Patterns of differential introgression in a salamander hybrid zone: inferences from genetic data and ecological niche modelling

Hybrid zones have yielded considerable insight into many evolutionary processes, including speciation and the maintenance of species boundaries. Presented here are analyses from a hybrid zone that occurs among three salamanders –Plethodon jordani, Plethodon metcalfi and Plethodon teyahalee– from the southern Appalachian Mountains. Using a novel statistical approach for analysis of non‐clinal, multispecies hybrid zones, we examined spatial patterns of variation at four markers: single‐nucleotide polymorphisms (SNPs) located in the mtDNA ND2 gene and the nuclear DNA ILF3 gene, and the morphological markers of red cheek pigmentation and white flecks. Concordance of the ILF3 marker and both morphological markers across four transects is observed. In three of the four transects, however, the pattern of mtDNA is discordant from all other markers, with a higher representation of P. metcalfi mtDNA in the northern and lower elevation localities than is expected given the ILF3 marker and morphology. To explore whether climate plays a role in the position of the hybrid zone, we created ecological niche models for P. jordani and P. metcalfi. Modelling results suggest that hybrid zone position is not determined by steep gradients in climatic suitability for either species. Instead, the hybrid zone lies in a climatically homogenous region that is broadly suitable for both P. jordani and P. metcalfi. We discuss various selective (natural selection associated with climate) and behavioural processes (sex‐biased dispersal, asymmetric reproductive isolation) that might explain the discordance in the extent to which mtDNA and nuclear DNA and colour‐pattern traits have moved across this hybrid zone.     

Spatiotemporally consistent genomic signatures of reproductive isolation in a moving hybrid zone

Studies of hybrid zone dynamics often investigate a single sampling period and draw conclusions from that temporal snapshot. Stochasticity can, however, result in loci with spurious outlier patterns, which is exacerbated by limited temporal or geographic sampling. Comparing admixed populations from different geographic regions is one way to detect repeatedly divergent genomic regions potentially involved in reproductive isolation. Temporal comparisons also allow us to control partially for the role of stochasticity, but the power of temporal sampling has not yet been adequately explored. In North America, black‐capped (Poecile atricapillus) and Carolina (P. carolinensis) chickadees hybridize in a contact zone extending from New Jersey to Kansas. The hybrid zone is likely maintained by strong intrinsic selection against hybrids, and it is moving north. We used a reduced representation genomic approach and temporally spaced sampling—two samples of ～80 individuals separated by a decade—to determine the pattern and consistency of selection and genomic introgression in the chickadee hybrid zone. We report consistently low introgression for highly divergent loci between P. atricapillus and P. carolinensis in this moving hybrid zone. This is strong evidence that these loci may be linked to genomic regions involved in reproductive isolation between chickadees.     

Kin selection may contribute to lek evolution and trait introgression across an avian hybrid zone

Understanding the mechanism(s) that favour cooperation among individuals competing for the same resources provides direct insights into the evolution of grouping behaviour. In a hybrid zone between golden-/yellow-collared (Manacus vitellinus) and white-collared (Manacus candei) manakins, males form aggregations composed of white and yellow males solely to attract females ('mixed leks'). Previous work shows that yellow males in these mixed leks experience a clear mating advantage over white males, resulting in the preferential introgression of yellow plumage allele(s) into the white species. However, the yellow male mating advantage only occurs in mixed leks with high frequencies of yellow males, and only a few of these males probably mate. Hence, it remains unclear why unsuccessful males join leks. Here, we used microsatellite markers to estimate pairwise relatedness among males within and between leks to test whether indirect genetic benefits of helping kin ('kin selection') can promote grouping. We found that yellow males are significantly more related to each other within than between leks, while relatedness among white males did not differ within and between leks. This suggests that yellow males may indirectly enhance their own reproductive success by preferentially lekking with relatives because yellow plumage is under positive frequency-dependent selection (positive FDS). Our results are consistent with the hypothesis that kin selection may promote grouping and facilitate positive FDS for yellow males, mediating the movement of yellow plumage across this hybrid zone.     

