Inconsistent reproductive isolation revealed by interactions between Catostomus fish species

Phylogeography and paleoclimatic range dynamics explain variable outcomes to contact across a species' range

Replicability of divergence after contact is a poorly characterized process, particularly in the contexts of phylogeography and postglacial range dynamics within species. Using contact zones located at the leading-, mid- and rear-edges of a species' range, we examined variation in outcomes to contact between divergent lineages of Campanula americana. We investigated whether contact zones vary in quantity and directionality of gene flow, how phylogeographic structure differs between contact zones, and how historic range dynamics may affect outcomes to contact. We found that all contact zones formed at similar times via primary contact yet detected significant admixture in only the rear-edge (RE) contact zone. In the northern leading-edge contact zone and the mid-range Virginia contact zone, gene flow was minimal and asymmetric. In the southern RE contact zone, gene flow was strong and symmetric. Asymmetric admixture in the leading-edge and Virginia contact zones matches the directionality of a known cosmopolitan cytonuclear incompatibility between lineages of C. americana. Our results emphasize the dependence of speciation processes on phylogeographic structure, evolutionary history and range dynamics.

Influence of dams on sauger population structure and hybridization with introduced walleye

Dams have negatively affected freshwater biodiversity throughout the world. These negative effects tend to be exacerbated for aquatic taxa with migratory life histories, and for taxa whose habitat is fundamentally altered by the formation of large reservoirs. Sauger (Sander candadensis; Percidae), large-bodied migratory fishes native to North America, have seen population declines over much of the species' range, and dams are often implicated for their role in blocking access to spawning habitat and otherwise negatively affecting river habitat. Furthermore, hybridization appears to be more frequent between sauger and walleye in the reservoirs formed by large dams. In this study, we examine the role of dams in altering sauger population connectivity and facilitating hybridization with introduced walleye in Wyoming's Wind River and Bighorn River systems. We collected genomic data from individuals sampled over a large spatial scale and replicated sampling throughout the spawning season, with the intent to capture potential variation in hybridization prevalence or genomic divergence between sauger with different life histories. The timing of sampling was not related to hybridization prevalence or population divergence, suggesting limited genetic differences between sauger spawning in different time and places. Overall, there was limited hybridization detected, however, hybridization was most prevalent in Boysen Reservoir (a large impounded section of the Wind River). Dams in the lower Wind River and upper Bighorn River were associated with population divergence between sauger upstream and downstream of the dams, and demographic models suggest that this divergence has occurred in concordance with the construction of the dam. Sauger upstream of the dams exhibited substantially lower estimates of genetic diversity, which implies that disrupted connectivity between Wind River and Bighorn River sauger populations may already be causing negative demographic effects. This research points towards the importance of considering the evolutionary consequences of dams on fish populations in addition to the threats they pose to population persistence.

Selection leads to remarkable variability in the outcomes of hybridisation across replicate hybrid zones

Hybrid zones have been viewed as an opportunity to see speciation in action. When hybrid zones are replicated, it is assumed that if the same genetic incompatibilities are maintaining reproductive isolation across all instances of secondary contact, those incompatibilities should be identifiable by consistent patterns in the genome. In contrast, changes in allele frequencies due to genetic drift should be idiosyncratic for each hybrid zone. To test this assumption, we simulated 20 replicates of each of 12 hybrid zone scenarios with varied genetic incompatibilities, rates of migration, selection and different starting population size ratios of parental species. We found remarkable variability in the outcomes of hybridisation in replicate hybrid zones, particularly with Bateson-Dobzhansky-Muller incompatibilities and strong selection. We found substantial differences among replicates in the overall genomic composition of individuals, including admixture proportions, inter-specific ancestry complement and number of ancestry junctions. Additionally, we found substantial variation in genomic clines among replicates at focal loci, regardless of locus-specific selection. We conclude that processes other than selection are responsible for some consistent outcomes of hybridisation, whereas selection on incompatibilities can lead to genomically widespread and highly variable outcomes. We highlight the challenge of mapping between pattern and process in hybrid zones and call attention to how selection against incompatibilities will commonly lead to variable outcomes. We hope that this study informs future research on replicate hybrid zones and encourages further development of statistical techniques, theoretical models and exploration of additional axes of variation to understand reproductive isolation.

