Physical linkage and mate preference generate linkage disequilibrium for behavioral isolation in two parapatric crickets

Behavioral isolation is a potent barrier to gene flow and a source of striking diversity in the animal kingdom. However, it remains unclear if the linkage disequilibrium (LD) between sex‐specific traits required for behavioral isolation results mostly from physical linkage between signal and preference loci or from directional mate preferences. Here, we test this in the field crickets Gryllus rubens and G. texensis. These closely related species diverged with gene flow and have strongly differentiated songs and preference functions for the mate calling song rhythm. We map quantitative trait loci for signal and preference traits (pQTL) as well as for gene expression associated with these traits (eQTL). We find strong, positive genetic covariance between song traits and between song and preference. Our results show that this is in part explained by incomplete physical linkage: although both linked pQTL and eQTL couple male and female traits, major effect loci for different traits were never on the same chromosome. We suggest that the finely tuned, highly divergent preference functions are likely an additional source of LD between male and female traits in this system. Furthermore, pleiotropy of gene expression presents an underappreciated mechanism to link sexually dimorphic phenotypes.     

Characterization of female preference functions for Drosophila montana courtship song and a test of the temperature coupling hypothesis

Female mate preferences are a major cause of diversity and elaboration in male sexual traits. Here we characterize the shape of female preference functions for pulse length and carrier frequency of the courtship song of Drosophila montana by fitting both parametric and nonparametric functions to the incidence of female receptive gestures to synthetic song. Preference functions for both traits are strongly directional. That for pulse length is linear and favors short pulses, whereas that for carrier frequency is stabilizing in shape, but would exert directional preferences favoring males with high carrier frequency. The preference for carrier frequency has probably evolved under sexual selection, but reasons for the preference for short pulses are less apparent. We also examine the effect of ambient temperature on the carrier frequency of male song and on the preference function for carrier frequency. For many similar acoustic communication systems, temperature coupling, a compensatory effect of temperature on preference functions, is thought to maintain coordination between preferences and signals. However, although the carrier frequency of D. montana song is highly dependent on environmental temperature, there is no temperature coupling of the female preference function. We suggest that temperature coupling may often arise due to a common effect of temperature on song and preference, rather than be an advantageous characteristic whose function is to maintain coordination in temperature-affected communication systems.     

Female preference functions drive interpopulation divergence in male signalling: call diversity in the bushcricket Ephippiger diurnus

Female preferences play a major role in the elaboration and diversification of male traits: as a selective pressure on males, variation in female preferences can generate population divergence and ultimately, speciation. We studied how interpopulation differences in the shape of female mate preference functions may have shaped male advertisement signals in the bushcricket Ephippiger diurnus. This species is distributed as geographically isolated populations with striking interpopulation variation in male acoustic signals, most notably in the number of syllables per call. Here, we asked whether differences in the shape of preference functions exist among populations and whether those differences may have driven male signal evolution resulting in the observed differences in syllable numbers. Our results reveal fundamental differences in female preferences among populations, with differences in the overall preference function shape corresponding to differences in male signals. These differences in female preferences best explain the major differences in male signals among populations. The interpopulation variation in signals and preferences potentially reflects the evolutionary history of the species and may contribute to further divergence among populations and subsequent speciation.     

Reproductive interference via display signals: the challenge of multiple receivers

Sexually selected traits important in both mate and competitor recognition provide an opportunity to understand the tradeoffs associated with reproductive and competitive interference. When co-occurring species compete over similar resources, selection may promote signal similarity to facilitate competitive interactions in opposition to selection for signal divergence to maintain assortative mating. Bird song provides a classic example of contrasting selection on signal design, because songs function both in mate discrimination and in territorial advertisement. Similarity in songs aids competitor recognition both within and across species, and song convergence or mixing is widespread in the songbirds. Two related mechanisms can maintain mate recognition in the face of song convergence. First, multiple recognition signals, both across and within signaling modalities, provide a basis for mate and competitor discrimination using different sets of cues. Second, stricter female song preferences may allow interspecific male–male competitive communication without compromising female mate discrimination. I suggest that increased understanding of the neurobiology underlying song recognition will provide insight into the relative importance and prevalence of these different mechanisms along a continuum of species divergence.     

