David A. Gray

Animal mating signals can communicate both species-specific recognition information and male phenotypic quality persuasion information to females. Here we show that the recognition components of cricket song diverge early and rapidly among species pairs, especially so among those pairs with the potential for gene flow compared to allopatric pairs. Divergence in the persuasion components of song is less closely linked to speciation. Together these results suggest that the key mating signal recognition traits involved in speciation evolve in a punctuated manner, and at equilibrium are subject to stabilizing selection. Persuasion traits, on the other hand, are typically under directional sexual selection, but are not the traits closely tied to speciation. This suggests that the mechanistic link between sexual selection and speciation is ephemeral, based on rapid change in recognition traits during speciation. An important implication is that equilibrium within-population sexual selection is uncoupled from interspecific divergence during speciation. We expect that the distinction between recognition and persuasion traits, and the particular importance of divergence in recognition traits for speciation, should be a widespread phenomenon. Significance Statement: Analyses of macroevolutionary patterns of cricket mating signal traits reveal mechanistically how and when rapid mating signal evolution facilitates speciation and challenges the prevailing view that traits experiencing intrapopulation sexual selection are likely to promote interspecific divergence. More than just documenting a pattern in a single system, our study provides a general framework for studying the interplay of sexual selection and speciation and for mating signal evolution generally.

Communication signals often contain behaviorally relevant structure on multiple temporal scales. Short-timescale features are commonly thought to be recognized first and then integrated into longer-timescale representations. Whether a shared circuit can instead recognize multiple timescales remains unclear. Cricket calling songs provide a tractable test because millisecond-scale pulses are grouped into chirps lasting hundreds of milliseconds. We combined female phonotaxis with a mechanistic model of the identified pulse-recognition network to ask whether this circuitry can also generate chirp selectivity. In Gryllus bimaculatus , the model, originally parameterized from pulse-scale electrophysiology and behavior, recovered the main boundaries of chirp preference without fitting to chirp behavior. Coincidence detection generated the minimum chirp-duration requirement, whereas suppressive computations in the network’s input neuron shaped the upper boundaries for chirp duty cycle and period. Across 14 species, varying these suppressive computations captured major differences in chirp tuning. These findings reveal a temporally interleaved serial circuit in which long-timescale selectivity begins upstream of completed short-timescale selectivity, offering an alternative to both conventional short-first hierarchies and parallel pathways.

Dispersal and hybridization can reinforce one another in a feedback loop when species that disperse farther are more likely to encounter and mate with close relatives, resulting in gene flow that can reintroduce dispersal-enhancing alleles in their descendants. This dynamic could help explain why complex dispersal-related traits, such as flight in insects, appear to evolve repeatedly within a clade, but this idea has rarely been tested with genomic data. North American Gryllus field crickets, in which flight capability appears to have been regained from flightless ancestors at least nine independent times, offer a powerful system to do so. Using a genome-wide dataset, we test three predictions about the origins of phylogenetic discordance in Gryllus and the role of dispersal-related traits. First, we find that both incomplete lineage sorting (ILS) and hybridization contribute to extensive gene tree conflict observed across the Gryllus phylogeny, but several conflicting patterns cannot be explained by ILS alone. Multiple lines of evidence reveal introgression at both deep and recent nodes, producing a mosaic evolutionary history with reticulation across many lineages. Second, hybridization is disproportionately concentrated among flight-capable lineages, supporting the idea that dispersal ability increases opportunities for interspecific gene flow. Third, introgression from flight-capable lineages could help explain repeated trait reversal across the phylogeny, suggesting that allele flow may have contributed to the re-emergence of flight in descendants of flightless ancestors. These results are consistent with dispersal-hybridization feedback shaping both the reticulated phylogenetic history of Gryllus and the repeated emergence of flight. More broadly, this species group reveals how models of character evolution built on bifurcating trees may overestimate the number of independent trait origins when reticulate processes are ignored. Teaser text: Evolutionary histories are often networks rather than bifurcating trees, with important consequences for how we interpret repeated trait evolution. In North American field crickets, we show that both incomplete lineage sorting and hybridization contribute to extensive discordance among gene trees, with introgression detectable across deep and recent timescales. Hybridization disproportionately involves flight-capable lineages and several introgression events coincide with lineages that regained flight from flightless ancestors. This supports a feedback loop, in which dispersal promotes hybridization, and hybridization in turn reintroduces genetic variation that can restore flight capability. More broadly, our results suggest that repeated origins of complex traits inferred from bifurcating phylogenies may instead partly reflect reticulate evolutionary histories.

