Acoustic preference functions and song variability in the Hawaiian cricket Laupala cerasina

Within-population variation in preference functions reveals substantial among-female disagreement in mate assessment

The mate choice behaviours of females can greatly affect patterns of reproductive success in males and influence the evolution of sexually selected male traits. Population-level estimates of display preferences may provide an accurate estimate of the strength and direction of selection by female choice if all females in the population show homogeneous preferences. However, population-level estimates may yield misleading estimates if there is within-population variation in mate preferences. While it is increasingly clear that the latter situation is common in nature, empirical data on the magnitude of variation in female preferences are required to improve our current understanding of its potential evolutionary consequences. We explored variations in female preference functions for 3 male call properties in a treefrog. We document substantial within-population variation not only in peak preferences but also in preference function shape (open, closed, flat), with at best 62% of females sharing a preference function shape with the respective population curve. Our findings suggest that population curves may accurately capture the direction of sexual selection, but depending on the properties of the constituting individual functions they may over- or underestimate the strength of selection. Particularly population estimates suggesting weak selection may in fact hide the presence of individual females with strong but opposing preferences. Moreover, due to the high within-population variation in both peak preferences and preference function shapes, the population functions drastically underestimate the predicted variation in male mating success in the population.

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.

A limit to sustained performance constrains trill length in birdsong

In birds, song performance determines the outcome of contests over crucial resources. We hypothesized that 1) sustained performance is limited within song, resulting in a performance decline towards the end and 2) the impact of song length is compromised if performance declines. To test these hypotheses, we analyzed the songs of 597 bird species (26 families) and conducted a playback experiment on blue tits (Cyanistes caeruleus). Our multi-species analysis showed that song performance declines after sustained singing, supporting our hypothesis. If the performance decline is determined by individual attributes (i.e., physical condition), our results explain how trill length can honestly signal quality. Our experiment showed that longer trills of high performance elicited a stronger response during territorial interactions. However, long trills that declined in performance elicited a weaker response than short, high-performance trills. A trade-off between the duration and performance quality of a motor display can be an important aspect in communication across taxa.

Local prothoracic auditory neurons in Ensifera

A new method for individually staining insect neurons with metal ions was described in the late 60s, closely followed by the introduction of the first bright fluorescent dye, Lucifer Yellow, for the same purpose. These milestones enabled an unprecedented level of detail regarding the neuronal basis of sensory processes such as hearing. Due to their conspicuous auditory behavior, orthopterans rapidly established themselves as a popular model for studies on hearing (first identified auditory neuron: 1974; first local auditory interneuron: 1977). Although crickets (Ensifera, Gryllidae) surpassed grasshoppers (Caelifera) as the main model taxon, surprisingly few neuronal elements have been described in crickets. More auditory neurons are described for bush crickets (Ensifera, Tettigoniidae), but due to their great biodiversity, the described auditory neurons in bush crickets are scattered over distantly related groups, hence being confounded by potential differences in the neuronal pathways themselves. Our review will outline all local auditory elements described in ensiferans so far. We will focus on one bush cricket species, Ancistrura nigrovittata (Phaneropterinae), which has the so-far highest diversity of identified auditory interneurons within Ensifera. We will present one novel and three previously described local prothoracic auditory neuron classes, comparing their morphology and aspects of sensory processing. Finally, we will hypothesize about their functions and evolutionary connections between ensiferan insects.

Population genomics and sexual signals support reproductive character displacement in Uperoleia (Anura: Myobatrachidae) in a contact zone

When closely related species come into contact via range expansion, both may experience reduced fitness as a result of the interaction. Selection is expected to favour traits that minimize costly interspecies reproductive interactions (such as mismating) via a phenomenon called reproductive character displacement (RCD). Research on RCD frequently assumes secondary contact between species, but the geographical history of species interactions is often unknown. Population genomic data permit tests of geographical hypotheses about species origins and secondary contact through range expansion. We used population genomic data from single nucleotide polymorphisms (SNPs), mitochondrial sequence data, advertisement call data and morphological data to investigate a species complex of toadlets (Uperoleia borealis, U. crassa, U. inundata) from northern Australia. Although the three species of frogs were morphologically indistinguishable in our analysis, we determined that U. crassa and U. inundata form a single species (synonymized here) based on an absence of genomic divergence. SNP data identified the phylogeographical origin of U. crassa as the Top End, with subsequent westward invasion into the range of U. borealis in the Kimberley. We identified six F1 hybrids, all of which had the U. borealis mitochondrial haplotype, suggesting unidirectional hybridization. Consistent with the RCD hypothesis, U. borealis and U. crassa sexual signals differ more in sympatry than in allopatry. Hybrid males have intermediate calls, which probably reduces attractiveness to females. Integrating population genomic data, mitochondrial sequencing, morphology and behavioural approaches provides an unusually detailed collection of evidence for reproductive character displacement following range expansion and secondary contact.