Coevolution of male mating signal and female preference during early lineage divergence of the Hawaiian cricket, Laupala cerasina

Sexual selection is a powerful evolutionary force shaping mate choice phenotypes, initiating phenotypic shifts resulting in (or reinforcing) population divergence and speciation when such shifts reduce mating probabilities among divergent populations. In the Hawaiian cricket genus Laupala, pulse rate of male calling song, a conspicuous mating signal, differs among species, potentially behaving as a speciation phenotype. Populations of the widespread species Laupala cerasina show variation in pulse rate. We document the degree of population differentiation in three features of calling song: pulse rate, pulse duration, and carrier frequency. All show significant population differentiation, with pulse rate showing the greatest heterogeneity. A Mantel test found no relationship between geographic distance and pulse rate divergence, indicating that a simple model of greater divergence with increasing distance cannot explain the observed pattern of differentiation. We demonstrate that female preference functions for pulse rate are unimodal, and that preference means show significant differentiation among populations. Furthermore, estimates of pulse rate preference correlate significantly with mean pulse rates across populations, indicating song and preference coevolve in a stepwise manner. This correlated divergence between signal and preference suggests that sexual selection facilitates the establishment of sexual isolation, reduced gene flow, and population differentiation, prerequisites for speciation.     

Acoustic preference functions and sexual selection on the male calling song in the grasshopper Chorthippus biguttulus

Male courtship songs have two functions in species recognition and intraspecific mate choice. Female preference might thus exert different types of selection pressure on male song traits. We used a combination of acoustic mate choice experiments and statistical analyses to examine how traits of the calling songs of male nightingale grasshoppers,Chorthippus biguttulus , are influenced by different sexual selection pressures. We recorded calling songs of males and tested their attractiveness to females in acoustic mate choice experiments. The attractiveness values were a good estimate of the potential male mating success. In experiments with a pair of males, females copulated significantly more often with the male that had the higher attractiveness value. To detect directional, stabilizing, disruptive or correlative selection acting on male song properties we used linear and nonlinear regressions between male song traits and female response behaviour. Three signal traits were revealed to be under directional selection: song loudness, pause to syllable ratio and the mean duration of gaps within syllables. A nonlinear regression testing for correlative selection showed that a fourth song trait, rhythm, in combination with mean gap duration was also important for female mate choice. With these traits and trait combinations we were able to explain 42% of the variance in attractiveness between males. Since we found no evidence for stabilizing selection, but ample evidence for directional selection, we conclude that selection on the traits examined is related to mate choice mainly in the context of intraspecific sexual selection and probably less so in species recognition. Copyright 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.      

Testing sensory drive speciation in cichlid fish: Linking light conditions to opsin expression,opsin genotype and female mate preference

Ecological speciation is facilitated when divergent adaptation has direct effects on selective mating. Divergent sensory adaptation could generate such direct effects, by mediating both ecological performance and mate selection. In aquatic environments, light attenuation creates distinct photic environments, generating divergent selection on visual systems. Consequently, divergent sensory drive has been implicated in the diversification of several fish species. Here, we experimentally test whether divergent visual adaptation explains the divergence of mate preferences in Haplochromine cichlids. Blue and red Pundamilia co‐occur across south‐eastern Lake Victoria. They inhabit different photic conditions and have distinct visual system properties. Previously, we documented that rearing fish under different light conditions influences female preference for blue versus red males. Here, we examine to what extent variation in female mate preference can be explained by variation in visual system properties, testing the causal link between visual perception and preference. We find that our experimental light manipulations influence opsin expression, suggesting a potential role for phenotypic plasticity in optimizing visual performance. However, variation in opsin expression does not explain species differences in female preference. Instead, female preference covaries with allelic variation in the long‐wavelength‐sensitive opsin gene (LWS), when assessed under broad‐spectrum light. Taken together, our study presents evidence for environmental plasticity in opsin expression and confirms the important role of colour perception in shaping female mate preferences in Pundamilia. However, it does not constitute unequivocal evidence for the direct effects of visual adaptation on assortative mating.     

