Distribution and habitat use of water frog hybrid complexes in France

Hybrid zones are either distributed along clines or in a mosaic of patches. This distribution may depend upon variation in taxon habitat use. Habitat use and distribution of diverse taxa of water frogs (Rana ridibunda, R. lessonae, R. perezi, R. kl. grafi and R. kl. esculenta) in France are analysed to determine whether water frog complexes conform to the mosaic or clinal model. Biogeographical scenarios may be invoked in order to explain the distribution of water frogs. However, the distribution of R. perezi and R. kl. grafi, being restricted to regions characterized by Mediterranean or Oceanic climatic conditions, suggests that these frogs do not endure cold winters. R. ridibunda is widespread in Southern France and its distribution suggests multiple introductions. It is concluded that water frogs conform to the mosaic zone model rather than to the tension zone model because: (i) taxa exhibited differences in habitat use, (ii) pure parental species were documented and (iii) hybrids are not unfit relative to parental species.     

Call Variation in the Bufo microscaphus Complex: Implications for Species Boundaries and the Evolution of Mate Recognition

Since the Modern Synthesis, many species concepts have considered characters mediating mate recognition essential to both the identification and definition of species. To explore divergence in mate recognition systems, calls were recorded and toads measured from all three species of the monophyletic Bufo microscaphus species complex including five populations of Bufo californicus, three populations of B. microscaphus, and one population of B. mexicanus. Call duration, dominant frequency, and pulse rate were significantly related to temperature, but not snout-vent length. When adjusted for temperature, calls of B. californicus had a significantly longer call duration, higher dominant frequency, and slower pulse rate than B. mexicanus and B. microscaphus, which did not differ from one another. However, the magnitude of variation among populations of B. californicus was similar to that between species. Discriminant analysis using call variables provided some separation of B. californicus and B. mexicanus from B. microscaphus, and cross-validation analysis correctly classified approximately 75% of B. californicus and B. mexicanus. Given only slight divergence in mate recognition systems between B. californicus and the other two taxa, the biological significance of this difference remains ambiguous. Comparisons among B. americanus group members indicate that hybridizing taxa may or may not exhibit divergence in advertisement calls. Maintenance of independently evolving lineages may be driven by other evolutionary mechanisms. These results support the notion that species recognition is best viewed as an effect of mate recognition.     

Strong premating divergence in a unimodal hybrid zone between two subspecies of the house mouse

Although selection against hybridization is expected to generate prezygotic divergence in unimodal hybrid zones, such a pattern has been seldom described. This study aims to better understand how prezygotic mechanisms may evolve in such zones. We investigated prezygotic divergence between populations of two subspecies of mice (Mus musculus musculus and M. m. domesticus) located at the edges of their unimodal hybrid zone in Denmark, and we developed an original multiple-population choice-test design, which allows assessment of within and between subspecies variation. Our study demonstrates that a strong assortative preference characterises one of the two subspecies (musculus) and that urinary signals are involved in this subspecies recognition. Taking into account the specific genetic and geographical characteristics of the Danish hybrid zone, we discuss the influence of the above pattern on its fate and the mechanisms that could have favoured this prezygotic divergence, among which the role of recombined populations constituting the core of the zone.     

Stream Noise,Hybridization, and Uncoupled Evolution of Call Traits in Two Lineages of Poison Frogs: Oophaga histrionica and Oophaga lehmanni

According to the acoustic adaptation hypothesis, communication signals are evolutionary shaped in a way that minimizes its degradation and maximizes its contrast against the background noise. To compare the importance for call divergence of acoustic adaptation and hybridization, an evolutionary force allegedly promoting phenotypic variation, we compared the mate recognition signal of two species of poison frogs (Oophaga histrionica and O. lehmanni) at five localities: two (one per species) alongside noisy streams, two away from streams, and one interspecific hybrid. We recorded the calls of 47 males and characterized the microgeographic variation in their spectral and temporal features, measuring ambient noise level, body size, and body temperature as covariates. As predicted, frogs living in noisy habitats uttered high frequency calls and, in one species, were much smaller in size. These results support a previously unconsidered role of noise on streams as a selective force promoting an increase in call frequency and pleiotropic effects in body size. Regarding hybrid frogs, their calls overlapped in the signal space with the calls of one of the parental lineages. Our data support acoustic adaptation following two evolutionary routes but do not support the presumed role of hybridization in promoting phenotypic diversity.     