Sexual selection and 'species recognition' revisited: serial processing and order-of-operations in mate choice

Following the modern synthesis, mating signals were thought of principally as species recognition traits, a view later challenged by a burgeoning interest in sexual selection-specifically mate choice. In the 1990s, these different signal functions were proposed to represent a single process driven by the shape of female preference functions across both intra- and interspecific signal space. However, the properties of reliable 'recognition' signals (stereotyped; low intraspecific variation) and informative 'quality' signals (condition dependent; high intraspecific variation) seem at odds, perhaps favouring different signal components for different functions. Surprisingly, the idea that different components of mating signals are evaluated in series, first to recognize generally compatible mates and then to select for quality, has never been explicitly tested. Here I evaluate patterns of (i) intraspecific signal variation, (ii) female preference function shape and (iii) phylogenetic signal for male cricket call components known to be processed in series. The results show that signal components processed first tend to have low variation, closed preference functions and low phylogenetic signal, whereas signal components processed later show the opposite, suggesting that mating signal evaluation follows an 'order-of-operations'. Applicability of this finding to diverse groups of organisms and sensory modalities is discussed.

Inter-individual differences of calling and exploratory behaviour in a lebinthine cricket species hint at different mate-finding strategies

Individual fitness can be boosted by behavioural strategies that maximise mate-finding probability while minimising predation risk. Animals that use acoustics to find mates may benefit from using both stationary calling and active exploration, but these also expose them to different types of predators. Studying calling and searching behaviours concurrently allows us to understand their evolutionary trade-offs between survival and reproduction. Unlike most other crickets, lebinthine males alternate between singing and exploration to find females, which offer a unique and excellent opportunity to test for inter-individual differences and behavioural syndrome between call properties and exploratory behaviours. Our data demonstrate that call properties and exploratory behaviour were repeatable. We did not, however, find that call properties correlate with exploration as some consistently exploratory individuals produce longer calls while others produce shorter calls. Our study suggests that lebinthine males use different combinations of calling and exploratory behaviours to cope with unpredictable risk–benefit scenarios.

Axes of multivariate sexual signal divergence among incipient species: Concordance with selection, genetic variation and phenotypic plasticity

Sexual signalling traits are often observed to diverge rapidly among populations, thereby playing a potentially key early role in the evolution of reproductive isolation. While often assumed to reflect divergent sexual selection among populations, patterns of sexual trait diversification might sometimes be biased along axes of standing additive genetic variation and covariation among trait components. Additionally, theory predicts that environmentally induced phenotypic variation might facilitate rapid trait evolution, suggesting that patterns of divergence between populations should mirror phenotypic plasticity within populations. Here, we evaluate the concordance between observed axes of multivariate sexual trait divergence and predicted divergence based on (1) interpopulation variation in sexual selection, (2) additive genetic variances and (3) temperature-related phenotypic plasticity in male courtship song among geographically isolated populations of the Hawaiian swordtail cricket, Laupala cerasina, which exhibit sexual isolation due acoustic signalling traits. The major axis of multivariate divergence, d_max , accounted for 76% of variation among population male song trait means and was moderately correlated with interpopulation differences in directional sexual selection based on female preferences. However, the majority of additive genetic variance was largely oriented away from the direction of divergence, suggesting that standing genetic variation may not play a dominant role in the patterning of signal divergence. In contrast, the axis of phenotypic plasticity strongly mirrored patterns of interpopulation phenotypic divergence, which is consistent with a role for temperature-related plasticity in facilitating instead of inhibiting male song evolution and sexual isolation in these incipient species. We propose potential mechanisms by which sexual selection might interact with phenotypic plasticity to facilitate the rapid acoustic diversification observed in this species and clade.