QTL analysis of a rapidly evolving speciation phenotype in the Hawaiian cricket Laupala

In mate recognition systems, the functional necessity to coordinate traits involved in sexual communication should result in reduced pairing potential for new variants outside the distribution of common reproductive signals. Yet, many closely related, sexual species differ in mate recognition traits, suggesting that directional selection influences the divergence of mate recognition systems. Species of the endemic Hawaiian cricket genus Laupala are morphologically and ecologically cryptic, although both male calling song and female acoustic preference have diverged rapidly between closely related species. These mate recognition phenotypes are therefore often likely to be speciation phenotypes, i.e. traits whose divergence contributes, directly or indirectly, to a reduction of gene flow during speciation, given their frequent participation in early lineage divergence. We conducted a replicated, quantitative trait loci (QTL) mapping study of the genetic basis of differences in male calling song between two closely related species, Laupala paranigra and Laupala kohalensis, allowing us to examine the genetic basis of traits involved in rapid speciation. We found statistical support for eight QTL in one replicate, with at least four of these QTL mapping to the same regions in a second replicate. QTL effects ranged between 3.0% and 10.7% of the difference in pulse rate between L. paranigra and L. kohalensis, and are thus of moderate to small effect. All QTL identified show directional effects consistent with the hypothesis of directional selection. Thus, we conclude that rapid speciation can occur under the influence of many genes of moderate to small effect. This study implicates the role of directional selection in the divergence of mate recognition traits and speciation the Hawaiian cricket genus Laupala.     

Genetic Architecture of Sexual Selection: QTL Mapping of Male Song and Female Receiver Traits in an Acoustic Moth

Models of indirect (genetic) benefits sexual selection predict linkage disequilibria between genes that influence male traits and female preferences, owing to non-random mate choice or physical linkage. Such linkage disequilibria can accelerate the evolution of traits and preferences to exaggerated levels. Both theory and recent empirical findings on species recognition suggest that such linkage disequilibria may result from physical linkage or pleiotropy, but very little work has addressed this possibility within the context of sexual selection. We studied the genetic architecture of sexually selected traits by analyzing signals and preferences in an acoustic moth, Achroia grisella, in which males attract females with a train of ultrasound pulses and females prefer loud songs and a fast pulse rhythm. Both male signal characters and female preferences are repeatable and heritable traits. Moreover, female choice is based largely on male song, while males do not appear to provide direct benefits at mating. Thus, some genetic correlation between song and preference traits is expected. We employed a standard crossing design between inbred lines and used AFLP markers to build a linkage map for this species and locate quantitative trait loci (QTL) that influence male song and female preference. Our analyses mostly revealed QTLs of moderate strength that influence various male signal and female receiver traits, but one QTL was found that exerts a major influence on the pulse-pair rate of male song, a critical trait in female attraction. However, we found no evidence of specific co-localization of QTLs influencing male signal and female receiver traits on the same linkage groups. This finding suggests that the sexual selection process would proceed at a modest rate in A. grisella and that evolution toward exaggerated character states may be tempered. We suggest that this equilibrium state may be more the norm than the exception among animal species.     

Phenotypic variation and covariation indicate high evolvability of acoustic communication in crickets

Studying the genetic architecture of sexual traits provides insight into the rate and direction at which traits can respond to selection. Traits associated with few loci and limited genetic and phenotypic constraints tend to evolve at high rates typically observed for secondary sexual characters. Here, we examined the genetic architecture of song traits and female song preferences in the field crickets Gryllus rubens and Gryllus texensis. Song and preference data were collected from both species and interspecific F1 and F2 hybrids. We first analysed phenotypic variation to examine interspecific differentiation and trait distributions in parental and hybrid generations. Then, the relative contribution of additive and additive‐dominance variation was estimated. Finally, phenotypic variance–covariance ( P ) matrices were estimated to evaluate the multivariate phenotype available for selection. Song traits and preferences had unimodal trait distributions, and hybrid offspring were intermediate with respect to the parents. We uncovered additive and dominance variation in song traits and preferences. For two song traits, we found evidence for X‐linked inheritance. On the one hand, the observed genetic architecture does not suggest rapid divergence, although sex linkage may have allowed for somewhat higher evolutionary rates. On the other hand, P matrices revealed that multivariate variation in song traits aligned with major dimensions in song preferences, suggesting a strong selection response. We also found strong covariance between the main traits that are sexually selected and traits that are not directly selected by females, providing an explanation for the striking multivariate divergence in male calling songs despite limited divergence in female preferences.     

Interspecific genetics of speciation phenotypes: song and preference coevolution in Hawaiian crickets

Understanding the genetic architecture of traits involved in premating isolation between recently diverged lineages can provide valuable insight regarding the mode and tempo of speciation. The repeated coevolution of male courtship song and female preference across the species radiation of Laupala crickets presents an unusual opportunity to compare the genetic basis of divergence across independent evolutionary histories. Previous studies of one pair of species revealed a polygenic basis (including a significant X chromosome contribution) to quantitative differences in male song and female acoustic preference. Here, we studied interspecific crosses between two phenotypically less-diverged species that represents a phylogenetically independent occurrence of intersexual signalling evolution. We found patterns consistent with an additive polygenic basis to differentiation in both song and preference (n(E) = 5.3 and 5.1 genetic factors, respectively), and estimate a moderate contribution of the X chromosome (7.6%) of similar magnitude to that observed for Laupala species with nearly twice the phenotypic divergence. Together, these findings suggest a similar genetic architecture underlying the repeated evolution of sexual characters in this genus and provide a counterexample to prevailing theory predicting an association between early lineage divergence and sex-linked 'major genes'.     

