Narrow Hybrid Zone Research Articles

Different life strategies of closely related louse species in sympatry: specialist and “generalist“ lineages of Polyplax serrata

The origin and significance of host specificity are intriguing questions in parasitology. In the case of single-host versus multiple-host parasites, this topic integrates with the concept of the specialist/generalist trade-off. We use the model of sucking lice Polyplax serrata and rodent hosts Apodemus, to address these concepts. Polyplax serrata was shown to form a complex genetic structure, with a strictly specific S lineage living on Apodemus flavicollis, and a less specific N lineage on A. flavicollis and Apodemus sylvaticus. Moreover, the S lineage formed two mitochondrial clades with geographically exclusive distributions and a narrow hybrid zone, providing an opportunity to test the hypothesis that hybrids suffer a decrease in fitness. We sampled 451 individual lice from two host species at 103 localities. We used prevalences and intensities as proxies of fitness, which the parasites realize on their host. The S lineage, strictly specific to Apodemus flavicollis, reached significantly higher prevalences and intensities on its host compared with the N lineage. Conversely, the N lineage occurred with high prevalence and intensity on A. sylvaticus but tended to use also A. flavicollis when the louse populations became too dense. We discuss possible mechanisms behind this difference (particularly interspecific competition as a typical phenomenon in the specialist/generalist systems). We conclude that a parasite’s “choice”, not accessibility of the host or interspecific competition, is the main factor affecting the louse prevalences. We suggest that historical differences in geographic distribution of both lice and mice may provide a possible explanation for the observed life strategy differences. In contrast to the convincing picture in S and N lineage prevalences, we did not detect an expected drop in fitness in hybrids. We consider instability of the hybrid zone, or decline in abundance of the respective hosts, as possible explanations for this result.

Genetic and telomeric variability: Insights from a tropical avian hybrid zone.

Telomere lengths and telomere dynamics can correlate with lifespan, behaviour and individual quality. Such relationships have spurred interest in understanding variation in telomere lengths and their dynamics within and between populations. Many studies have identified how environmental processes can influence telomere dynamics, but the role of genetic variation is much less well characterized. To provide a novel perspective on how telomeric variation relates to genetic variability, we longitudinally sampled individuals across a narrow hybrid zone (n = 127 samples), wherein two Manacus species characterized by contrasting genome-wide heterozygosity interbreed. We measured individual (n = 66) and population (n = 3) differences in genome-wide heterozygosity and, among hybrids, amount of genetic admixture using RADseq-generated SNPs. We tested for population differences in telomere lengths and telomere dynamics. We then examined how telomere lengths and telomere dynamics covaried with genome-wide heterozygosity within populations. Hybrid individuals exhibited longer telomeres, on average, than individuals sampled in the adjacent parental populations. No population differences in telomere dynamics were observed. Within the parental population characterized by relatively low heterozygosity, higher genome-wide heterozygosity was associated with shorter telomeres and higher rates of telomere shortening-a pattern that was less apparent in the other populations. All of these relationships were independent of sex, despite the contrasting life histories of male and female manakins. Our study highlights how population comparisons can reveal interrelationships between genetic variation and telomeres, and how naturally occurring hybridization and genome-wide heterozygosity can relate to telomere lengths and telomere dynamics.

Maintenance of a narrow hybrid zone between native and introduced red foxes (Vulpes vulpes) despite conspecificity and high dispersal capabilities.

Human-facilitated introductions of nonnative populations can lead to secondary contact between allopatric lineages, resulting in lineage homogenisation or the formation of stable hybrid zones maintained by reproductive barriers. We investigated patterns of gene flow between the native Sacramento Valley red fox (Vulpes vulpes patwin) and introduced conspecifics of captive-bred origin in California's Central Valley. Considering their recent divergence (20-70 kya), we hypothesised that any observed barriers to gene flow were primarily driven by pre-zygotic (e.g. behavioural differences) rather than post-zygotic (e.g. reduced hybrid fitness) barriers. We also explored whether nonnative genes could confer higher fitness in the human-dominated landscape resulting in selective introgression into the native population. Genetic analysis of red foxes (n = 682) at both mitochondrial (cytochrome b + D-loop) and nuclear (19,051 SNPs) loci revealed narrower cline widths than expected under a simulated model of unrestricted gene flow, consistent with the existence of reproductive barriers. We identified several loci with reduced introgression that were previously linked to behavioural divergence in captive-bred and domestic canids, supporting pre-zygotic, yet possibly hereditary, barriers as a mechanism driving the narrowness and stability of the hybrid zone. Several loci with elevated gene flow from the nonnative into the native population were linked to genes associated with domestication and adaptation to human-dominated landscapes. This study contributes to our understanding of hybridisation dynamics in vertebrates, particularly in the context of species introductions and landscape changes, underscoring the importance of considering how multiple mechanisms may be maintaining lineages at the species and subspecies level.