Drosophila females receive male substrate-borne signals through specific leg neurons during courtship

Substrate-borne vibratory signals are thought to be one of the most ancient and taxonomically widespread communication signals among animal species, including Drosophila flies.1-9 During courtship, the male Drosophila abdomen tremulates (as defined in Busnel et al.10) to generate vibrations in the courting substrate.8,9 These vibrations coincide with nearby females becoming immobile, a behavior that facilitates mounting and copulation.8,11-13 It was unknown how the Drosophila female detects these substrate-borne vibratory signals. Here, we confirm that the immobility response of the female to the tremulations is not dependent on any air-borne cue. We show that substrate-borne communication is used by wild Drosophila and that the vibrations propagate through those natural substrates (e.g., fruits) where flies feed and court. We examine transmission of the signals through a variety of substrates and describe how each of these substrates modifies the vibratory signal during propagation and affects the female response. Moreover, we identify the main sensory structures and neurons that receive the vibrations in the female legs, as well as the mechanically gated ion channels Nanchung and Piezo (but not Trpγ) that mediate sensitivity to the vibrations. Together, our results show that Drosophila flies, like many other arthropods, use substrate-borne communication as a natural means of communication, strengthening the idea that this mode of signal transfer is heavily used and reliable in the wild.3,4,7 Our findings also reveal the cellular and molecular mechanisms underlying the vibration-sensing modality necessary for this communication.

Spatial Mixing between Calling Males of Two Closely Related, Sympatric Crickets Suggests Beneficial Heterospecific Interactions in a NonAdaptive Radiation

Sympatry among closely related species occurs in both adaptive and nonadaptive radiations. Among closely related, sympatric species of a nonadaptive radiation, the lack of ecological differentiation brings species into continual contact where individuals are exposed to the risk of reproductive interference. Selection thus should cause divergence in multiple components mediating the reproductive boundary. Besides differentiation of reproductive signals per se, spatial segregation is a commonly proposed mechanism that can mitigate reproductive interference. Studying a pair of broadly sympatric, closely related cricket species from a nonadaptive radiation in Hawaii, we 1) quantified acoustic divergence of male songs and 2) tested alternative hypotheses of spatial distribution of calling males of the 2 species. Acoustic analyses of the recorded songs showed that, while the 2 species differed substantially in pulse rate, no spectral or fine temporal segregation of the pulse structure was evident, indicating the potential for acoustic masking. Moreover, we found that calling males of the 2 species are highly mixed both vertically and horizontally and showed the same preference for calling sites. More surprisingly, calling males were found to form mixed-species calling clusters where heterospecific males are closer to each other than conspecific males. Such an individual spacing pattern suggests low heterospecific aggression and/or high conspecific competition. Because females prefer higher sound intensity, heterospecific males may benefit, rather than interfere, with each other in attracting females. These findings offer a potential mechanism enabling species coexistence in sympatry.

Extensive Linkage and Genetic Coupling of Song and Preference Loci Underlying Rapid Speciation in Laupala Crickets

In nature, closely related species commonly display divergent mating behaviors, suggesting a central role for such traits in the origin of species. Elucidating the genetic basis of divergence in these traits is necessary to understand the evolutionary process leading to reproductive barriers and speciation. The rapidly speciating Hawaiian crickets of the genus Laupala provides an ideal system for dissecting the genetic basis of mating behavior divergence. In Laupala, closely related species differ markedly in male song pulse rate and female preference for pulse rate. These behaviors play an important role in determining mating patterns. Previous studies identified a genetic architecture consisting of numerous small to moderate effect loci causing interspecific differences in pulse rate and preference, including colocalizing pulse rate and preference QTL on linkage group one (LG1). To further interrogate these QTL, we conduct a fine mapping study using high-density SNP linkage maps. With improved statistical power and map resolution, we provide robust evidence for genetic coupling between song and preference, along with two additional pulse rate QTL on LG1, revealing a more resolved picture of the genetic architecture underlying mating behavior divergence. Our sequence-based genetic map, along with dramatically narrowed QTL confidence intervals, allowed us to annotate genes within the QTL regions and identify several exciting candidate genes underlying variation in pulse rate and preference divergence. Such knowledge suggests potential molecular mechanisms underlying the evolution of behavioral barriers.