Interspecific genetics of mate recognition: inheritance of female acoustic preference in Hawaiian crickets

Abstract. Female mating behavior plays a fundamental role in the divergent evolution of mate recognition systems that may lead to speciation. Despite this important role, the phenotypic and genetic bases of female mating behavior remain poorly understood. In this study, I examine the shape of the female acoustic preference function and estimate values for pulse rate preference in two species of Hawaiian crickets, Laupala kohalensis and L. paranigra . In addition, I examine how preference differences are inherited in hybrid crosses between these species. Females expressed unimodal preference functions and were generally more attracted to pulse rates characterizing their own species. Unimodal preference functions also characterized F1 and backcross generations, with hybrid females expressing preferences for intermediate pulse rates. Pulse rate preferences segregated in the backcross generation. Mean pulse rate preference matched mean pulse rate in both parental and hybrid generations. Based on F1 hybrids and segregation patterns in backcross females, I show that changes in both signal and receiver components of the mate recognition system are consistent with a multilocus model of change through incremental steps. The results therefore suggest that ancestors of the current species also expressed unimodal preference functions and that changes in acoustic communication signals occurred through shifts in mean pulse rates and pulse rate preferences among populations.     

Male courtship song and female preference variation between phylogeographically distinct populations of Drosophila montana

Understanding the variation within and between populations in important male mating traits and female preferences is crucial to theories concerning the origin of sexual isolation by coevolution or other processes. There have been surprisingly few studies on the extent of variation and covariation within and between populations, especially where the evolutionary relationships between populations are understood. Here we examine variation in female preferences and a sexually selected male song trait, the carrier frequency of the song, within and between populations from different phylogeographic clusters of Drosophila montana. Song is obligatory for successful mating in this species, and both playback and field studies implicate song carrier frequency as the most important parameter in male song. Carrier frequency varied among three recently collected populations from Oulanka (Finland), Vancouver (Canada), and Colorado (central United States), which represent the main phylogeographic groups in D. montana. Males from Colorado had the most distinct song frequency, which did not follow patterns of genetic differentiation. There was considerable variation in preference functions within, and some variation between, populations. Surprisingly, females from three lines from Colorado seem to have preferences disfavoring the extreme male trait found in this population. We discuss sources of selection on male song and female preference.     

QUANTITATIVE GENETICS OF SEXUAL SELECTION IN THE FIELD CRICKET,GRYLLUS INTEGER

Major theories of sexual selection predict heritable variation in female preferences and male traits and a positive genetic correlation between preference and trait. Here we show that female Texas field crickets, Gryllus integer, have heritable genetic variation for the male calling song stimulus level that produces the greatest phonotactic response. Approximately 34% of the variation in female preferences was due to additive genetic effects. Female choosiness, that is, the strength of the female response to her most preferred stimulus relative to her average response to all stimuli, did not show significant genetic effects. The male calling song character was not related to male size or age but did show significant genetic effects. Approximately 39% of the variation in the number of pulses per trill was due to additive genetic variation. The genetic correlation estimated for the field population was 0.51 ± 0.17. The number of pulses per trill produced by males is under stabilizing sexual selection.     

The inheritance of female preference functions in a mate recognition system

Mate recognition systems (MRSs) play a major role in sexual selection and speciation, yet few studies have analysed both male and female components in detail. Here, female preference functions have been characterized for the tettigoniid bushcricket Ephippiger ephippiger, and the inheritance of male song and female preference functions followed in crosses between subspecies. Songs are disproportionately determined by sex-linked genes. However, there is no evidence for a role of maternally derived sex-linked genes in female preference or of maternal effects. At the genetic level, there is a mismatch between peak preferences and male song, consistent with an evolutionary history of persistent directional preferences. Such a pattern of inheritance could contribute to the process of speciation via the evolution of new MRSs.     