Geographic variation of genetic and behavioral traits in northern and southern tüngara frogs

We use a combination of microsatellite marker analysis and mate-choice behavior experiments to assess patterns of reproductive isolation of the túngara frog Physalaemus pustulosus along a 550-km transect of 25 populations in Costa Rica and Panama. Earlier studies using allozymes and mitochondrial DNA defined two genetic groups of túngara frogs, one ranging from Mexico to northern Costa Rica (northern group), the second ranging from Panama to northern South America (southern group). Our more fine-scale survey also shows that the northern and southern túngara frogs are genetically different and geographically separated by a gap in the distribution in central Pacific Costa Rica. Genetic differences among populations are highly correlated with geographic distances. Temporal call parameters differed among populations as well as between genetic groups. Differences in calls were explained better by geographic distance than by genetic distance. Phonotaxis experiments showed that females preferred calls of males from their own populations over calls of males from other populations in about two-thirds to three-fourths of the contrasts tested. In mating experiments, females and males from the same group and females from the north with males from the south produced nests and tadpoles. In contrast, females from the south did not produce nests or tadpoles with males from the north. Thus, northern and southern túngara frogs have diverged both genetically and bioacoustically. There is evidence for some prezygotic isolation due to differences in mate recognition and fertilization success, but such isolation is hardly complete. Our results support the general observation that significant differences in sexual signals are often not correlated with strong genetic differentiation.     

Character convergence in advertisement call and mate choice in two genetically distinct water frog hybridogenetic lineages (Rana kl esculenta, Rana kl grafi)

Patterns of advertisement call were investigated in two genetically distinct water frog lineages ( Rana kl esculenta, Rana. kl grafi ), which were identified by starch gel electrophoresis – the aim being to determinate the role of vocalization in the hybridogenetic process. Both hybrids displayed major modifications from the basic structure of the Rana ridibunda call. In Rana kl grafi , the call structure tended to correspond to that of Rana perezi in most of the studied parameters (frequency, duration, number of pulses) whereas the call of Rana kl esculenta tended to resemble that of Rana lessonae . The ascendant hierarchical classification clearly revealed such a convergence toward parental species and accounted for a divergence between hybrids. Changes in call patterns might result from both the expression of the non- ridibunda genome and the sexual selective pressure through female mate choice. Only few non- ridibunda females exhibited a preference for hybrid calls which, however, allowed some hybridogenetic males to obtain successful mates. Thus, hybridogenesis induced character convergence in courtship signal with the non- ridibunda species in hybrid zones. In any case, these changes in the courtship signal favoured the particular hybridogenetic process, which constitutes a quasi-parasitism of the non- ridibunda genome.     

Genomic basis for an informed conservation management of Pelophylax water frogs in Luxembourg

Genetic identification methods have become increasingly important for species that are difficult to identify in the field. A case in point is Pelophylax water frogs. While their morphological determination is highly complex, they include species protected under EU law and some that are classified as invasive. Additionally, genetic data can provide insights into their complex breeding systems, which may or may not involve the reproductive dependency of one species on another. Here, we generate baseline data for water frog monitoring in Luxembourg. We applied a countrywide sampling approach and used SNPs generated by ddRAD sequencing to identify individuals and infer the breeding systems present in the country. We found Pelophylax lessonae and P. kl. esculentus throughout Luxembourg, mostly living in syntopy. In general, a reproductive dependency of P. kl. esculentus on P. lessonae (L‐E system) was revealed. Besides this general system, we detected triploid P. kl. esculentus in six ponds. This indicates a modified L‐E system with reproductive dependency of the triploids on the diploid P. kl. esculentus. The invasive P. cf. bedriagae was detected in three ponds in southern Luxembourg, with evidence for hybridization with native water frogs. In addition to the ddRAD data, we tested a simple genetic method for future monitoring based on the MND1 marker. It showed in almost all cases, an identical species identification as the ddRAD data and was successfully applied to DNA extracts from mouth swabs. Combining this method with our baseline data will enable informed choices for the protection of native water frog species in Luxembourg.     

