Acoustic signalling for mate attraction in crickets: Abdominal ganglia control the timing of the calling song pattern

Quantitative trait loci underlying a speciation phenotype

Sexual signalling traits and their associated genetic components play a crucial role in the speciation process, as divergence in these traits can contribute to sexual isolation. Despite their importance, our understanding of the genetic basis of variable sexual signalling traits linked to speciation remains limited. In this study, we present new genetic evidence of Quantitative Trait Loci (QTL) underlying divergent sexual signalling behaviour, specifically pulse rate, in the Hawaiian cricket Laupala. By performing RNA sequencing on the brain and central nervous system of the parental species, we annotate these QTL regions and identify candidate genes associated with pulse rate. Our findings provide insights into the genetic processes driving reproductive isolation during speciation, with implications for understanding the mechanisms underlying species diversity.

The central organisation of courtship and rivalry behaviour in Gryllus bimaculatus (deGeer) as revealed by lesions of abdominal connectives

Behaviour is rooted in the organization and activity of an animal's nervous system. As male crickets use their front wings for sound production, the neural circuits underlying singing had been suggested to be housed in the thoracic ganglia. However, systematic lesion experiments of the CNS demonstrated that the abdominal nervous system is essential for their calling song behaviour. As male crickets also generate a courtship and rivalry song, we explored which parts of the abdominal central nervous system control the underlying motor patterns. A combination of systematic lesions to the abdominal nerve cord and video recording of courtship and rivalry behaviour revealed that most components of male courtship and rivalry behaviour were not affected by the lesions, except for the generation of courtship song, rivalry song, and the male's ability to copulate with the female. Any lesion to the abdominal nerve cord abolished copulations. Generation of courtship song initially failed when the connection to abdominal ganglion A6 was severed but in few males recovered after a week. For rivalry song production a central nerve cord extending to abdominal ganglion A4 was sufficient. These findings indicate that in the bispotted cricket the neural organization of courtship song is different from calling and rivalry song, while calling song and rivalry song might share a common network for generating the song patterns.

Wing movements underlying sound production in calling, rivalry, and courtship songs of the cricket Gryllus bimaculatus (DeGeer)

We recorded the wing movements and sound signals during the production of calling, rivalry, and courtship song in the bispotted field cricket Gryllus bimaculatus. Recordings confirm that salient sound pulses during calling and rivalry song are generated during the closing movements of the wings. Wing movements for calling and rivalry song start from an elevated wing position and are performed with a very similar opening-closing movement, indicating that both types of songs may be generated by the same neuronal network. Wing movements for courtship song start from a low wing position; rapid closing movements generate high-frequency ticks and low-amplitude wing oscillations lead to low-amplitude pulses, generated during the opening and closing movements with a carrier frequency corresponding to the calling song. The two types of wing movements underlying courtship song indicate a different motor control as compared to calling song and may represent an early evolutionary phenotype.

Evolutionary dynamics of sex-biased genes expressed in cricket brains and gonads

Sex-biased gene expression, particularly sex-biased expression in the gonad, has been linked to rates of protein sequence evolution (nonsynonymous to synonymous substitutions, dN/dS) in animals. However, in insects, sex-biased expression studies remain centred on a few holometabolous species. Moreover, other major tissue types such as the brain remain underexplored. Here, we studied sex-biased gene expression and protein evolution in a hemimetabolous insect, the cricket Gryllus bimaculatus. We generated novel male and female RNA-seq data for two sexual tissue types, the gonad and somatic reproductive system, and for two core components of the nervous system, the brain and ventral nerve cord. From a genome-wide analysis, we report several core findings. Firstly, testis-biased genes had accelerated evolution, as compared to ovary-biased and unbiased genes, which was associated with positive selection events. Secondly, although sex-biased brain genes were much less common than for the gonad, they exhibited a striking tendency for rapid protein sequence evolution, an effect that was stronger for the female than male brain. Further, some sex-biased brain genes were linked to sexual functions and mating behaviours, which we suggest may have accelerated their evolution via sexual selection. Thirdly, a tendency for narrow cross-tissue expression breadth, suggesting low pleiotropy, was observed for sex-biased brain genes, suggesting relaxed purifying selection, which we speculate may allow enhanced freedom to evolve adaptive protein functional changes. The findings of rapid evolution of testis-biased genes and male and female-biased brain genes are discussed with respect to pleiotropy, positive selection and the mating biology of this cricket.