Sharp acoustic boundaries across an altitudinal avian hybrid zone despite asymmetric introgression

Birdsong is a sexually selected trait that could play an important evolutionary role when related taxa come into secondary contact. Many songbird species, however, learn their songs through copying one or more tutors, which complicates the evolutionary outcome of such contact. Two subspecies of a presumed vocal learner, the grey‐breasted wood‐wren (Henicorhina leucophrys), replace each other altitudinally across the western slope of the Ecuadorian Andes. These subspecies are morphologically very similar, but show striking differences in their song. We examined variation in acoustic traits and genetic composition across the altitudinal range covered by both subspecies and between two allopatric populations. The acoustic boundary between the subspecies was found to be highly abrupt across a narrow elevational range with virtually no evidence of song convergence. Mixed singing and use of hetero‐subspecific song occurred in the contact zone and was biased towards the use of leucophrys song types. Hetero‐subspecific song copying by hilaris and not by leucophrys reflected a previously found asymmetric pattern of response to song playback. Using amplified fragment length polymorphisms (AFLP) markers, we detected hybridization in the contact zone and asymmetric introgression in parapatric populations, with more leucophrys alleles present in hilaris populations than vice versa. This pattern may be a trail of introgression due to upslope displacement of leucophrys by hilaris. Our data suggest that song learning may impact speciation and hybridization in contrasting ways at different spatial scales: although learning may speed up population divergence in songs, thereby enhancing assortative mating and reducing gene flow, it may at a local level also lead to the copying of heterospecific songs, therefore allowing some level of hybridization and introgression.     

Changing climate mediates sapsucker (Aves: Sphyrapicus) hybrid zone movement

Hybrid zones, where two divergent taxa meet and interbreed, offer unique opportunities to investigate how climate contributes to reproductive isolation between closely related taxa and how these taxa may respond to climatic changes. Red‐naped (Sphyrapicus nuchalis) and Red‐breasted (Sphyrapicus ruber) sapsuckers (Aves: Picidae) hybridize along a narrow contact zone that stretches from northern California to British Columbia. The hybrid zone between these species has been studied extensively for more than 100 years and represents an excellent system for investigations of the evolution of reproductive isolation. Shifts in the proportions of phenotypes at hybrid localities since 1910 that were inferred using specimens from museum collections were confirmed using species distribution models. We predicted the historical, current, and future distributions of parental and hybrid sapsuckers using Random Forests models to quantify how climate change is affecting hybrid zone movement in the Pacific Northwest. We found observed distribution shifts of parental sapsuckers were likely the result of climate change over the past 100 years, with these shifts predicted to continue for both sapsuckers over the next 80 years. We found Red‐breasted Sapsuckers are predicted to continue to expand, while Red‐naped Sapsuckers are predicted to contract substantially under future climate scenarios. As a result of the predicted changes, the amount of overlap in the distribution of these sapsuckers may decrease. Using hybrid phenotypes, we found the climate niche occupied by the hybrid zone is predicted to disappear under future conditions. The disappearance of this climate niche where the two parental species come into contact and hybridize may lead to a substantial reduction in genetic introgression. Understanding the impacts of global climate change on hybrid zones may help us to better understand how speciation has been shaped by climate in the past, as well as how evolution may respond to climate change in the future.     