Mechanisms of incomplete prezygotic reproductive isolation in an intertidal snail: testing behavioural models in wild populations

Two morphs (ecotypes) of the marine snail Littorina saxatilis coexist along Galician exposed rocky shores. They hybridize, but gene flow is impeded by a partial prezygotic reproductive barrier, and we have earlier suggested that this is a case of incipient sympatric speciation. To assess the mechanisms of prezygotic reproductive isolation, we estimated deviations from random mating (sexual selection and sexual isolation) of sympatric snails in 13 localities on the shore, and performed mate choice experiments in the laboratory. We also investigated the microdistribution of both morphs over patches of barnacles and blue mussels in the hybridization zone. We used computer simulations to separate the mechanisms contributing to reproductive isolation. On the shores sampled, male–female pairs were strongly assortative both with respect to morphs (mean Yule's V = 0.77) and size (mean Pearson's r = 0.47). In the laboratory, males of both morphs mounted other snails and mated other males and juveniles at random. However, mature females of equal sizes mated assortatively with respect to morph. The two morphs were nonrandomly distributed over barnacle and mussel patches in the hybridization zone. Monte Carlo simulations showed that this microdistribution could explain about half the morph and size relationships in male–female pairs, while a simple rejection mechanism, rejecting the first 1–3 mates if they were of contrasting morphs, accounted for the remaining part of the reproductive isolation, and for parts of the size relationships found between mates. A size discriminant mate choice mechanism may also, to a lesser extent, contribute to the sexual isolation. Sexual selection was observed for female size (larger ones being favoured) and among certain morphs, but distinct biological mechanisms may cause these processes.     

Reproductive isolation in birds: postcopulatory prezygotic barriers

Reproductive isolation is an important part of the speciation process. Recent studies of birds have highlighted not only the significance of postcopulatory postzygotic barriers, but also the almost complete absence of information about postcopulatory prezygotic barriers. Here, we draw attention to studies that provide an opportunity to test whether prezygotic barriers to heterospecific sperm exist in birds. We show that, compared with other taxa, such barriers in birds are relatively inefficient, possibly because, similar to postcopulatory postzygotic barriers, they take a long time to evolve. These data also raise questions about the mechanisms of sperm-female and sperm-egg recognition in birds. Future research will serve the dual purpose of providing more detail of the mechanisms of both heterospecific and conspecific prezygotic processes.     

Aposematism increases acoustic diversification and speciation in poison frogs

Multimodal signals facilitate communication with conspecifics during courtship, but they can also alert eavesdropper predators. Hence, signallers face two pressures: enticing partners to mate and avoiding detection by enemies. Undefended organisms with limited escape abilities are expected to minimize predator recognition over mate attraction by limiting or modifying their signalling. Alternatively, organisms with anti-predator mechanisms such as aposematism (i.e. unprofitability signalled by warning cues) might elaborate mating signals as a consequence of reduced predation. We hypothesize that calls diversified in association with aposematism. To test this, we assembled a large acoustic signal database for a diurnal lineage of aposematic and cryptic/non-defended taxa, the poison frogs. First, we showed that aposematic and non-aposematic species share similar extinction rates, and aposematic lineages diversify more and rarely revert to the non-aposematic phenotype. We then characterized mating calls based on morphological (spectral), behavioural/physiological (temporal) and environmental traits. Of these, only spectral and temporal features were associated with aposematism. We propose that with the evolution of anti-predator defences, reduced predation facilitated the diversification of vocal signals, which then became elaborated or showy via sexual selection.     