Genomic islands of speciation harbor genes underlying coloration differences in a pair of Neotropical seedeaters.

Incomplete speciation can be leveraged to associate phenotypes with genotypes, thus providing insights into the traits relevant to the reproductive isolation of diverging taxa. We investigate the genetic underpinnings of the phenotypic differences between Sporophila plumbea and Sporophila beltoni. Sporophila beltoni has only recently been described based, most notably, based on differences in bill coloration (yellow vs. black in S. plumbea). Both species are indistinguishable through mtDNA or reduced-representation genomic data, and even whole-genome sequencing revealed low genetic differentiation. Demographic reconstructions attribute this genetic homogeneity to gene flow, despite divergence in the order of millions of generations. We found a narrow hybrid zone in southern Brazil where genetically, yet not phenotypically, admixed individuals appear to be prevalent. Despite the overall low genetic differentiation, we identified 3 narrow peaks along the genome with highly differentiated SNPs. These regions harbor 6 genes, one of which is involved in pigmentation (EDN3) and is a candidate for controlling bill color. Within the outlier peaks, we found signatures of resistance to gene flow, as expected for islands of speciation. Our study shows how genes related to coloration traits are likely involved in generating prezygotic isolation and establishing species boundaries early in speciation.

Molecular Markers and Taxonomic Explosion in Herpetology: More or Less Robust Taxonomy? True and False Advantages of DNA Markers

Despite the increasing deficit of taxonomic expertise, the number of newly described species since the early 2010s has grown exponentially. This growth is related to the increased use of DNA markers in taxonomic descriptions. However, routine use of DNA markers in taxonomy did not bring practical taxonomy closer to the theory. Species are unique lineages with irreversible evolutionary pathways, and only the presence of distinct populations within the same geographic range, or at least the presence of narrow hybrid zones between the parapatric ranges is a conclusive evidence of evolutionary irreversibility. In the case of allopatric populations, only very high genetic distances, suggesting several tens of millions of years of independent evolution, can be used for validation of species status. This problem cannot be solved by the broader introduction of genomic phylogenies, which also fail to provide robust criteria for evolutionary irreversibility. We can hardly suppose that robust validation of species status is applicable to all or most of hundreds of thousands of animal species, including 20,000 amphibians and reptiles. Instead, practical taxonomy should concentrate on describing recognizable species, maintaining a trade-off between sufficiently detailed descriptions of world biodiversity and the applicability of these descriptions for practical use and metaanalyses, not pretending that formally described species reflect real lineages with independent and irreversible evolutionary pathways. Simultaneously, the non-critical elevation of the taxonomic status of individual geographic populations, contrary to the declared purpose of better-focusing conservation efforts, often has the opposite effect, leaving many formally described taxa outside the conservation umbrella.

Phylogeography, hybrid zones and contemporary species boundaries in the south-eastern Australian smooth frogs (Anura: Myobatrachidae: Geocrinia)

Paleo-climatic fluctuations have driven episodic changes in species distributions, providing opportunities for populations to diverge in isolation and hybridise following secondary contact. Studies of phylogeographic diversity and patterns of gene flow across hybrid zones can provide insight into contemporary species boundaries and help to inform taxonomic and conservation inferences. Here we explore geographic diversity within the acoustically divergent yet morphologically conserved south-eastern Australian smooth frog complex and assess gene flow across a narrow hybrid zone using mitochondrial nucleotide sequences and nuclear genome-wide single nucleotide polymorphisms. Our analyses reveal the presence of an evolutionarily distinct taxon restricted to the Otway Plains and Ranges, Victoria, which forms a narrow (9–30 km wide), spatiotemporally stable (>50 years) hybrid zone with Geocrinia laevis, which we describe herein as a new species.

Hybrid zone analysis confirms cryptic species of banded newt and does not support competitive displacement since secondary contact.

