Sexual selection is responsible for a great diversity of elaborate male traits. A general female preference for males that have exaggerated traits drives this process, but the reasons females exhibit this preference are often unclear. Recent advances in understanding signal evolution have emerged from studies of receiver psychology that focus on how receivers perceive and process communication signals. I apply the perspective of receiver psychology to understand female preference for elaborate signals in túngara frogs (Physalaemus pustulosus). Male túngara frogs produce advertisement calls of variable complexity. Females exhibit a strong preference for complex to simple calls, but previous studies have not found consistent patterns of preference between calls of variable complexity. In my doctoral research, I investigate the function of variable complexity in túngara frogs. Specifically, I address the following questions: 1) Are calls of variable complexity especially relevant to females in certain contexts? Do males respond to female behavior by increasing their production of complex calls? 2) Does male to female proximity influence female response to call complexity? 3) Are females constrained by their perceptual biology in discriminating differences in call complexity? 4) Can females remember attractive males over silences between bouts of advertising? Is working memory for attractive males dependent upon signal complexity? And 5) Does signal memorability increase with signal complexity in a linear relationship? These studies provide several new perspectives to an understanding of female preference for elaborate signals. Phonotaxis experiments demonstrate that females use elicitation behaviors to influence male production of complex calls, that proximity influences female response to signal elaboration, that females are constrained by their perceptual biology in discriminating between complex calls, that memory can influence the evolution of signal complexity, and that memorability and signal complexity share a non-linear relationship. / text

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What processes contribute to the evolution of reproductive isolation and the coexistence of interfertile species in the same habitat? This thesis investigates the relative roles of species interactions and intraspecific processes in contributing to reproductive isolation. I combine behavioural and genomic approaches to test hypotheses about what mechanisms maintain the general species boundary between two closely related field cricket species: Teleogryllus oceanicus and T. commodus. These species are a classic study system for sexual communication and readily hybridize in the laboratory, however little is known about species interactions in sympatric populations. I examine patterns of geographic variation in two key sexual traits: calling song and cuticular hydrocarbons (CHCs), and the geographic distribution of genetic variation across a broad sample of allopatric and sympatric populations. I test whether X chromosomes play a pronounced role in population divergence and reproductive isolation. Using close range mating trials and hybridization experiments I identify numerous pre-mating and post-mating barriers between the species. The results indicate that the species are currently reproductively isolated and the pattern of population differentiation does not strongly support contemporary species interactions contributing to phenotypic diversity. Numerous barriers exist between the species, in particular hybrid females are sterile in both cross directions, while hybrid males are relatively fertile. This provides a rare exception to Haldane's rule which is central to many genetic theories of speciation. Established theory predicts that X chromosomes should play a pronounced role in the evolution of both pre- and postzygotic barriers. Contrary to this, I found no evidence that X chromosomes contribute to hybrid female sterility. Moreover, X-linked loci exhibited an unexpected pattern of reduced population differentiation within species, but increased species divergence compared to autosomal loci, which may indicate selective sweeps or sex-biased processes. Taken together, the results suggest that the causes and consequences of X chromosome evolution, in particular among XO taxa, may contradict some of the established theories.

In dieser Arbeit identifizierte und quantifizierte ich zunächst mehrere Isolationsbarrieren zwischen den nah verwandten Feldheuschreckenarten Chorthippus biguttulus und C. mollis (Kapitel 2). Meine Ergebnisse deuten auf eine wichtige Rolle von chemischen Signalen bei der reproduktiven Isolation zwischen diesen Arten hin. Durch die Kombination von verschiedenen Ansätzen untersuchte ich die ultimaten und proximaten Ursachen von chemischen Signalen auf das Fortpflanzungsverhalten. Im dritten Kapitel zeigte ich, dass die kutikulären Kohlenwasserstoff Profile (CHC) von C. biguttulus und C. mollis art- und geschlechtsspezifisch sind. Mit Hilfe eines RNA-seq Ansatzes untersuchte ich transkriptionelle Unterschiede in Kandidatengenen, die für die Divergenz in den CHC Profilen zwischen den Arten und den Geschlechtern verantwortlich sein könnten. Ein solches Gen zeigte artspezifische Expression und trägt möglicherweise zur reproduktiven Isolation zwischen den Arten bei. Darüber hinaus fand ich Expressionsunterschiede zwischen den Geschlechtern in vier Kandidatengenen. Zwei von diesen Genen zeigten eine erhöhte Expression in Männchen, was eventuell in Verbindung mit dem höheren Anteil von dimethyl-verzweigten Kohlenwasserstoffen in Männchen steht. Ich fand keine Hinweise für positive Selektion in den Kandidatengenen, was vermuten lässt, dass die Unterschiede in CHC Profilen durch transkriptionelle Unterschiede entstehen. In Kapitel 4 erforschte ich mit Hilfe eines Bioassays, wie sich verschiedene CHC Signale auf das Balzverhalten von Männchen auswirkten. Zusammenfassend zeigt diese Arbeit, dass der Genfluss zwischen C. biguttulus und C. mollis durch verschiedene Barrieren unterbrochen ist und dass diese Feldheuschrecken multimodale Kanäle im Paarungsverhalten verwenden. Zusätzlich lassen meine Ergebnisse eine zentrale Rolle von kutikulären Kohlenwasserstoffen in der reproduktiven Isolation beider Arten und in der Artbildung vermuten. / In this thesis, I first conducted several experiments to identify and quantify reproductive isolation at multiple stages in the life history of the closely related species Chorthippus biguttulus and C. mollis (chapter 2). My results indicated a crucial role of chemical cues in the maintenance of species isolation. I combined multiple approaches to examine the ultimate and proximate causes of chemical cues on reproductive behavior in these species. In chapter 3, I demonstrated that the cuticular hydrocarbon (CHC) profiles of C. biguttulus and C. mollis provide species- and sex-specific cues. I used a RNA-seq approach to examine transcriptional differences of candidate genes, which might cause the divergence in CHC profiles between species and sex. One candidate gene showed species-specific transcriptional differences and may contribute to reproductive isolation. In addition, four candidate genes were differentially expressed between the sexes. Two of them exhibited a strong male-biased expression, which may be linked to higher proportions of dimethyl-branched CHCs in males. I found no evidence for positive selection acting on these genes, suggesting that differences in CHC profiles are presumably mediated at transcriptional level. In chapter 4, I developed a bioassay to determine if female CHCs act as chemical cues that induce courtship behavior in males. In summary, this thesis demonstrated that various reproductive isolating mechanisms reduce the gene flow between C. biguttulus and C. mollis and that in these species the courtship display consists of multimodal signals. In addition, my results suggest a key role of chemical cues in reproductive isolation and speciation.