Acoustic communication signals often contain a complex temporal structure, yet how this temporal structure contributes to signal recognition is poorly understood, particularly in eavesdropping receivers. The parasitoid fly Ormia ochracea localises host crickets by eavesdropping on their calling songs. In Florida, preferred host songs consist of sound pulses repeated at ∼50 pulses s−1, and flies exhibit matching preferences. However, it is unclear whether this preference reflects sensitivity to individual temporal parameters or to derived temporal relationships that emerge from their combination. We independently varied pulse duration and interpulse interval across a broad stimulus space and quantified tethered-walking phonotaxis using a switch-following paradigm. Behavioural responses formed a structured tuning surface, with elevated responses for pulse durations of ∼5–15 ms across a broad range of interpulse intervals, together with high performance along a diagonal corresponding to 50 pulses s−1. Responses failed to collapse across stimuli sharing the same pulse rate or pulse period, indicating that these parameters alone do not determine recognition. Models allowing behavioural responses to vary jointly across combinations of temporal parameters substantially outperformed models based on single parameters. Although pulse duration strongly constrained recognition, behavioural responses also varied systematically with interpulse interval and could not be explained by pulse duration, interpulse interval, pulse rate, pulse period or duty cycle alone. The apparent preference of O. ochracea for ∼50 pulses s−1 therefore represents one component of a broader temporal recognition landscape rather than recognition based on pulse rate alone.

Acoustic communication signals often contain complex temporal structure, yet the features underlying signal recognition remain poorly understood, particularly in eavesdropping receivers. The parasitoid fly Ormia ochracea localises host crickets by eavesdropping on their calling songs. In Florida, preferred host songs consist of sound pulses repeated at ~50 pulses/s, and flies exhibit matching preferences. However, it remains unclear whether this preference reflects sensitivity to individual temporal features (e.g., pulse duration, interpulse interval) or to derived temporal relationships (e.g., pulse rate, pulse period, duty cycle) that emerge from their combination. We independently varied pulse duration and interpulse interval across a broad stimulus space and quantified tethered-walking phonotaxis using a switch-following paradigm. Behavioural responses formed a structured tuning surface, with high performance along a diagonal corresponding to 50 pulses/s, as well as elevated responses for a restricted range of pulse durations across a wide range of interpulse intervals. Responses failed to collapse across stimuli sharing the same pulse rate or pulse period, indicating that these features alone do not determine recognition. Instead, behaviour was best explained by the interacting effects of pulse duration and interpulse interval. These results demonstrate that song recognition in O. ochracea is multidimensional, with pulse rate tuning emerging from an underlying feature space rather than a single encoded parameter.

Mayr proposed that the stringency of stabilizing selection on animal mating signals is context-dependent: in species lacking sympatric congeners, relaxed selection pressure would afford greater intraspecific variability. This idea has rarely (if ever) been directly tested. Here, I evaluate Mayr's conjecture using a comprehensive dataset on acoustic mating signals from 39 taxa (33 named species and 6 unnamed lineages) of North American Gryllus field crickets. In doing so, I distinguish between 2 distinct functions of mating signal components: recognition versus persuasion. Contrary to Mayr's prediction, intraspecific variation in recognition mating signals did not increase in species with fewer or no sympatric congeners. Stabilizing selection on recognition traits appears to be maintained across both isolated and sympatric populations, possibly due to selection for efficient intraspecific communication—aligning with some aspects of Paterson's “Specific Mate Recognition System” model. Persuasion traits, on the other hand, show elevated levels of variation consistent with directional sexual selection promoting condition dependence. Together these results reveal the ubiquity of stabilizing selection on recognition traits when at evolutionary equilibrium, and the critical importance of distinguishing between recognition and persuasion functions of animal mating signals.