Insights into the genomic evolution of insects from cricket genomes

Most of our knowledge of insect genomes comes from Holometabolous species, which undergo complete metamorphosis and have genomes typically under 2 Gb with little signs of DNA methylation. In contrast, Hemimetabolous insects undergo the presumed ancestral process of incomplete metamorphosis, and have larger genomes with high levels of DNA methylation. Hemimetabolous species from the Orthopteran order (grasshoppers and crickets) have some of the largest known insect genomes. What drives the evolution of these unusual insect genome sizes, remains unknown. Here we report the sequencing, assembly and annotation of the 1.66-Gb genome of the Mediterranean field cricket Gryllus bimaculatus, and the annotation of the 1.60-Gb genome of the Hawaiian cricket Laupala kohalensis. We compare these two cricket genomes with those of 14 additional insects and find evidence that hemimetabolous genomes expanded due to transposable element activity. Based on the ratio of observed to expected CpG sites, we find higher conservation and stronger purifying selection of methylated genes than non-methylated genes. Finally, our analysis suggests an expansion of the pickpocket class V gene family in crickets, which we speculate might play a role in the evolution of cricket courtship, including their characteristic chirping.

Ylla, Extavour et al. use genomic data from crickets to investigate the evolution of large genome sizes and DNA methylation events in insects. Their findings indicate that transposable element activity drove genome expansion in hemimetabolous insects, such as crickets and grasshoppers, and that DNA methylation is predominant in conserved genes.

Lesions of abdominal connectives reveal a conserved organization of the calling song central pattern generator (CPG) network in different cricket species

Although crickets move their front wings for sound production, the abdominal ganglia house the network of the singing central pattern generator. We compared the effects of specific lesions to the connectives of the abdominal ganglion chain on calling song activity in four different species of crickets, generating very different pulse patterns in their calling songs. In all species, singing activity was abolished after the connectives between the metathoracic ganglion complex and the first abdominal ganglion A3 were severed. The song structure was lost and males generated only single sound pulses when connectives between A3 and A4 were cut. Severing connectives between A4 and A5 had no effect in the trilling species, it led to an extension of chirps in a chirping species and to a loss of the phrase structure in two Teleogryllus species. Cutting the connectives between A5 and A6 caused no or minor changes in singing activity. In spite of the species-specific pulse patterns of calling songs, our data indicate a conserved organisation of the calling song motor pattern generating network. The generation of pulses is controlled by ganglia A3 and A4 while A4 and A5 provide the timing information for the chirp and/or phrase structure of the song.

Descending and Ascending Signals That Maintain Rhythmic Walking Pattern in Crickets

The cricket is one of the model animals used to investigate the neuronal mechanisms underlying adaptive locomotion. An intact cricket walks mostly with a tripod gait, similar to other insects. The motor control center of the leg movements is located in the thoracic ganglia. In this study, we investigated the walking gait patterns of the crickets whose ventral nerve cords were surgically cut to gain an understanding of how the descending signals from the head ganglia and ascending signals from the abdominal nervous system into the thoracic ganglia mediate the initiation and coordination of the walking gait pattern. Crickets whose paired connectives between the brain and subesophageal ganglion (SEG) (circumesophageal connectives) were cut exhibited a tripod gait pattern. However, when one side of the circumesophageal connectives was cut, the crickets continued to turn in the opposite direction to the connective cut. Crickets whose paired connectives between the SEG and prothoracic ganglion were cut did not walk, whereas the crickets exhibited an ordinal tripod gait pattern when one side of the connectives was intact. Crickets whose paired connectives between the metathoracic ganglion and abdominal ganglia were cut initiated walking, although the gait was not a coordinated tripod pattern, whereas the crickets exhibited a tripod gait when one side of the connectives was intact. These results suggest that the brain plays an inhibitory role in initiating leg movements and that both the descending signals from the head ganglia and the ascending signals from the abdominal nervous system are important in initiating and coordinating insect walking gait patterns.