Strong philopatry in an estuarine-dependent fish

Understanding fish movement is critical in determining the spatial scales in which to appropriately manage wild populations. Genetic markers provide a natural tagging approach to assess the degree of gene flow and population connectivity across a species distribution. We investigated the genetic structure of black bream Acanthopagrus butcheri across its entire distribution range in Australia, as well as regional scale gene flow across south-eastern Australia by undertaking a comprehensive analysis of the populations in estuaries across the region. We applied genome-wide sampling of single-nucleotide polymorphism (SNP) markers generated from restriction site-associated DNA sequencing. Genetic structure and potential gene flow was assessed using principal component analyses and admixture analyses (STRUCTURE). Using 33,493 SNPs, we detected broad scale genetic structuring, with limited gene flow among regional clusters (i.e. Western Australia, South Australia and western Victoria; and eastern Victoria, Tasmania and New South Wales). This is likely the result of unsuitable habitats, strong ocean currents (e.g. the Leeuwin Current and the East Australian Current), large water bodies (e.g. Bass Strait) and known biogeographical provinces across the continent. Local-scale genetic structuring was also identified across the south-eastern Australian estuaries sampled, reflecting that the coexistence of both migratory and resident individuals within populations (i.e. partial migration), and the movement of fish into coastal waters, still results in strong philopatry across the region. Instances of movement among estuaries at this spatial scale were primarily found between adjacent estuaries and were likely attributed to lone migrants utilising inshore coastal currents for movement beyond nearby habitats. Targeting SNP markers in A. butcheri at this continental scale highlighted how neither spatial proximity of estuaries nor black bream's ability to move into coastal waters reflects increased gene flow. Overall, our findings highlight the importance of location-specific management.

Quantitative Analyses of Coupling in Hybrid Zones

In hybrid zones, whether barrier loci experience selection mostly independently or as a unit depends on the ratio of selection to recombination as captured by the coupling coefficient. Theory predicts a sharper transition between an uncoupled and coupled system when more loci affect hybrid fitness. However, the extent of coupling in hybrid zones has rarely been quantified. Here, we use simulations to characterize the relationship between the coupling coefficient and variance in clines across genetic loci. We then reanalyze 25 hybrid zone data sets and find that cline variances and estimated coupling coefficients form a smooth continuum from high variance and weak coupling to low variance and strong coupling. Our results are consistent with low rates of hybridization and a strong genome-wide barrier to gene flow when the coupling coefficient is much greater than 1, but also suggest that this boundary might be approached gradually and at a near constant rate over time.

A complex genomic architecture underlies reproductive isolation in a North American oriole hybrid zone

Natural hybrid zones provide powerful opportunities for identifying the mechanisms that facilitate and inhibit speciation. Documenting the extent of genomic admixture allows us to discern the architecture of reproductive isolation through the identification of isolating barriers. This approach is particularly powerful for characterizing the accumulation of isolating barriers in systems exhibiting varying levels of genomic divergence. Here, we use a hybrid zone between two species-the Baltimore (Icterus galbula) and Bullock's (I. bullockii) orioles-to investigate this architecture of reproductive isolation. We combine whole genome re-sequencing with data from an additional 313 individuals amplityped at ancestry-informative markers to characterize fine-scale patterns of admixture, and to quantify links between genes and the plumage traits. On a genome-wide scale, we document several putative barriers to reproduction, including elevated peaks of divergence above a generally high genomic baseline, a large putative inversion on the Z chromosome, and complex interactions between melanogenesis-pathway candidate genes. Concordant and coincident clines for these different genomic regions further suggest the coupling of pre- and post-mating barriers. Our findings of complex and coupled interactions between pre- and post-mating barriers suggest a relatively rapid accumulation of barriers between these species, and they demonstrate the complexities of the speciation process.