Differential Evolution of Advertisement Call Traits in Dart-Poison Frogs (Anura: Dendrobatidae)

The ability to recognise and discriminate between heterospecific and conspecific individuals plays an essential role in mate choice, reproductive isolation and thus species diversification. Many animals discriminate based on advertisement calls, whose evolution may be driven by a variety of forces such as natural selection, sexual selection or stochastic processes. The relative importance of stochastic processes acting on a given trait is usually correlated with its phylogenetic signal. Mate-recognition signals are complex traits composed of multiple features that could potentially respond independently to evolutionary forces. The advertisement call of anurans is used in species recognition and mate choice. In this study, we estimate the phylogenetic signal for body size and a suite of traits describing the male advertisement call from dart-poison frogs (Anura: Dendrobatidae). We found a surprisingly high phylogenetic signal for all call traits. In addition, call traits varied in their degree of phylogenetic signal, suggesting that evolutionary forces have been acting differently on different traits. Pulse duration showed the strongest phylogenetic signal. Peak frequency and body size were correlated and presented high phylogenetic signal indicating that the evolution of one trait may be driving or constraining the other. Since most variation in call traits can be explained by the phylogenetic history of the species, we cannot reject the hypothesis that stochastic processes account for significant evolutionary divergence in frog calls.     

Reproductive isolation among deep-water cichlid fishes of Lake Malawi differing in monochromatic male breeding dress

Male nuptial colour hues are important for the maintenance of reproductive isolation among cichlid fish species, and environmental changes that lead to narrower light spectra can lead to hybridization. However, cichlid species can naturally co-occur in narrow light spectrum habitats, such as turbid shallow lakes and the deep benthic zones of African rift lakes. Closely related species from narrow light spectrum habitats tend to differ little in the palette of male nuptial colours, thus for these taxa differences in colour patterns may be more important than differences in colour hue for species recognition. To investigate this hypothesis we examined morphometric and genetic differentiation among males of four sympatric putative species within the deep-water genus Diplotaxodon. These taxa live in a narrow-light spectrum environment where only blue light is present, and males differ primarily in 'monochromatic' black, white and silver patterning of the body and fins. Significant genetic differentiation was present among taxa in both microsatellite DNA and mitochondrial DNA, including one pair with no significant morphometric differentiation. Thus, these taxa represent reproductively isolated biological species, a result consistent with male nuptial patterning being important for species recognition and assortative mating. As such, we suggest that narrow-light spectra need not always represent barriers to effective visually mediated mate recognition.     

Reproductive isolating mechanisms and molecular phylogenetic relationships among Palearctic and Oriental brown frogs

Crossing experiments were made among various brown frog species and populations collected from Japan, China, Russia and Taiwan. The main purpose of these experiments was to confirm the existence of reproductive isolating mechanisms among Rana pirica from Japan, R. chensinensis from China and R. chensinensis from Russia, and between these three taxa and the other brown frogs distributed in the Palearctic and Oriental regions. It was found that there was no or a slight gametic isolation among the three taxa. While there was a nearly equal number of male and female offspring in the control groups, the hybrid frogs were all males, and completely sterile upon attaining sexual maturity. Thus, each of the Japanese R. pirica and the Russian R. chensinensis is a valid species, distinct from the Chinese R. chensinensis. The phylogenetic tree based on nucleotide sequence data from the mitochondrial 12S and 16S rRNA genes of the Palearctic and Oriental brown frogs showed that the three taxa are included in a cluster together with the other species with 2n=24 chromosomes. The present crossing experiments and molecular data support the hypothesis that each of them is a separate but closely related species.     