Modular timer networks: abdominal interneurons controlling the chirp and pulse pattern in a cricket calling song

Chirping male crickets combine a 30 Hz pulse pattern with a 3 Hz chirp pattern to drive the rhythmic opening-closing movements of the front wings for sound production. Lesion experiments suggest two coupled modular timer-networks located along the chain of abdominal ganglia, a network in A3 and A4 generating the pulse pattern, and a network organized along with ganglia A4–A6 controlling the generation of the chirp rhythm. We analyzed neurons of the timer-networks and their synaptic connections by intracellular recordings and staining. We identified neurons spiking in phase with the chirps and pulses, or that are inhibited during the chirps. Neurons share a similar “gestalt”, regarding the position of the cell body, the dendritic arborizations and the contralateral ascending axon. Activating neurons of the pulse-timer network elicits ongoing motor activity driving the generation of pulses; this activity is not structured in the chirp pattern. Activating neurons of the chirp-timer network excites pulse-timer neurons; it drives the generation of chirps and during the chirps the pulse pattern is produced. Our results support the hypothesis that two modular networks along the abdominal ganglion chain control the cricket calling song, a pattern generating network in the mesothoracic ganglion may not be required.

Neuroethology of acoustic communication in field crickets - from signal generation to song recognition in an insect brain

Field crickets are best known for the loud calling songs produced by males to attract conspecific females. This review aims to summarize the current knowledge of the neurobiological basis underlying the acoustic communication for mate finding in field crickets with emphasis on the recent research progress to understand the neuronal networks for motor pattern generation and auditory pattern recognition of the calling song in Gryllus bimaculatus. Strong scientific interest into the neural mechanisms underlying intraspecific communication has driven persistently advancing research efforts to study the male singing behaviour and female phonotaxis for mate finding in these insects. The growing neurobiological understanding also inspired many studies testing verifiable hypotheses in sensory ecology, bioacoustics and on the genetics and evolution of behaviour. Over last decades, acoustic communication in field crickets served as a very successful neuroethological model system. It has contributed significantly to the scientific process of establishing, reconsidering and refining fundamental concepts in behavioural neurosciences such as command neurons, central motor pattern generation, corollary discharge processing and pattern recognition by sensory feature detection, which are basic building blocks of our modern understanding on how nervous systems control and generate behaviour in all animals.

Feedforward discharges couple the singing central pattern generator and ventilation central pattern generator in the cricket abdominal central nervous system

We investigated the central nervous coordination between singing motor activity and abdominal ventilatory pumping in crickets. Fictive singing, with sensory feedback removed, was elicited by eserine-microinjection into the brain, and the motor activity underlying singing and abdominal ventilation was recorded with extracellular electrodes. During singing, expiratory abdominal muscle activity is tightly phase coupled to the chirping pattern. Occasional temporary desynchronization of the two motor patterns indicate discrete central pattern generator (CPG) networks that can operate independently. Intracellular recordings revealed a sub-threshold depolarization in phase with the ventilatory cycle in a singing-CPG interneuron, and in a ventilation-CPG interneuron an excitatory input in phase with each syllable of the chirps. Inhibitory synaptic inputs coupled to the syllables of the singing motor pattern were present in another ventilatory interneuron, which is not part of the ventilation-CPG. Our recordings suggest that the two centrally generated motor patterns are coordinated by reciprocal feedforward discharges from the singing-CPG to the ventilation-CPG and vice versa. Consequently, expiratory contraction of the abdomen usually occurs in phase with the chirps and ventilation accelerates during singing due to entrainment by the faster chirp cycle.