Differential introgression across newt hybrid zones: Evidence from replicated transects

Genomic heterogeneity of divergence between hybridizing species may reflect heterogeneity of introgression, but also processes unrelated to hybridization. Heterogeneous introgression and its repeatability can be directly tested in natural hybrid zones by examining multiple transects. Here, we studied hybrid zones between the European newts Lissotriton montandoni and two lineages of Lissotriton vulgaris, with replicate transects within each zone. Over 1,000 nuclear genes located on a linkage map and mitochondrial DNA were investigated using geographical and genomic clines. Overall, the five transects were all similar, showing hallmarks of strong reproductive isolation: bimodal distribution of genotypes in central populations and narrow allele frequency clines. However, the extent of introgression differed between the zones, possibly as a consequence of their different ages, as suggested by the analysis of heterozygosity runs in diagnostic markers. In three transects genomic signatures of small‐scale (~2 km) zone movements were detected. We found limited overlap of cline outliers between transects, and only weak evidence of stronger differentiation of introgression between zones than between transects within zones. Introgression was heterogeneous across linkage groups, with patterns of heterogeneity similar between transects and zones. Predefined candidates for increased or reduced introgression exhibited only a subtle tendency in the expected direction, suggesting that interspecific differentiation is not a reliable indicator for the strength of introgression. These hierarchically sampled hybrid zones of apparently different ages show how introgression unfolds with time and offer an excellent opportunity to dissect the dynamics of hybridization and architecture of reproductive isolation at advanced stages of speciation.     

Ecological and genetic associations across a Heliconius hybrid zone

Differences in habitat use can bridge early and late stages of speciation by initiating assortative mating. Heliconius colour pattern races might select habitats over which each pattern confers a relative fitness advantage because signal efficacy of wing patterns can vary by environment. Thus habitat preferences could serve to promote the evolution of mimetic colour patterns for mate choice. Here I compare colour pattern genotype and phenotype frequencies to environmental variation across the H. erato hydara x H. erato erato hybrid zone in French Guiana to determine whether races exhibit habitat preferences. I found that genotype and phenotype frequencies correspond to differences in land cover moreso than to other environmental factors. Temporal shifts in colour pattern genotypes, phenotypes and land cover also were associated at individual sample sites, which further suggests that H. erato races differ in habitat use and that habitat preferences may promote speciation among Heliconius butterflies.     

Spatial variation in gene flow across a hybrid zone reveals causes of reproductive isolation and asymmetric introgression in wall lizards*

Hybrid zones provide insights into the evolution of reproductive isolation. Sexual selection can contribute to the evolution of reproductive barriers, but it remains poorly understood how sexual traits impact gene flow in secondary contact. Here, we show that a recently evolved suite of sexual traits that function in male-male competition mediates gene flow between two lineages of wall lizards (Podarcis muralis). Gene flow was relatively low and asymmetric in the presence of exaggerated male morphology and coloration compared to when the lineages share the ancestral phenotype. Putative barrier loci were enriched in genomic regions that were highly differentiated between the two lineages and showed low concordance between the transects. The exception was a consistently low genetic exchange around ATXN1, a gene that modulates social behavior. We suggest that this gene may contribute to the male mate preferences that are known to cause lineage-assortative mating in this species. Although female choice modulates the degree of reproductive isolation in a variety of taxa, wall lizards demonstrate that both male-male competition and male mate choice can contribute to the extent of gene flow between lineages.     

Patterns of genetic,phenotypic, and acoustic variation across a chiffchaff (Phylloscopus collybita abietinus/tristis) hybrid zone