The evolution of reproductive character displacement conflicts with how sexual selection operates within a species

Processes that affect the evolution of female preferences or male display traits involved in mating decisions in different geographic areas have the potential to result in within-species divergence. This could occur via reinforcement of mate recognition in species using the same traits for species recognition and sexual selection. Sympatric individuals experience reinforcement of female preferences and male display traits, whereas allopatric individuals do not, creating the potential for divergent sexual selection in sympatric and allopatric populations. Sexual selection operates on the cuticular hydrocarbons (CHCs) of Drosophila serrata, and reinforcement on the CHCs of populations sympatric with D. birchii. Here, we manipulate sexual selection in D. serrata populations generated by hybridizing natural sympatric and allopatric populations. Under the influence of sexual selection, male CHCs evolved from an intermediate phenotype to resemble an allopatric phenotype, which was driven by female choice. Additionally, female choice resulted in evolution of an allopatric female preference, so that allopatric males were preferred to sympatric males. Allopatric CHCs and preferences represent a sexual selection optimum via female choice. Sympatric populations display suboptimal phenotypes relative to their allopatric conspecifics. The combination of reinforcement and sexual selection can therefore generate divergence in female preferences and male display traits.     

Mate preference across the speciation continuum in a clade of mimetic butterflies

Premating behavioral isolation is increasingly recognized as an important part of ecological speciation, where divergent natural selection causes the evolution of reproductive barriers. A number of studies have now demonstrated that traits under divergent natural selection also affect mate preferences. However, studies of single species pairs only capture a snapshot of the speciation process, making it difficult to assess the role of mate preferences throughout the entire process. Heliconius butterflies are well known for their brightly colored mimetic warning patterns, and previous studies have shown that these patterns are also used as mate recognition cues. Here, we present mate preference data for four pairs of sister taxa, representing different stages of divergence, which together allow us to compare diverging mate preferences across the continuum of Heliconius speciation. Using a novel Bayesian approach, our results support a model of ecological speciation in which strong premating isolation arises early, but continues to increase throughout the continuum from polymorphic populations through to "good," sympatric ecologically divergent species.     

Sex differences in developmental plasticity and canalization shape population divergence in mate preferences

Sexual selection of high-quality mates can conflict with species recognition if traits that govern intraspecific mate preferences also influence interspecific recognition. This conflict might be resolved by developmental plasticity and learned mate preferences, which could drive preference divergence in populations that differ in local species composition. We integrate field and laboratory experiments on two calopterygid damselfly species with population genetic data to investigate how sex differences in developmental plasticity affect population divergence in the face of gene flow. Whereas male species recognition is fixed at emergence, females instead learn to recognize heterospecifics. Females are therefore more plastic in their mate preferences than males. We suggest that this results from sex differences in the balance between sexual selection for high-quality mates and selection for species recognition. As a result of these sex differences, females develop more pronounced population divergence in their mate preferences compared with males. Local ecological community context and presence of heterospecifics in combination with sex differences in plasticity and canalization therefore shape population divergence in mate preferences. As ongoing environmental change and habitat fragmentation bring formerly allopatric species into secondary contact, developmental plasticity of mate preferences in either or both sexes might facilitate coexistence and prevent local species extinction.     

Divergence of a speciation trait through artificial selection: Insights into constraints,by-product effects and sexual isolation

Speciation and sexual isolation often occur when divergent female mating preferences target male secondary sexual traits. Despite the importance of such male signals, little is known about their evolvability and genetic linkage to other traits during speciation. To answer these questions, we imposed divergent artificial selection for 10 non-overlapping generations on the Inter-Pulse-Interval (IPI) of male courtship songs; which has been previously shown to be a major species recognition trait for females in the Drosophila athabasca species complex. Focusing on one of the species, Drosophila mahican (previously known as EA race), we examined IPI's: (1) rate of divergence, (2) response to selection in different directions, (3) genetic architecture of divergence and (4) by-product effects on other traits that have diverged in the species complex. We found rapid and consistent response for higher IPI but less response to lower IPI; implying asymmetrical constraints. Genetic divergence in IPI differed from natural species in X versus autosome contribution and in dominance, suggesting that evolution may take different paths. Finally, selection on IPI did not alter other components of male songs, or other ecological traits, and did not cause divergence in female preferences, as evidenced by lack of sexual isolation. This suggests that divergence of male courtship song IPI is unconstrained by genetic linkage with other traits in this system. This lack of linkage between male signals and other traits implies that female preferences or ecological selection can co-opt and mould specific male signals for species recognition free of genetic constraints from other traits.