Male advertisement call characters as phylogeographical indicators in European water frogs

Anuran vocalizations are valuable in the determination of species identity and have the potential to discriminate intraspecific variation. We developed novel bioacoustic sampling techniques, based on Fast Fourier Transforms, to increase the precision and sensitivity of male advertisement call analysis and applied the method to European water frogs. This approach robustly separated the three types of north European water frogs (Rana ridibunda, R. lessonae and their viable, fertile hybrid R. esculenta) by their call subunit characteristics. The hybrid frog exhibited a high frequency call component absent from both the parental species. Furthermore, call analysis demonstrated significant intraspecific differences among populations of all three frog types. Call characters of R. ridibunda changed systematically as a function of longitude. This trend may reflect either clinal variation in selection pressures across Europe, or the consequences of drift following postglacial colonization from eastern refugia. High resolution vocalization analysis therefore provides a potentially useful method for investigating intraspecific differentiation and the phylogeographical origins of anuran distributions. © 2002 The Linnean Society of London. Biological Journal of the Linnean Society, 2002, 77 , 355?365.     

Advertisement Call Variation in the Golden Rocket Frog (Anomaloglossus beebei): Evidence for Individual Distinctiveness

Individual distinctiveness in acoustic signals can allow receivers to respond appropriately to different individuals. The aim of this research was to examine signal variation and to investigate the relative importance of different acoustic properties for coding individual distinctiveness in the advertisement calls of male golden rocket frogs (Anomaloglossus beebei). We examined patterns of within‐individual and among‐individual variability in 760 advertisement calls of 40 males as well as repeatability in 16 additional males that were recorded on two different occasions. We examined eight call properties, and all properties exhibited significant among‐individual variation and moderate to high repeatability across days. We employed discriminant function analysis to examine individual distinctiveness statistically. These analyses assigned 79% of calls to the correct individual. Fine temporal properties, including pulse duration, pulse rate, and pulse interval, as well as the spectral property of dominant frequency, contributed most toward discrimination among individuals. These results indicate that individual male golden rocket frogs can be distinguished statistically by their advertisement calls. We use these findings to evaluate results of previous playback studies in A. beebei and to generate testable predictions regarding the potential for specific call properties to function in mate selection and social recognition.     

Why do animals hybridize?

Animal hybridization is increasingly recognized as common and evolutionarily important, but the role of behavior in promoting hybridization events is not well understood. Understanding the behavioral causes of hybridization requires understanding the ecological, demographic, and phenotypic influences on mate choice in hybridizing taxa. Here, I review how these influences on mate choice can contribute to hybridization by (1) circumventing (female) choice, (2) bringing formerly isolated species into sympatry, (3) masking cues important for mate recognition, (4) altering the traits or preferences involved in mate recognition, or (5) altering the costs and benefits of mate choice. In particular, hybridization in response to either high direct costs of mate choice or high direct benefits of heterospecific mating may be widespread, a possibility that awaits further testing. Adopting a mate choice perspective to the study of hybridization challenges the assumption of hybrids as reproductive “mistakes” and allows for functional explanations regarding the occurrence and maintenance of hybridization. As evidence of the prevalence and evolutionary importance of animal hybridization accumulates, investigations into the role of behavior will be increasingly important for our understanding of diversification and for conservation applications.     

Mechanisms of prezygotic reproductive isolation between two sympatric species, Gelsemium rankinii and G. sempervirens (Gelsemiaceae), in the southeastern United States