Characterizing patterns of evolution of genetic and phenotypic divergence between incipient species is essential to understand how evolution of reproductive isolation proceeds. Hybrid zones are excellent for studying such processes, as they provide opportunities to assess trait variation in individuals with mixed genetic background and to quantify gene flow across different genomic regions. Here, we combine plumage, song, mtDNA and whole‐genome sequence data and analyze variation across a sympatric zone between the European and the Siberian chiffchaff (Phylloscopus collybita abietinus/tristis) to study how gene exchange between the lineages affects trait variation. Our results show that chiffchaff within the sympatric region show more extensive trait variation than allopatric birds, with a large proportion of individuals exhibiting intermediate phenotypic characters. The genomic differentiation between the subspecies is lower in sympatry than in allopatry and sympatric birds have a mix of genetic ancestry indicating extensive ongoing and past gene flow. Patterns of phenotypic and genetic variation also vary between regions within the hybrid zone, potentially reflecting differences in population densities, age of secondary contact, or differences in mate recognition or mate preference. The genomic data support the presence of two distinct genetic clades corresponding to allopatric abietinus and tristis and that genetic admixture is the force underlying trait variation in the sympatric region—the previously described subspecies (“fulvescens”) from the region is therefore not likely a distinct taxon. In addition, we conclude that subspecies identification based on appearance is uncertain as an individual with an apparently distinct phenotype can have a considerable proportion of the genome composed of mixed alleles, or even a major part of the genome introgressed from the other subspecies. Our results provide insights into the dynamics of admixture across subspecies boundaries and have implications for understanding speciation processes and for the identification of specific chiffchaff individuals based on phenotypic characters.     

Discordant patterns of introgression across a narrow hybrid zone between two cryptic lineages of an Iberian endemic newt

The study of natural hybrid zones can illuminate aspects of lineage divergence and speciation in morphologically cryptic taxa. We studied a hybrid zone between two highly divergent but morphologically similar lineages (south‐western and south‐eastern) of the Iberian endemic Bosca's newt (Lissotriton boscai) in SW Iberia with a multilocus dataset (microsatellites, nuclear and mitochondrial genes). STRUCTURE and NEWHYBRIDS analyses retrieved few admixed individuals, which classified as backcrosses involving parental individuals of the south‐western lineage. Our results show asymmetric introgression of mtDNA beyond the contact from this lineage into the south‐eastern lineage. Analysis of nongeographic introgression patterns revealed asymmetries in the direction of introgression, but except for mtDNA, we did not find evidence for nonconcordant introgression patterns across nuclear loci. Analysis of a 150‐km transect across the hybrid zone showed broadly coincident cline widths (ca. 3.2–27.9 km), and concordant cline centres across all markers, except for mtDNA that is displaced ca. 60 km northward. Results from ecological niche modelling show that the hybrid zone is in a climatically homogenous area with suitable habitat for the species, suggesting that contact between the two lineages is unlikely to occur further south as their distributions are currently separated by an extensive area of unfavourable habitat. Taken together, our findings suggest the genetic structure of this hybrid zone results from the interplay of historical (biogeographic) and population‐level processes. The narrowness and coincidence of genetic clines can be explained by weak selection against hybrids and reflect a degree of reproductive isolation that is consistent with cryptic speciation.     

Plumage and mitochondrial DNA haplotype variation across a moving hybrid zone

We analyze variation in phenotypes and mitochondrial DNA (mtDNA) haplotypes over the breeding ranges of hermit and Townsend's warblers and across two of their three hybrid zones. Within these two hybrid zones, we demonstrate that the placement, shape, and width of transitions in seven plumage characters are remarkably similar, suggesting that a balance between dispersal and sexual selection keeps these hybrid zones narrow. A consistent asymmetry in these character transition curves suggests that Townsend's warblers have a selective advantage over hermit warblers, which is presumably due to the aggressive superiority of Townsend's over hermit males (Pearson and Rohwer 2000). An association between plumage and mtDNA haplotypes shows that pure Townsend's warblers, but not pure hermit warblers, immigrate into these hybrid zones, further supporting the competitive superiority of Townsend's warblers over hermit warblers. The mitochondrial haplotype transitions across these hybrid zones are much wider than the phenotypic transitions and provide no indication that the mtDNA haplotypes representing these two warblers are selectively maintained. More importantly, the phenotypically pure populations of Townsend's warblers throughout a 2,000-km coastal strip north of the Washington hybrid zones contain a preponderance of hermit warbler mtDNA haplotypes. This result suggests massive movement of the hybrid zone between these warblers during the 5,000 years since their most recent interglacial contact. We develop a model to explain the phenotypic and genetic divergence between these warblers and the evolution of their dramatic differences in aggressiveness; we also show how differences in male aggression, in combination with biased pairing patterns, can explain the haplotype footprint recording the historical movement of this hybrid zone.     