Speciation across the Tree of Life

Much of what we know about speciation comes from detailed studies of well-known model systems. Although there have been several important syntheses on speciation, few (if any) have explicitly compared speciation among major groups across the Tree of Life. Here, we synthesize and compare what is known about key aspects of speciation across taxa, including bacteria, protists, fungi, plants, and major animal groups. We focus on three main questions. Is allopatric speciation predominant across groups? How common is ecological divergence of sister species (a requirement for ecological speciation), and on what niche axes do species diverge in each group? What are the reproductive isolating barriers in each group? Our review suggests the following patterns. (i) Based on our survey and projected species numbers, the most frequent speciation process across the Tree of Life may be co-speciation between endosymbiotic bacteria and their insect hosts. (ii) Allopatric speciation appears to be present in all major groups, and may be the most common mode in both animals and plants, based on non-overlapping ranges of sister species. (iii) Full sympatry of sister species is also widespread, and may be more common in fungi than allopatry. (iv) Full sympatry of sister species is more common in some marine animals than in terrestrial and freshwater ones. (v) Ecological divergence of sister species is widespread in all groups, including ~70% of surveyed species pairs of plants and insects. (vi) Major axes of ecological divergence involve species interactions (e.g. host-switching) and habitat divergence. (vii) Prezygotic isolation appears to be generally more widespread and important than postzygotic isolation. (viii) Rates of diversification (and presumably speciation) are strikingly different across groups, with the fastest rates in plants, and successively slower rates in animals, fungi, and protists, with the slowest rates in prokaryotes. Overall, our study represents an initial step towards understanding general patterns in speciation across all organisms.

Inconsistent sexual signaling degrades optimal mating decisions in animals

Like political stump speeches and product advertisements, animal signals are highly repetitive and function to persuade receivers to adopt behaviors benefiting the signaler. And like potential constituents and consumers, receivers assess signals to inform their behavioral decisions. However, inconsistency in sexual signals is widespread and potentially injects uncertainty into mating decisions. Here, we show that females fail to make optimal mating decisions based on assessments of signal quality due to inconsistency in signal production. Natural levels of inconsistency markedly reduced female preference expression for a nonarbitrary signal of male quality. Inconsistency reshaped preferences even more profoundly than the better-known impediment of ambient noise. To our knowledge, this is the first demonstration of how inconsistent messaging degrades optimal decision-making in animals, with implications for understanding signal evolution.

Genetic coupling of signal and preference facilitates sexual isolation during rapid speciation

The divergence of sexual signals is ultimately a coevolutionary process: while signals and preferences diverge between lineages, they must remain coordinated within lineages for matings to occur. Divergence in sexual signals makes a major contribution to evolving species barriers. Therefore, the genetic architecture underlying signal-preference coevolution is essential to understanding speciation but remains largely unknown. In Laupala crickets where male song pulse rate and female pulse rate preferences have coevolved repeatedly and rapidly, we tested two contrasting hypotheses for the genetic architecture underlying signal-preference coevolution: linkage disequilibrium between unlinked loci and genetic coupling (linkage disequilibrium resulting from pleiotropy of a shared locus or tight physical linkage). Through selective introgression and quantitative trait locus (QTL) fine mapping, we estimated the location of QTL underlying interspecific variation in both female preference and male pulse rate from the same mapping populations. Remarkably, map estimates of the pulse rate and preference loci are as close as 0.06 cM apart, the strongest evidence to date for genetic coupling between signal and preference loci. As the second pair of colocalizing signal and preference loci in the Laupala genome, our finding supports an intriguing pattern, pointing to a major role for genetic coupling in the quantitative evolution of a reproductive barrier and rapid speciation in Laupala. Owing to its effect on suppressing recombination, a coupled, quantitative genetic architecture offers a powerful and parsimonious genetic mechanism for signal-preference coevolution and the establishment of positive genetic covariance on which the Fisherian runaway process of sexual selection relies.

On the architecture of mate choice decisions: preference functions and choosiness are distinct traits

Mate choice is an important cause of sexual selection; it can drive the evolution of extravagant ornaments and displays, and promote speciation through the reproductive isolation generated by rapid divergence of sexual traits. Understanding mate choice requires knowledge of the traits involved in generating mate-choice decisions, and how those traits may interact with each other. It has been proposed that mate-choice decisions are the outcome of two components that vary independently: the preference function (the ranking of the attractiveness of prospective mates) and choosiness (the effort invested in mate assessment). Here we test this hypothesis by examining individual variation in female preference functions and choosiness in green treefrogs ( Hyla cinerea). We show that measures describing preference functions and choosiness are not correlated. We also show that both components are influenced differently by variation in female body size, and that preference function shape (closed and preferring intermediate values or open-ended and preferring extremes) has a strong influence on this relationship: function traits are positively correlated with body size only for individuals with closed functions, while choosiness is positively correlated with body size for individuals with open functions, but negatively for those with closed functions.

