and accessible tool for annotating acoustic communication signals.

Auditory activity is diverse and widespread throughout the central brain of Drosophila

Sensory pathways are typically studied by starting at receptor neurons and following postsynaptic neurons into the brain. However, this leads to a bias in analyses of activity toward the earliest layers of processing. Here, we present new methods for volumetric neural imaging with precise across-brain registration to characterize auditory activity throughout the entire central brain of Drosophila and make comparisons across trials, individuals and sexes. We discover that auditory activity is present in most central brain regions and in neurons responsive to other modalities. Auditory responses are temporally diverse, but the majority of activity is tuned to courtship song features. Auditory responses are stereotyped across trials and animals in early mechanosensory regions, becoming more variable at higher layers of the putative pathway, and this variability is largely independent of ongoing movements. This study highlights the power of using an unbiased, brain-wide approach for mapping the functional organization of sensory activity.

To court or not to court - a multimodal sensory decision in Drosophila males

When Drosophila males encounter another fly, they have to make a rapid assessment to ensure the appropriate response: should they court, fight or pursue a different action entirely? Previous work has focused on the significance of sensory cues detected by the male during these encounters; however, recent evidence highlights the importance of the male's own internal state in shaping his responses. Additionally, once triggered, courtship is not a rigid sequence of motor actions, but rather a finely tuned behavioural display that must continually update in response to sensory feedback. Here, we review recent findings highlighting how sensory information and internal states are integrated ensuring appropriate action selection, and how they sustain and fine-tune motor output. We further discuss recent advances in our understanding of species differences in sensory processing that may contribute to reproductive isolation.

Anthropogenic noise and the bioacoustics of terrestrial invertebrates

Anthropogenic noise is an important issue of environmental concern owing to its wide-ranging effects on the physiology, behavior and ecology of animals. To date, research has focused on the impacts of far-field airborne noise (i.e. pressure waves) on vertebrates, with few exceptions. However, invertebrates and the other acoustic modalities they rely on, primarily near-field airborne and substrate-borne sound (i.e. particle motion and vibrations, respectively) have received little attention. Here, we review the literature on the impacts of different types of anthropogenic noise (airborne far-field, airborne near-field, substrate-borne) on terrestrial invertebrates. Using literature on invertebrate bioacoustics, we propose a framework for understanding the potential impact of anthropogenic noise on invertebrates and outline predictions of possible constraints and adaptations for invertebrates in responding to anthropogenic noise. We argue that understanding the impacts of anthropogenic noise requires us to consider multiple modalities of sound and to cultivate a broader understanding of invertebrate bioacoustics.

Sound localization behavior in Drosophila melanogaster depends on inter-antenna vibration amplitude comparisons

Drosophila melanogaster hear with their antennae: sound evokes vibration of the distal antennal segment, and this vibration is transduced by specialized mechanoreceptor cells. The left and right antennae vibrate preferentially in response to sounds arising from different azimuthal angles. Therefore, by comparing signals from the two antennae, it should be possible to obtain information about the azimuthal angle of a sound source. However, behavioral evidence of sound localization has not been reported in Drosophila Here, we show that walking D. melanogaster do indeed turn in response to lateralized sounds. We confirm that this behavior is evoked by vibrations of the distal antennal segment. The rule for turning is different for sounds arriving from different locations: flies turn toward sounds in their front hemifield, but they turn away from sounds in their rear hemifield, and they do not turn at all in response to sounds from 90 or -90 deg. All of these findings can be explained by a simple rule: the fly steers away from the antenna with the larger vibration amplitude. Finally, we show that these behaviors generalize to sound stimuli with diverse spectro-temporal features, and that these behaviors are found in both sexes. Our findings demonstrate the behavioral relevance of the antenna's directional tuning properties. They also pave the way for investigating the neural implementation of sound localization, as well as the potential roles of sound-guided steering in courtship and exploration.