Genetic structure, introgression, and a narrow hybrid zone between northern and California spotted owls (Strix occidentalis)

The northern spotted owl (Strix occidentalis caurina) is a threatened subspecies and the California spotted owl (Strix occidentalis occidentalis) is a subspecies of special concern in the western United States. Concern for their continued viability has arisen because of habitat loss caused by timber harvesting. The taxonomic status of the northern subspecies has been the subject of continuing controversy. We investigated the phylogeographical and population genetic structure of northern and California spotted owls with special reference to their region of contact. Mitochondrial DNA (mtDNA) control region sequences confirmed the existence of two well-differentiated lineages connected by a narrow hybrid zone in a region of low population density in north central California. Maximum-likelihood estimates indicated bidirectional gene flow between the lineages but limited introgression outside the region of contact. The lengths of both the mtDNA hybrid zone and the reduced density patch were similar and slightly exceeded estimates of natal dispersal distances. This suggests that the two subspecies were in secondary contact in a hybrid zone trapped by a population density trough. Consequently, the zone of interaction is expected to be geographically stable. We discovered a third, rare clade of haplotypes, which we interpreted to be a result of incomplete lineage sorting; those haplotypes result in a paraphyletic northern spotted owl with respect to the California spotted owl. A congeneric species, the barred owl (Strix varia), occasionally hybridizes with spotted owls; our results indicated an upper bound for the frequency of barred owl mtDNA haplotypes in northern spotted owl populations of 3%.     

Sharp genetic discontinuity across a unimodal Heliconius hybrid zone

Hybrid zones are powerful natural systems to study evolutionary processes to gain an understanding of adaptation and speciation. In the Cauca Valley (Colombia), two butterfly races, Heliconius cydno cydnides and Heliconius cydno weymeri, meet and hybridize. We characterized this hybrid zone using a combination of mitochondrial DNA (mtDNA) sequences, amplified fragment length polymorphisms (AFLPs), microsatellites and sequences for nuclear loci within and outside of the genomic regions that cause differences in wing colour pattern. The hybrid zone is largely composed of individuals of mixed ancestry. However, there is strong genetic discontinuity between the hybridizing races in mtDNA and, to a lesser extent, in all nuclear markers surveyed. The mtDNA clustering of H. c. cydnides with the H. cydno race from the Magdalena Valley and H. c. weymeri with the H. cydno race from the pacific coast suggests that H. c. cydnides colonized the Cauca Valley from the north, whereas H. c. weymeri did so by crossing the Andes in the southern part, implying a secondary contact origin. Colonization of the valley by H. cydno was accompanied by mimicry shift. Strong ecological isolation, driven by locally adaptive differences in mimetic wing patterns, is playing an important role in maintaining the hybrid zone. However, selection on wing pattern alone is not sufficient to explain the genetic discontinuity observed. There is evidence for differences in male mating preference, but the contribution of additional barriers needs further investigation. Overall, our results support the idea that speciation is a cumulative process, where the combination of multiple isolation barriers, combined with major phenotypic differences, facilitates population divergence in face of gene flow.     

Genetical and ecological differentiation across a hybrid zone

ABSTRACT.

• 1 There is a narrow hybrid zone between two chromosomal races of the grasshopper Podisma pedestris (L.) in the French Alps.
• 2 There are no gross differences in habitat across the zone, and a multivariate analysis of the vegetation composition detected no significant transition either.
• 3 The hatch and development rates of Podisma in hybrid populations were lower than those found in matched habitat types outside the zone.
• 4 The simplest explanation for the persistence of the parapatric distribution of the two races is that hybrid disadvantage is maintaining a tension zone between them.