The Genetics of Mating Song Evolution Underlying Rapid Speciation: Linking Quantitative Variation to Candidate Genes for Behavioral Isolation

Differences in mating behaviors evolve early during speciation, eventually contributing to reproductive barriers between species. Knowledge of the genetic and genomic basis of these behaviors is therefore integral to a causal understanding of speciation. Acoustic behaviors are often part of the mating ritual in animal species. The temporal rhythms of mating songs are notably species-specific in many vertebrates and arthropods and often underlie assortative mating. Despite discoveries of mutations that disrupt the temporal rhythm of these songs, we know surprisingly little about genes affecting naturally occurring variation in the temporal pattern of singing behavior. In the rapidly speciating Hawaiian cricket genus Laupala, the striking species variation in song rhythms constitutes a behavioral barrier to reproduction between species. Here, we mapped the largest-effect locus underlying interspecific variation in song rhythm between two Laupala species to a narrow genomic region, wherein we find no known candidate genes affecting song temporal rhythm in Drosophila Whole-genome sequencing, gene prediction, and functional annotation of this region reveal an exciting and promising candidate gene, the putative cyclic nucleotide-gated ion channel-like gene, for natural variation in mating behavior. Identification and molecular characterization of the candidate gene reveals a nonsynonymous mutation in a conserved binding domain, suggesting that ion channels are important targets of selection on rhythmic signaling during establishment of behavioral isolation and rapid speciation.

The impact of age and egg-laying cycle on female grasshoppers' preference functions for acoustic signals

Female responsiveness and the shape of preference functions for male signal traits are important determinants for male mating success. We observed the responsiveness and the selectivity of virgin grasshopper females (Chorthippus biguttulus L.) for different features of males' acoustic signals throughout their life span to detect possible influences of age on the females' preference functions. In particular, we explored the hypothesis that the females may become less selective with increasing age and, therefore, would start to accept songs that are normally rejected. Such an age effect could relieve the selection pressure on male signal traits. In the majority of animals tested, the general responsiveness decreased with age although a few individuals exhibited an opposite trend. Contrary to the above expectation, there was no indication of a loss of selectivity in older females or an increased acceptance of normally unattractive song models. The timing within the oviposition cycle had a strong effect on responsiveness: near oviposition the general responsiveness increased and with it also the half width of the preference functions. However, highly unattractive song models remained unattractive also near oviposition.

Evolutionarily conserved coding properties favour the neuronal representation of heterospecific signals of a sympatric katydid species

To function as a mechanism in premating isolation, the divergent and species-specific calling songs of acoustic insects must be reliably processed by the afferent auditory pathway of receivers. Here, we analysed the responses of interneurons in a katydid species that uses long-lasting acoustic trills and compared these with previously reported data for homologous interneurons of a sympatric species that uses short chirps as acoustic signals. Some interneurons of the trilling species respond exclusively to the heterospecific chirp due to selective, low-frequency tuning and "novelty detection". These properties have been considered as evolutionary adaptations in the sensory system of the chirper, which allow it to detect signals effectively during the simultaneous calling of the sympatric sibling species. We propose that these two mechanisms, shared by the interneurons of both species, did not evolve in the chirper to guarantee its ability to detect the chirp under masking conditions. Instead we suggest that chirpers evolved an additional, 2-kHz component in their song and exploited pre-existing neuronal properties for detecting their song under masking noise. The failure of some interneurons to respond to the conspecific song in trillers does not prevent intraspecific communication, as other interneurons respond to the trill.