When two putatively cryptic species meet in nature, hybrid zone analysis can be used to estimate the extent of gene flow between them. Two recently recognized cryptic species of banded newt (genus Ommatotriton) are suspected to meet in parapatry in Anatolia, but a formal hybrid zone analysis has never been conducted. We sample populations throughout the range, with a focus on the supposed contact zone, and genotype them for 31 nuclear DNA SNP markers and mtDNA. We determine the degree of genetic admixture, introgression, and niche overlap. We reveal an extremely narrow hybrid zone, suggesting strong selection against hybrids, in line with species status. The hybrid zone does not appear to be positioned at an ecological barrier, and there is significant niche overlap. Therefore, the hybrid zone is best classified as a tension zone, maintained by intrinsic selection against hybrids. While the two banded newt species can evidently backcross, we see negligible introgression and the pattern is symmetric, which we interpret as supporting the fact that the hybrid zone has been practically stationary since its origin (while extensive hybrid zone movement has been suggested in other newt genera in the region). Our study illustrates the use of hybrid zone analysis to test cryptic species status.

A new species of Pandanus-dwelling frog from northern Madagascar related to Guibemantis pulcher.

Populations of phytotelmic frogs from northern Madagascar assigned to Guibemantis (Pandanusicola) pulcher are known to differ genetically from populations further south in the eastern rainforest belt of the island, but to date, their status has not been analyzed in depth. We combined molecular genetic data with an examination of color pattern to clarify the taxonomy of these frogs. DNA sequences of both mitochondrial and nuclear-encoded genes were consistently differentiated between the northern populations and those occurring further south. Uncorrected pairwise distance in the 16S rRNA gene was 3.7‒4.3% and thus at a level usually characterizing distinct frog species in Madagascar. Furthermore, the northern specimens were characterized by more and smaller purplish-brown spots on their green dorsal surface, and a less distinct brown patch on the flanks. Although fully conclusive evidence for the species status of the northern lineage from bioacoustic differences, sympatric occurrence or narrow hybrid zone is currently lacking, such species-level distinctness is currently the most likely hypothesis. We therefore name the northern populations as Guibemantis (Pandanusicola) pulcherrimus sp. nov. The new species is known from Makira (type locality) and Bemanevika, and specimens morphologically assignable to this taxon have also been recorded from Masoala, Marojejy and Anjanaharibe-Sud.

Genomics reveals broad hybridization in deeply divergent Palearctic grass and water snakes (Natrix spp.)

Understanding speciation is one of the cornerstones of biological diversity research. Currently, speciation is often understood as a continuous process of divergence that continues until genetic or other incompatibilities minimize or prevent interbreeding. The Palearctic snake genus Natrix is an ideal group to study speciation, as it comprises taxa representing distinct stages of the speciation process, ranging from widely interbreeding parapatric taxa through parapatric species with very limited gene flow in narrow hybrid zones to widely sympatric species. To understand the evolution of reproductive isolation through time, we have sequenced the genomes of all five species within this genus and two additional subspecies. We used both long-read and short-read methods to sequence and de-novo-assemble two high-quality genomes (Natrix h. helvetica, Natrix n. natrix) to their 1.7 Gb length with a contig N50 of 4.6 Mbp and 1.5 Mbp, respectively, and used these as references to assemble the remaining short-read-based genomes. Our phylogenomic analyses yielded a well-supported dated phylogeny and evidence for a surprisingly complex history of interspecific gene flow, including between widely sympatric species. Furthermore, evidence for gene flow was also found for currently allopatric species pairs. Genetic exchange among these well-defined, distinct, and several million-year-old reptile species emphasizes that speciation and maintenance of species distinctness can occur despite continued genetic exchange.

Craniodental divergence associated with bite force between hybridizing pine squirrels (Tamiasciurus).

Bite force can be a limiting factor in foraging and can significantly affect the competitive ability and lifetime fitness of mammals. Tamiasciurus squirrels feed primarily on conifer seeds and have a strong bite force to mechanically extract seeds from conifer cones with their mouths. In the North Cascades region, Douglas squirrels (Tamiasciurus douglasii) and red squirrels (T. hudsonicus) occupy ecologically different forests with different hardnesses in conifer cones. The ranges of these species overlap in a narrow hybrid zone where these forests meet near the crest of the North Cascades. We examined interspecific divergence in dietary ecomorphology in allopatry, in sympatry within the hybrid zone, and between hybrids and each parental species. We focused on three craniodental traits, including the incisor-strength index as a proxy measure for maximal bite force, cranial-suture complexity, and mandible shape. We find that these sister squirrel species differ in bite force and suture complexity in allopatry and sympatry and that mandible shape changes with the expected hardness of accessed food items, but is not significantly different between species. Furthermore, we find that hybrids display morphologies that overlap with hybrid zone red squirrels but not with hybrid zone Douglas squirrels. This work shows how important ecological processes at shallow evolutionary timescales can impact the divergence of morphological traits in taxa with extreme conservation of craniomandibular shape.