Acoustic Classification of Singing Insects Based on MFCC/LFCC Fusion

This work introduces a new approach for automatic identification of crickets, katydids and cicadas analyzing their acoustic signals. We propose the building of a tool to identify this biodiversity. The study proposes a sound parameterization technique designed specifically for identification and classification of acoustic signals of insects using Mel Frequency Cepstral Coefficients (MFCC) and Linear Frequency Cepstral Coefficients (LFCC). These two sets of coefficients are evaluated individually as has been done in previous studies and have been compared with the fusion proposed in this work, showing an outstanding increase in identification and classification at species level reaching a success rate of 98.07% on 343 insect species.

Innate releasing mechanisms and fixed action patterns: basic ethological concepts as drivers for neuroethological studies on acoustic communication in Orthoptera

This review addresses the history of neuroethological studies on acoustic communication in insects. One objective is to reveal how basic ethological concepts developed in the 1930s, such as innate releasing mechanisms and fixed action patterns, have influenced the experimental and theoretical approaches to studying acoustic communication systems in Orthopteran insects. The idea of innateness of behaviors has directly fostered the search for central pattern generators that govern the stridulation patterns of crickets, katydids or grasshoppers. A central question pervading 50 years of research is how the essential match between signal features and receiver characteristics has evolved and is maintained during evolution. As in other disciplines, the tight interplay between technological developments and experimental and theoretical advances becomes evident throughout this review. While early neuroethological studies focused primarily on proximate questions such as the implementation of feature detectors or central pattern generators, later the interest shifted more towards ultimate questions. Orthoptera offer the advantage that both proximate and ultimate questions can be tackled in the same system. An important advance was the transition from laboratory studies under well-defined acoustic conditions to field studies that allowed to measure costs and benefits of acoustic signaling as well as constraints on song evolution.

Structure, Activity and Function of a Singing CPG Interneuron Controlling Cricket Species-Specific Acoustic Signaling

The evolution of species-specific song patterns is a driving force in the speciation of acoustic communicating insects. It must be closely linked to adaptations of the neuronal network controlling the underlying singing motor activity. What are the cellular and network properties that allow generating different songs? In five cricket species, we analyzed the structure and activity of the identified abdominal ascending opener interneuron, a homologous key component of the singing central pattern generator. The structure of the interneuron, based on the position of the cell body, ascending axon, dendritic arborization pattern, and dye coupling, is highly similar across species. The neuron's spike activity shows a tight coupling to the singing motor activity. In all species, current injection into the interneuron drives artificial song patterns, highlighting the key functional role of this neuron. However, the pattern of the membrane depolarization during singing, the fine dendritic and axonal ramifications, and the number of dye-coupled neurons indicate species-specific adaptations of the neuronal network that might be closely linked to the evolution of species-specific singing.SIGNIFICANCE STATEMENT A fundamental question in evolutionary neuroscience is how species-specific behaviors arise in closely related species. We demonstrate behavioral, neurophysiological, and morphological evidence for homology of one key identified interneuron of the singing central pattern generator in five cricket species. Across-species differences of this interneuron are also observed, which might be important to the generation of the species-specific song patterns. This work offers a comprehensive and detailed comparative analysis addressing the neuronal basis of species-specific behavior.

The Genetics of a Behavioral Speciation Phenotype in an Island System

Mating behavior divergence can make significant contributions to reproductive isolation and speciation in various biogeographic contexts. However, whether the genetic architecture underlying mating behavior divergence is related to the biogeographic history and the tempo and mode of speciation remains poorly understood. Here, we use quantitative trait locus (QTL) mapping to infer the number, distribution, and effect size of mating song rhythm variations in the crickets Laupala eukolea and Laupala cerasina, which occur on different islands (Maui and Hawaii). We then compare these results with a similar study of an independently evolving species pair that diverged within the same island. Finally, we annotate the L. cerasina transcriptome and test whether the QTL fall in functionally enriched genomic regions. We document a polygenic architecture behind the song rhythm divergence in the inter-island species pair that is remarkably similar to that previously found for an intra-island species pair in the same genus. Importantly, the QTL regions were significantly enriched for potential homologs of the genes involved in pathways that may be modulating the cricket song rhythm. These clusters of loci could constrain the spatial genomic distribution of the genetic variation underlying the cricket song variation and harbor several candidate genes that merit further study.