Natural hybridization plays a critical role in speciation, the maintenance of reproductive isolation, and genetic introgression. While many plant species have hybrid swarms in areas of sympatry, the lack of hybrids among closely related sympatrically distributed species suggests that strong pre- and/or postzygotic barriers exist to hybridization. Gelsemium sempervirens and G. rankinii (Gelsemiaceae) are sympatrically distributed southeastern sister taxa that have strong postzygotic barriers to hybrid formation and high levels of genetic differentiation. In this study, two sympatric populations in Lowndes County, Georgia were surveyed from 1999-2005 to assess the role of temporal and pollinator isolation as potential prezygotic barriers. The populations had mostly non-overlapping flowering periods in 2003-2005, with significant differences in time of peak flowering and length of flowering. Both species shared a similar community of flower visitors, with the apid bee Habropoda laboriosa the dominant visitor to both species. A choice experiment found that H. laboriosa visited both species but preferred G. sempervirens. The primary prezygotic barrier is temporal isolation preventing hybridization in spite of the shared pollinators. This study suggests that reliance on a shared pollinator during speciation may limit opportunity for divergent selection on flowering time.     

SPECIES RECOGNITION AND SEXUAL SELECTION AS A UNITARY PROBLEM IN ANIMAL COMMUNICATION

We investigated patterns of mating call preference and mating call recognition by examining phonotaxis of female túngara frogs, Physalaemus pustulosus, in response to conspecific and heterospecific calls. There are four results: females always prefer conspecific calls; most heterospecific calls do not elicit phonotaxis; some heterospecific calls do elicit phonotaxis and thus are effective mate recognition signals; and females prefer conspecific calls to which a component of a heterospecific call has been added to a normal conspecific call. We use these data to illustrate how concepts of species recognition and sexual selection can be understood in a unitary framework by comparing the distribution of signal traits to female preference functions.     

Multiple hybridization in the Aristolochia kaempferi group (Aristolochiaceae): evidence from reproductive isolation and molecular phylogeny

Hybridization via distributional changes should be an important factor for plant speciation. Previous cpDNA analyses of the Aristolochia kaempferi group, comprising six taxa in East Asia, showed a distinct phylogeographic structure resulting from distributional changes brought about by paleoclimatic oscillations. However, the cpDNA phylogeny was incongruent with morphologically defined taxa. To explore the evolutionary processes responsible for the inconsistency between cpDNA and morphology, we made artificial crosses and performed phylogenetic analyses using multiple nuclear markers. All crosses among different taxa or cpDNA clades set fruit, if crossing direction is not considered. The five nuclear phylogenies mostly did not support either the taxa or the cpDNA clades. A combined analysis of cpDNA and the PI exon revealed the two major lineages in the group, lacking a prezygotic isolating barrier between them. However, an asymmetric prezygotic isolating barrier occurs between populations of the Japanese main islands and of other areas that belong to different cpDNA subclades. It seems reasonable to conclude that the development of a prezygotic isolating mechanism is not necessarily proportional to the degree of genetic divergence. These results suggested that species boundaries within the group are blurred due to speciational processes associated with multiple hybridization and introgression resulting from repeated contacts among differentiated populations.     

Female spadefoot toads compromise on mate quality to ensure conspecific matings

When high-quality conspecifics resemble heterospecifics, femalesmay be unable to engage effectively in both species recognition(identification of conspecifics) and mate-quality recognition(identification of high-quality mates). Consequently, femalesthat engage primarily in mate-quality recognition may riskheterospecific matings, and females that engage primarily inspecies recognition may risk mating with low-quality mates.I examined the evolutionary consequences of this conflict betweenspecies and mate-quality recognition in spadefoot toads, Speamultiplicata. I compared mate preferences and the fitness consequencesof these preferences in spadefoot toad populations that didand did not overlap with congeners. In non-overlapping populations,S. multiplicata females preferred an extreme call characterresembling that of heterospecifics, and they had more eggsfertilized. In overlapping populations, S. multiplicata females preferred those call characteristics that were closest to thenorm for their population, and they did not receive the benefitof enhanced fertilization success. Thus, S. multiplicata femalesappear to trade off species and mate-quality recognition, suchthat those co-occurring with heterospecifics forgo the benefitsof high-quality matings to ensure conspecific matings. Theseresults suggest that the interaction between species and mate-qualityrecognition may influence mate choice decisions in importantand nonintuitive ways.