Temporal stability of the hybrid zone between Calocitta magpie-jays revealed through comparison of museum specimens and iNaturalist photos.

Hybrid zones are natural experiments for the study of avian evolution. Hybrid zones can be dynamic, moving as species adjust to new climates and habitats, with unknown implications for species and speciation. There are relatively few studies that have comparable modern and historic sampling to assess change in hybrid zone location and width over time, and those studies have generally found mixed results, with many hybrid zones showing change over time, but others showing stability. The white-throated magpie-jay (Calocitta formosa) and black-throated magpie-jay (Calocitta colliei) occur along the western coast of Mexico and Central America. The two species differ markedly in throat color and tail length, and prior observation suggests a narrow hybrid zone in southern Jalisco where individuals have mixed throat color. This study aims to assess the existence and temporal stability of this putative hybrid zone by comparing throat color between georeferenced historical museum specimens and modern photos from iNaturalist with precise locality information. Our results confirm the existence of a narrow hybrid zone in Jalisco, with modern throat scores gradually increasing from the parental ends of the cline toward the cline center in a sigmoidal curve characteristic of hybrid zones. Our temporal comparison suggests that the hybrid zone has not shifted its position between historical (pre-1973) and modern (post-2005) time periods-a surprising result given the grand scale of habitat change to the western Mexican lowlands during this time. An anomalous pocket of white-throated individuals in the northern range of the black-throated magpie-jay hints at the possibility of prehistorical long-distance introduction. Future genomic data will help disentangle the evolutionary history of these lineages and better characterize how secondary contact is affecting both the DNA and the phenotype of these species.

Exploring the speciation continuum of slow worms: location and extent of the Anguis fragilis/veronensis hybrid zone in southeastern France

Abstract With five currently recognized species that form several secondary contact zones, slow worms (Anguidae: Anguis) offer a valuable model to study the fate of evolutionary lineages in the face of hybridization and genetic introgression. The relationships between the Western Slow Worm Anguis fragilis and the Italian Slow Worm Anguis veronensis are particularly puzzling. Their respective distributions remain poorly known on the edges of their parapatric ranges, as both species lack external differentiation. Contra earlier mitochondrial phylogenies, new phylogenomic inferences have shown that A. fragilis and A. veronensis are sister taxa, thus casting doubts on their specific status. In this study, we analyze the A. fragilis/veronensis transition in southeastern France, based on one mitochondrial (ND2) and two nuclear (PRLR and HA1) genetic markers in 81 specimens from 61 localities. The ranges of A. fragilis and A. veronensis roughly extends northwest and southeast of the Rhône-Durance valleys, respectively, with clear signs of introgressive hybridization in the areas of contact (notably the eastern parts of the lower Rhône valley). Based on the three molecular markers analyzed, gene flow does not seem to reach outside the narrow hybrid zone, which likely indicates (incomplete) intrinsic reproductive isolation. Hence, we provisionally suggest maintaining A. veronensis as a separate species from A. fragilis. More generally, patterns of genetic divergence, external differentiation, and hybridization (both historical and contemporary) in Anguis ssp. supports a speciation continuum spanning from cryptic, genetically compatible alloparapatric lineages to phenotypically distinct, deeply diverged and fully reproductively isolated taxa able to coexist in sympatry.

Dynamic properties of the pinyon pine syngameon.

This article is a Commentary on Buck et al. (2023), 237: 2435–2449.

Tension zone trapped by exogenous cline: Analysis of a narrow hybrid zone between two parapatric Oxytropis species (Fabaceae).