Die vorliegende Dissertation verbindet Ergebnisse aus neuroethologischen, verhaltensbiologischen, quantitativ genetischen und genomischen Ansätzen bei Feldgrillen (Gryllus), um neue Erkenntnisse über die Rolle von sexueller Selektion bei Artbildung zu erlangen. Es wird gezeigt dass multivariate Gesangspräferenzen von Grillenweibchen von wenigen Merkmalen abhängen und zwischen Arten ähnlich sind, während sich Männchengesänge in allen Merkmalen unterschieden. Verschiedene Ebenen der Gesangserkennung sind durch unterschiedliche Präferenzfunktionen charakterisiert. Multivariate Präferenzen können also gleichzeitig verschiedene Indikatoren für Paarungspartnerqualität aus den Gesangsmerkmalen erkennen. Eine polygene genetische Architektur der Gesangsmerkmale und der Präferenz wurde beobachtet und weist auf eine eher langsamere Divergenz hin, obwohl gonosomale Vererbung mehrerer Gesangsmerkmale höhere Evolutionsraten zulässt. Starke Kovarianz zwischen den Merkmalen die direkt sexueller Selektion unterliegen und Merkmale, die nicht direkt von Weibchen gewählt werden, zeigen, dass indirekte Selektion teilweise für die markante Divergenz der Gesänge verantwortlich sein könnte, trotz begrenzter Divergenz der Präferenzen. Ferner zeigte ein Artvergleich der multivariaten Gesangsmerkmale, dass die Form der Präferenzfunktion die Ausrichtung der Kovarianzen und damit die erwartete Selektionsantwort der männlichen Gesänge beeinflussen kann. Simulationen ergaben starke Hinweise auf Genfluss zwischen zwei nahverwandten Arten über einen langen Zeitraum . Nur wenige Contigs zeigten hohe genetische Divergenz und hohe Raten nicht-synonymer Polymorphismen. Diese stimmten aber mit Genen überein, die experimentell nachgewiesene Funktionen in neuromuskulärer Entwicklung und im Paarungsverhalten haben. Zusammen zeigen die Ergebnisse das Potential von sexueller Selektion bei der Entstehung und Aufrechterhaltung von reproduktiver Isolation zwischen Arten. / This thesis integrates insights from neuro-ethological, behavioural, quantitative genetics, and genomic approaches in field crickets to provide novel insights in the role of sexual selection in speciation, in particular focusing on speciation with gene flow. It was shown that song preferences depend on few traits and are similar across species while the male song has diverged strongly in all traits. Because the different levels of song recognition are characterized by different types of preference functions, it is conceivable that multivariate preferences can extract various cues for mate quality from different traits simultaneously. A polygenic genetic architecture was found for song traits and preferences, probably limiting divergence rates. However, sex-chromosomal inheritance of some song traits may have allowed for somewhat higher rates. Strong covariance was found between traits that are under sexual selection and traits that are not directly selected by females. This indicates that indirect selection may be responsible in part for striking multivariate divergence in the male calling song despite limited divergence in female preferences. Furthermore, comparing multivariate song traits among species showed that the shape of the preference function can affect the orientation of trait covariance and thereby the selection responses of the male song. Coalescent simulations revealed evidence for a long history of gene flow between two closely related cricket species. Only few contigs with high genetic divergence and high rates of non-synonymous SNPs were found, but many of those that were highly diverged matched genes with experimentally proven functions in neuromuscular development and courtship behavior. Together, these findings underline the potential for sexual selection to drive reproductive isolation.