Mating signals are targets of conspecific signal recognition and sexual selection, but are also subject to abiotic temperature effects and to biotic interspecific eavesdroppers. In crickets, the male calling song becomes faster at warmer temperatures, and female crickets’ recognition of male song tracks temperature in a coordinated manner, termed ‘temperature coupling.’ But female crickets are not the only ecologically relevant listeners: some cricket species are parasitized by Ormia ochracea, a parasitoid fly which finds its cricket hosts by eavesdropping on male cricket song. How temperature affects parasitoid fly phonotaxis to song is largely unexplored, with only one previous study conducted under field conditions. Here we explore six possible patterns of thermal effects on fly responses to cricket song, including temperature coupling, using field playbacks of synthetic Gryllus lineaticeps songs designed to be species-typical at various temperatures. We find that temperature does affect fly response, but that the temperature deviation of songs from ambient does not impact numbers of flies caught. We extend this finding by comparing the temperatures of the air (where flies search for their hosts) and the ground (where their host crickets signal) to show that temperature coupling is unlikely to be effective given microhabitat variation and differential rates of cooling in the evening hours when flies are most active. Our results can be interpreted more broadly to suggest (i) temperature effects on intraspecific communication systems may be more tightly coupled than are effects on interspecific eavesdropping, and (ii) variation in thermal microhabitats in the field make it difficult to translate laboratory physiological responses to natural selection in the wild.

Mating signals are targets of conspecific signal recognition and sexual selection, but are also subject to abiotic temperature effects and to biotic interspecific eavesdroppers. In crickets, the male calling song becomes faster at warmer temperatures, and female crickets’ recognition of male song tracks temperature in a coordinated manner, termed ‘temperature coupling.’ But female crickets are not the only ecologically relevant listeners: some cricket species are parasitized by Ormia ochracea , a parasitoid fly which finds its cricket hosts by eavesdropping on male cricket song. How temperature affects parasitoid fly phonotaxis to song is largely unexplored, with only one previous study conducted under field conditions. Here we explore six possible patterns of thermal effects on fly responses to cricket song, including temperature coupling, using field playbacks of synthetic Gryllus lineaticeps songs designed to be species-typical at various temperatures. We find that temperature does affect fly response, but that the temperature deviation of songs from ambient does not impact numbers of flies caught. We extend this finding by comparing the temperatures of the air and ground to show that temperature coupling is unlikely to be effective given microhabitat variation and differential rates of cooling in the evening hours when flies are most active. Our results can be interpreted more broadly to suggest (i) temperature effects on intraspecific communication systems may be more tightly coupled than are effects on interspecific eavesdropping, and (ii) variation in thermal microhabitats in the field make it difficult to translate laboratory physiological responses to natural selection in the wild. Lay Summary: Mating signals and signal recognition change with temperature, and sometimes mating signals are intercepted by predators or parasites. By using playbacks of cricket song in the wild, we show that temperature changes do affect the response of a parasitoid fly to cricket song. However, parasitoid responses are not tightly coupled to temperature induced changes in cricket song, in part due to unpredictable variation in microhabitat temperatures typical of crickets and flies.

The order Orthoptera includes insects such as grasshoppers, katydids, and crickets, among which there are important species for ecosystem stability and pollination, as well as research organisms in different fields such as neurobiology, ecology, and evolution. Crickets, with more than 2,400 described species, are emerging as novel model research organisms, for their diversity, worldwide distribution, regeneration capacity, and their characteristic acoustic communication. Here we report the assembly and annotation of the first New World cricket, that of Gryllus longicercus Weissman & Gray 2019. The genome assembly, generated by combining 44.54 Gb of long reads from PacBio and 120.44 Gb of short Illumina reads, has a length of 1.85 Gb. The genome annotation yielded 19,715 transcripts from 14,789 gene models.

Patterns of morphological divergence across species’ ranges can provide insight into local adaptation and speciation. In this study, we compared phenotypic divergence among 4,221 crickets from 337 populations of two closely related species of field cricket, Gryllus firmus and G. pennsylvanicus, and their hybrids. We found that these species differ across their geographic range in key morphological traits, such as body size and ovipositor length, and we directly compared phenotype with genotype for a subset of crickets to demonstrate nuclear genetic introgression, phenotypic intermediacy of hybrids, and essentially unidirectional mitochondrial introgression. We discuss how these morphological traits relate to life history differences between the two species. Our comparisons across geographic areas support prior research suggesting that cryptic variation within G. firmus may represent different species. Our study highlights how variable morphology can be across wide-ranging species and the importance of studying reproductive barriers in more than one or two transects of a hybrid zone.