Hybrid zones have been widely highlighted for their interest in understanding evolutionary processes. It is generally accepted that hybrid zones can be maintained in a balance between dispersal and selection. However, the selective forces can either be endogenous (i.e., genetic incompatibilities between parental taxa) or exogenous (i.e., parental taxa are adapted to different environments). In this study, we evaluated these alternatives and determined the maintenance of a narrow hybrid zone between parapatric distributed Oxytropis diversifolia and O. leptophylla in Nei Mongol, China. For 507 individuals sampled from two populations in the hybrid zone, 12 O. diversifolia populations and five O. leptophylla populations, we measured leaf‐morphological characteristics, quantified genetic structure using 11 nuclear microsatellite loci and five chloroplast DNA intergenic regions, collected micro‐ and macrohabitat data, and conducted geographical cline analysis. We found that the two species differed in leaf morphology, and putative hybrids showed either intermediacy or a bias to O. diversifolia. Parental taxa formed two genetically distinct clusters, while populations in the hybrid zone consisted of both parental forms and various admixed individuals, exhibiting a bimodal pattern. The hybrid zone was coupled to ecological transitions of both microhabitat (i.e., the slope) and macroclimatic conditions. However, the genetic clines were significantly narrower than the environmental cline. Our results indicate that endogenous selection can be primarily responsible for maintaining the hybrid zone, while local adaptation accounts for the position of the zone. We further suggest the probable outcome of hybridization could be introgression.

Gene flow, genomic homogenization and the timeline to speciation in Amazonian manakins.

Phylogeographical studies of the most species-rich region of the planet-the Amazon basin-have repeatedly uncovered genetically distinctive, allopatric lineages within currently named species, but understanding whether such lineages are reproductively isolated species is challenging. Here we harness the power of genome-wide data sets together with detailed phylogeographical sampling to both characterize the number of unique lineages and infer levels of reproductive isolation for three parapatric manakin species that make up the genus Pipra. The mitochondrial and nuclear genomes both support six distinctive lineages. The youngest lineages are now highly admixed with each other across major portions of their geographical ranges with one lineage now extinct in a genomically unadmixed state. In contrast, the oldest sets of lineages-dated to 1.4 million years-exhibit narrow hybrid zones. By fitting demographic models to parapatric lineage pairs we found that levels of gene flow and genomic homogenization decline with increasing evolutionary age. Only lineages descending from the basal node at 1.4 million years ago in the genus experience negligible gene flow, possess genomes resistant to homogenization and are separated by narrow hybrid zones. We conclude that a million years or more were required for Pipra manakins to become reproductively isolated. We suggest the six lineages be reclassified as two or three reproductively isolated species. Our unique approach to quantifying reproductive isolation in parapatric lineages could be applied broadly to other phylogeographical studies and would help determine species classification of the plethora of newly identified lineages in the Amazon basin and other regions.

Pleiotropic opposing dominance within a color gene block contributes to a nascent species boundary via its influence on hybrid male territorial behavior.

The divergence of plumage color genes contributes to songbird radiation. However, the mechanisms by which color gene divergence counteracts gene flow to maintain reproductive isolation during the formation of new species boundaries remain elusive. The hybrid zone between Setophaga occidentalis (SOCC) and S. townsendi (STOW) in the Cascade Range provides a natural observatory to investigate potential behavioral mechanisms underlying divergent selection on color genes. Recently, we found that selection within a single gene block associated with plumage color variation has maintained a stable and narrow hybrid zone. Here, we investigated the potential role of plumage signals in moderating a behavioral mechanism of selection. Specifically, we assessed whether two plumage traits are associated with body size among breeding males and if trait mismatch predicted aggressive behavior within hybrid and parental individuals in response to simulated territorial intrusion. The two plumage signals, cheek and flank coloration, though associated with the same gene block, reflect opposing dominance of SOCC and STOW alleles. We found that both plumage traits significantly predict the body size in the territorial sex (i.e. males). The opposing dominance of the single color gene block resulted in plumage signal discordance in heterozygotes, which in turn was associated with reduced hybrid territorial performance, an important proxy of fitness in this system. Taken together, these observations point to a single-locus-two-alleles mechanism of incompatibility in shaping a natural species boundary in the early stage of speciation.

Widespread genomic signatures of reproductive isolation and sex-specific selection in the Eastern Yellow Robin, Eopsaltria australis.

How new species evolve is one of the most fundamental questions in biology. Population divergence, which may lead to speciation, may be occurring in the Eastern Yellow Robin, a common passerine that lives along the eastern coast of Australia. This species is composed of 2 parapatric lineages that have highly divergent mitochondrial DNA; however, similar levels of divergence have not been observed in the nuclear genome. Here we re-examine the nuclear genomes of these mitolineages to test potential mechanisms underlying the discordance between nuclear and mitochondrial divergence. We find that nuclear admixture occurs in a narrow hybrid zone, although the majority of markers across the genome show evidence of reproductive isolation between populations of opposing mitolineages. There is an 8 MB section of a previously identified putative neo-sex chromosome that is highly diverged between allopatric but not parapatric populations, which may be the result of a chromosomal inversion. The neo-sex chromosomal nature of this region, as well as the geographic patterns in which it exhibits divergence, suggest it is unlikely to be contributing to reproductive isolation through mitonuclear incompatibilities as reported in earlier studies. In addition, there are sex differences in the number of markers that are differentiated between populations of opposite mitolineages, with greater differentiation occurring in females, which are heterozygous, than males. These results suggest that, despite the absence of previously observed assortative mating, mitolineages of Eastern Yellow Robin experience at least some postzygotic isolation from each other, in a pattern consistent with Haldane’s Rule.