     

Patterns of introgression across an expanding hybrid zone: analysing historical patterns of gene flow using nonequilibrium approaches

Previous studies suggest that the hybrid zone between two taxa in the Piriqueta caroliniana complex in central Florida (south-eastern North America) has recently expanded with hybrids replacing parental genotypes across a broad region of the Florida peninsula. Here I use patterns of genetic disequilibria and levels of differentiation among populations to infer historical patterns of introgression across this broad hybrid zone. There were strong positive associations among taxon-specific alleles at the southern extent of hybridization, but disequilibria values were close to zero across the central and northern portions of the hybrid zone. Levels of among-population differentiation in the central portion of the hybrid zone were relatively low, and increased towards the northern extent of hybridization. The high levels of disequilibria are coincident with the sharp clines at the southern end of the hybrid zone, suggesting that there is a tension zone in this region that is maintained by selection against hybrids and dispersal from parental regions. The levels of disequilibria within populations and patterns of differentiation among populations are consistent with historical introgression and northward expansion of this hybrid zone, which may have slowed or ceased in recent generations.     

Genetic exchange across a hybrid zone within the Iberian endemic golden-striped salamander, Chioglossa lusitanica

The study of hybrid zones resulting from Pleistocene vicariance is central in examining the potential of genetically diverged evolutionary units either to introgress and merge or to proceed with further isolation. The hybrid zone between two mitochondrial lineages of Chioglossa lusitanica is located near the Mondego River in Central Portugal. We used mitochondrial and nuclear diagnostic markers to conduct a formal statistical analysis of the Chioglossa hybrid zone in the context of tension zone theory. Key results are: (i) cline centres are not coincident for all markers, with average widths of ca. 2-15 km; (ii) heterozygote deficit was not observed across loci near the transect centre; (iii) associations of parental allele combinations ('linkage disequilibrium'R) were not detected either across loci or across the transect. These observations suggest that the Chioglossa hybrid zone is not a tension zone with strong selection against hybrids but instead one shaped mostly by neutral mixing. The patterns uncovered suggest a complex history of populations over a small scale that may be common in southern Pleistocene refugia.     

Geographic structure of genetic and phenotypic variation in the hybrid zone between Quercus affinis and Q. laurina in Mexico

Analyzing the structure of hybrid zones is important for inferring their origin, dynamics and evolutionary significance. We examined the geographic structure of phenotypic and genetic variation in the contact zone between two Mexican red oaks, Quercus affinis and Q. laurina. A total of 105 individuals from seven populations were sampled along a 600‐km latitudinal gradient representing the distribution area of the two species and their contact zone. Individuals were genotyped for nine nuclear and four chloroplast DNA microsatellite loci (ncSSR and cpSSR, respectively), and characterized for several leaf and acorn traits. The cpSSR data revealed extensive haplotype sharing among populations of the two species, while a Bayesian assignment analysis based on ncSSRs identified two main genetic groups, each corresponding to one of the species, and two populations in the contact zone showing evidence of admixture. The proportion of genetic ancestry in the populations was strongly associated with latitude and showed a pattern of variation with the shape of a narrow sigmoidal cline. The variation in three of the seven phenotypic traits was partially congruent with molecular variation, while the other traits did not conform to a geographic cline but instead were correlated with environmental variables. In conclusion, the hybrid zone between the two oak species has some of the characteristics of a tension zone, but heterogeneous variation across traits suggests differential introgression and the action of extrinsic selection.     