A statistical approach to understanding reproductive isolation in two sympatric species of tree crickets

In acoustically communicating animals, reproductive isolation between sympatric species is usually maintained through species-specific calls. This requires that the receiver be tuned to the conspecific signal. Mapping the response space of the receiver onto the signal space of the conspecific investigates this tuning. A combinatorial approach to investigating the response space is more informative as the influence on the receiver of the interactions between the features is also elucidated. However, most studies have examined individual preference functions rather than the multivariate response space. We studied the maintenance of reproductive isolation between two sympatric tree cricket species (Oecanthus henryi and Oecanthus indicus) through the temporal features of the calls. Individual response functions were determined experimentally for O. henryi, the results from which were combined in a statistical framework to generate a multivariate quantitative receiver response space. The predicted response was higher for the signals of the conspecific than for signals of the sympatric heterospecific, indicating maintenance of reproductive isolation through songs. The model allows prediction of response to untested combinations of temporal features as well as delineation of the evolutionary constraints on the signal space. The model can also be used to predict the response of O. henryi to other heterospecific signals, making it a useful tool for the study of the evolution and maintenance of reproductive isolation via long-range acoustic signals.

Reinforcement and a cline in mating behaviour evolve in response to secondary contact and hybridization in shield-back katydids (Orthoptera: Tettigoniidae)

In a dispersal-limited species that has evolved reproductive character displacement at a contact zone, a cline in mating behaviour may result if gene flow diffuses alleles out of the contact zone into allopatric populations. Prior work has found such a clinal pattern in the shield-back katydid Aglaothorax morsei, in which the male calling songs in a sympatric population have a displaced, short interpulse interval that increases in length with increasing distance from the contact zone. In this study, molecular phylogenetic and female preference data show that (1) sympatric populations result from secondary contact, (2) hybridization in sympatry has resulted in unidirectional mitochondrial introgression and (3) female preferences are consistent with reproductive character displacement and could generate a cline in mating behaviour. These data together suggest a history of reinforcement, generally considered rare in acoustically communicating insects; thus, Aglaothorax represents an important example of a rarely documented evolutionary process.

Evolution of a Communication System by Sensory Exploitation of Startle Behavior

New communication signals can evolve by sensory exploitation if signaling taps into preexisting sensory biases in receivers [1, 2]. For mate attraction, signals are typically similar to attractive environmental cues like food [3-6], which amplifies their attractiveness to mates, as opposed to aversive stimuli like predator cues. Female field crickets approach the low-frequency calling song of males, whereas they avoid high-frequency sounds like predatory bat calls [7]. In one group of crickets (Eneopterinae: Lebinthini), however, males produce exceptionally high-frequency calling songs in the range of bat calls [8], a surprising signal in the context of mate attraction. We found that female lebinthines, instead of approaching singing males, produce vibrational responses after male calls, and males track the source of vibrations to find females. We also demonstrate that field cricket species closely related to the Lebinthini show an acoustic startle response to high-frequency sounds that generates substrate vibrations similar to those produced by female lebinthine crickets. Therefore, the startle response is the most likely evolutionary origin of the female lebinthine vibrational signal. In field crickets, the brain receives activity from two auditory interneurons; AN1 tuned to male calling song controls positive phonotaxis, and AN2 tuned to high-frequency bat calls triggers negative phonotaxis [9, 10]. In lebinthine crickets, however, we found that auditory ascending neurons are only tuned to high-frequency sounds, and their tuning matches the thresholds for female vibrational signals. Our results demonstrate how sensory exploitation of anti-predator behavior can evolve into a communication system that benefits both senders and receivers.

Do circadian genes and ambient temperature affect substrate-borne signalling during Drosophila courtship?

Courtship vibratory signals can be air-borne or substrate-borne. They convey distinct and species-specific information from one individual to its prospective partner. Here, we study the substrate-borne vibratory signals generated by the abdominal quivers of the Drosophila male during courtship; these vibrations travel through the ground towards courted females and coincide with female immobility. It is not known which physical parameters of the vibrations encode the information that is received by the females and induces them to pause. We examined the intervals between each vibratory pulse, a feature that was reported to carry information for animal communication. We were unable to find evidence of periodic variations in the lengths of these intervals, as has been reported for fly acoustical signals. Because it was suggested that the genes involved in the circadian clock may also regulate shorter rhythms, we search for effects of period on the interval lengths. Males that are mutant for the period gene produced vibrations with significantly altered interpulse intervals; also, treating wild type males with constant light results in similar alterations to the interpulse intervals. Our results suggest that both the clock and light/dark cycles have input into the interpulse intervals of these vibrations. We wondered if we could alter the interpulse intervals by other means, and found that ambient temperature also had a strong effect. However, behavioural analysis suggests that only extreme ambient temperatures can affect the strong correlation between female immobility and substrate-borne vibrations.