Genetic traces of hybrid zone movement across a fragmented habitat.

Theoretical and empirical studies suggest that the structure and position of hybrid zones can change over time. Evidence for moving hybrid zones has been directly inferred by repeated sampling over time, or indirectly through the detection of genetic footprints left by the receding species and the resulting asymmetric patterns of introgression across markers. We here investigate a hybrid zone formed by two subspecies of the Iberian golden-striped salamander, Chioglossa lusitanica, using a panel of 35 nuclear loci (31 SNPs and 4 allozymes) and one mitochondrial locus in a transect in central Portugal. We found concordant and coincident clines for most of the nuclear loci (n=22, 63%), defining a narrow hybrid zone of ca. 6km wide, with the centre positioned ca. 15km south of the Mondego River. Asymmetric introgression was observed at another 14loci. Their clines are displaced towards the north, with positions located either close to the Mondego River (n=6) or further northwards (n=8). We interpret these profiles as genetic traces of the southward displacement of C. lusitanica lusitanica by C. l. longipes over the wider Mondego River valley. We noted the absence of significant linkage disequilibrium, and we inferred low levels of effective selection per locus against hybrids, suggesting that introgression in the area of species replacement occurred under a neutral diffusion process. A species distribution model suggests that the C. lusitanica hybrid zone coincides with a narrow corridor of fragmented habitat. From the position of the displaced clines, we infer that patches of locally suitable habitat trapped some genetic variants that became disassociated from the southward moving hybrid zone. This study highlights the influence of habitat availability on hybrid zone movement.

Landscape and Climatic Variations Shaped Secondary Contacts amid Barn Owls of the Western Palearctic.

The combined actions of climatic variations and landscape barriers shape the history of natural populations. When organisms follow their shifting niches, obstacles in the landscape can lead to the splitting of populations, on which evolution will then act independently. When two such populations are reunited, secondary contact occurs in a broad range of admixture patterns, from narrow hybrid zones to the complete dissolution of lineages. A previous study suggested that barn owls colonized the Western Palearctic after the last glaciation in a ring-like fashion around the Mediterranean Sea, and conjectured an admixture zone in the Balkans. Here, we take advantage of whole-genome sequences of 94 individuals across the Western Palearctic to reveal the complex history of the species in the region using observational and modeling approaches. Even though our results confirm that two distinct lineages colonized the region, one in Europe and one in the Levant, they suggest that it predates the last glaciation and identify a secondary contact zone between the two in Anatolia. We also show that barn owls recolonized Europe after the glaciation from two distinct glacial refugia: a previously identified western one in Iberia and a new eastern one in Italy. Both glacial lineages now communicate via eastern Europe, in a wide and permeable contact zone. This complex history of populations enlightens the taxonomy of Tyto alba in the region, highlights the key role played by mountain ranges and large water bodies as barriers and illustrates the power of population genomics in uncovering intricate demographic patterns.

Genetic data reveal fine-scale ecological segregation between larval plethodontid salamanders in replicate contact zones

Contact zones present unique opportunities to investigate ecological divergence, reproductive barriers, and gene flow between species. The two-lined salamander (Eurycea bislineata) species complex is a group of semiaquatic plethodontid salamanders with a reticulate evolutionary history that reflects the reorganization of river drainage basins. Although evidence for widespread, ancient introgression suggests an absence of reproductive isolating mechanisms in the early evolutionary history of the group, modern contact zones reveal a broader diversity of outcomes—with some putative species pairs occurring in sympatry and others exhibiting narrow hybrid zones. Here, we used RADcap data to investigate gene flow and ecological divergence in replicate contact zones between two species in the Appalachian foothills. Our results demonstrate that gene flow between these species is absent or rare, and larvae show strong, fine-scale ecological segregation among riffles, runs, and pools in streams. These results reinforce the more ambiguous conclusions of previous studies that suggested the evolutionary distinctiveness of these two species and underscore the importance of ecological factors in shaping local distributions.