Stabilization of a salamander moving hybrid zone

When related species meet upon postglacial range expansion, hybrid zones are frequently formed. Theory predicts that such zones may move over the landscape until equilibrium conditions are reached. One hybrid zone observed to be moving in historical times (1950–1979) is that of the pond‐breeding salamanders Triturus cristatus and Triturus marmoratus in western France. We identified the ecological correlates of the species hybrid zone as elevation, forestation, and hedgerows favoring the more terrestrial T. marmoratus and pond density favoring the more aquatic T. cristatus. The past movement of the zone of ca. 30 km over three decades has probably been driven by the drastic postwar reduction of the “bocage” hedgerow landscape, favoring T. cristatus over T. marmoratus. No further hybrid zone movement was observed from 1979 to the present. To explain the changing dynamics of the hybrid zone, we propose that it stalled, either because an equilibrium was found at an altitude of ca. 140 m a.s.l. or due to pond loss and decreased population densities. While we cannot rule out the former explanation, we found support for the latter. Under agricultural intensification, ponds in the study area are lost at an unprecedented rate of 5.5% per year, so that remaining Triturus populations are increasingly isolated, hampering dispersal and further hybrid zone movement.     

Characterizing neutral and adaptive genomic differentiation in a changing climate: The most northerly freshwater fish as a model

Arctic freshwater ecosystems have been profoundly affected by climate change. Given that the Arctic charr (Salvelinus alpinus) is often the only fish species inhabiting these ecosystems, it represents a valuable model for studying the impacts of climate change on species life‐history diversity and adaptability. Using a genotyping‐by‐sequencing approach, we identified 5,976 neutral single nucleotide polymorphisms and found evidence for reduced gene flow between allopatric morphs from two high Arctic lakes, Linne'vatn (Anadromous, Normal, and Dwarf) and Ellasjøen (Littoral and Pelagic). Within each lake, the degree of genetic differentiation ranged from low (Pelagic vs. Littoral) to moderate (Anadromous and Normal vs. Dwarf). We identified 17 highly diagnostic, putatively adaptive SNPs that differentiated the allopatric morphs. Although we found no evidence for adaptive differences between morphs within Ellasjøen, we found evidence for moderate (Anadromous vs. Normal) to high genetic differentiation (Anadromous and Normal vs. Dwarf) among morphs within Linne'vatn based on two adaptive loci. As these freshwater ecosystems become more productive, the frequency of sympatric morphs in Ellasjøen will likely shift based on foraging opportunities, whereas the propensity to migrate may decrease in Linne'vatn, increasing the frequency of the Normal morph. The Dwarf charr was the most genetically distinct group. Identifying the biological basis for small body size should elucidate the potential for increased growth and subsequent interbreeding with sympatric morphs. Overall, neutral and adaptive genomic differentiation between allopatric and some sympatric morphs suggests that the response of Arctic charr to climate change will be variable across freshwater ecosystems.     

Molecular patterns of introgression in a classic hybrid zone between the Australian tree frogs,Litoria ewingii and L. paraewingi: evidence of a tension zone

Hybrid zones provide a rare opportunity to explore the processes involved in reproductive isolation and speciation. The southern hybrid zone between the southeastern Australian tree frogs Litoria ewingii and L. paraewingi has been comprehensively studied over the last 40 years, primarily using reproductive compatibility experiments and male advertisement calls. We used mitochondrial DNA (mtDNA) and eight nuclear microsatellite markers to characterize this hybrid zone along a historically studied transect and to test various dispersal‐dependent and dispersal‐independent hybrid zone models. The species are genetically distinct and the level of hybridization within the contact zone is low, with the majority of admixed individuals representing later‐generation hybrids. Based on previous experimental genetic compatibility studies, we predicted that hybrids with L. paraewingi mtDNA would be more frequent than hybrids with L. ewingii mtDNA. Surprisingly, a greater proportion of the identified hybrids had L. ewingii mtDNA. Geographical cline analyses showed a sharp transition in allele frequencies across the transect, and both the mtDNA and microsatellite data showed concordant cline centres, but were best supported by a model that allowed width to vary. Overall, the L. ewingii–L. paraewingi hybrid zone is best characterized as a tension zone, due to the narrow cline width, concordant genetic clines and low levels of hybridization.