The locus of sexual selection: moving sexual selection studies into the post-genomics era

Sexual selection drives fundamental evolutionary processes such as trait elaboration and speciation. Despite this importance, there are surprisingly few examples of genes unequivocally responsible for variation in sexually selected phenotypes. This lack of information inhibits our ability to predict phenotypic change due to universal behaviours, such as fighting over mates and mate choice. Here, we discuss reasons for this apparent gap and provide recommendations for how it can be overcome by adopting contemporary genomic methods, exploiting underutilized taxa that may be ideal for detecting the effects of sexual selection and adopting appropriate experimental paradigms. Identifying genes that determine variation in sexually selected traits has the potential to improve theoretical models and reveal whether the genetic changes underlying phenotypic novelty utilize common or unique molecular mechanisms. Such a genomic approach to sexual selection will help answer questions in the evolution of sexually selected phenotypes that were first asked by Darwin and can furthermore serve as a model for the application of genomics in all areas of evolutionary biology.

Conservation of multivariate female preference functions and preference mechanisms in three species of trilling field crickets

Divergence in mate recognition systems among closely related species is an important contributor to assortative mating and reproductive isolation. Here, we examine divergence in male song traits and female preference functions in three cricket species with songs consisting of long trills. The shape of female preference functions appears to be mostly conserved across species and follows the predictions from a recent model for song recognition. Multivariate preference profiles, combining the pulse and trill parameters, demonstrate selectivity for conspecific pulse rates and high trill duty cycles. The rules for integration across pulse and trill timescales were identical for all three species. Generally, we find greater divergence in male song traits than in associated female preferences. For pulse rate, we find a strong match between divergent male traits and female peak preferences. Preference functions for trill parameters and carrier frequency are similar between species and show less congruence between signal and preference. Differences among traits in the degree of trait-preference (mis)match may reflect the strength of preferences and the potential for linkage disequilibrium, selective constraints and alternative selective pressures, but appear unrelated to selection for mate recognition per se.

Evolution of divergent female mating preference in response to experimental sexual selection

Sexual selection is predicted to drive the coevolution of mating signals and preferences (mating traits) within populations, and could play a role in speciation if sexual isolation arises due to mating trait divergence between populations. However, few studies have demonstrated that differences in mating traits between populations result from sexual selection alone. Experimental evolution is a promising approach to directly examine the action of sexual selection on mating trait divergence among populations. We manipulated the opportunity for sexual selection (low vs. high) in populations of Drosophila pseudoobscura. Previous studies on these experimental populations have shown that sexual selection manipulation resulted in the divergence between sexual selection treatments of several courtship song parameters, including interpulse interval (IPI) which markedly influences male mating success. Here, we measure female preference for IPI using a playback design to test for preference divergence between the sexual selection treatments after 130 generations of experimental sexual selection. The results suggest that female preference has coevolved with male signal, in opposite directions between the sexual selection treatments, providing direct evidence of the ability of sexual selection to drive the divergent coevolution of mating traits between populations. We discuss the implications in the context sexual selection and speciation.

Selective phonotaxis of female crickets under natural outdoor conditions

Acoustic mate choice in insects has been extensively studied under laboratory conditions, using different behavioural paradigms. Ideally, however, mate choice designs should reflect natural conditions, including the physical properties of the transmission channel for the signal. Since little is known about the discrimination ability of females between male song variants under natural conditions, we performed phonotaxis experiments with female field crickets (Gryllus bimaculatus) outdoors, using two-choice decisions based on differences in carrier frequency, sound pressure level, and chirp rate. For all three song parameters, minimum differences necessary for a significant preference between two song models were considerably larger outdoors compared to laboratory conditions. A minimum amplitude difference of 5 dB was required for a significant choice in the field, compared to only 1-2 dB reported for lab-based experiments. Due to the tuned receiver system, differences in carrier frequency equal differences in perceived loudness, and the results on choice for differences in carrier frequency corroborate those in amplitude. Similarly, chirp rate differences of 50 chirps/min were required outdoors compared to only 20 chirps/min in the lab. For predictions about patterns of sexual selection, future studies need to consider the different outcomes of mate choice decisions in lab and field trials.