Ecological outcomes of hybridization vary extensively in Catostomus fishes

Hybridization outcomes vary geographically and can depend on the environment. Hybridization can also reshape biotic interactions, leading to ecological shifts. If hybrids function differently ecologically in ways that enhance or reduce fitness, and those ecological roles vary geographically, ecological factors might explain variation in hybridization outcomes. However, relatively few studies have focused on ecological traits of hybrids. We compared the feeding ecology of Catostomus fish species and hybrids by using stable isotopes (δ13 C and δ15 N) as a proxy for diet and habitat use, and compared two native species, an introduced species, and three interspecific hybrid crosses. We included hybrids and parental species from seven rivers where hybridization outcomes vary. Relative isotopic niches of native species varied geographically, but native species did not fully overlap in isotopic space in any river sampled, suggesting little overlap of resource use between historically sympatric species. The introduced species overlapped with one or both native species in every river, suggesting similar resource use and potential competition. Hybrids occupied intermediate, matching, or more transgressive isotopic niches, and varied within and among rivers. Ecological outcomes of hybridization varied across locations, implying that hybridization might have unpredictable, idiosyncratic ecological effects.

Variable hybridization between two Lake Tanganyikan cichlid species in recent secondary contact

Closely related taxa frequently exist in sympatry before the evolution of robust reproductive barriers, which can lead to substantial gene flow. Post-divergence gene flow can promote several disparate trajectories of divergence ranging from the erosion of distinctiveness and eventual collapse of the taxa to the strengthening of reproductive isolation. Among many relevant factors, understanding the demographic history of divergence (e.g. divergence time, extent of historical gene flow) can be particularly informative when examining contemporary gene flow between closely related taxa because this history can influence gene flow's prevalence and consequences. Here, we used genotyping-by-sequencing data to investigate speciation and contemporary hybridization in two closely related and sympatrically distributed Lake Tanganyikan cichlid species in the genus Petrochromis. Demographic modeling supported a speciation scenario involving divergence in isolation followed by secondary contact with bidirectional gene flow. Further investigation of this recent gene flow found evidence of ongoing hybridization between the species that varied in extent between different co-occurring populations. Relationships between abundance and the degree of admixture across populations suggest that the availability of conspecific mates may influence patterns of hybridization. These results, together with the observation that sets of recently diverged cichlid taxa are generally geographically separated in the lake, suggest that ongoing speciation in Lake Tanganyikan cichlids relies on initial spatial isolation. Additionally, the spatially heterogeneous patterns of admixture between the Petrochromis species illustrates the complexities of hybridization when species are in recent secondary contact.

Hybridization decreases native cutthroat trout reproductive fitness

Examining natural selection in wild populations is challenging, but crucial to understanding many ecological and evolutionary processes. Additionally, in hybridizing populations, natural selection may be an important determinant of the eventual outcome of hybridization. We characterized several components of relative fitness in hybridizing populations of Yellowstone cutthroat trout and rainbow trout in an effort to better understand the prolonged persistence of both parental species despite predictions of extirpation. Thousands of genomic loci enabled precise quantification of hybrid status in adult and subsequent juvenile generations; a subset of those data also identified parent-offspring relationships. We used linear models and simulations to assess the effects of ancestry on reproductive output and mate choice decisions. We found a relatively low number of late-stage (F3+) hybrids and an excess of F2 juveniles relative to the adult generation in one location, which suggests the presence of hybrid breakdown decreasing the fitness of F2+ hybrids later in life. Assessments of reproductive output showed that Yellowstone cutthroat trout are more likely to successfully reproduce and produce slightly more offspring than their rainbow trout and hybrid counterparts. Mate choice appeared to be largely random, though we did find statistical support for slight female preference for males of similar ancestry. Together, these results show that native Yellowstone cutthroat trout are able to outperform rainbow trout in terms of reproduction and suggests that management action to exclude rainbow trout from spawning locations may bolster the now-rare Yellowstone cutthroat trout.

Stage-specific disruption of X chromosome expression during spermatogenesis in sterile house mouse hybrids

Hybrid sterility is a complex phenotype that can result from the breakdown of spermatogenesis at multiple developmental stages. Here, we disentangle two proposed hybrid male sterility mechanisms in the house mice, Mus musculus domesticus and M. m. musculus, by comparing patterns of gene expression in sterile F1 hybrids from a reciprocal cross. We found that hybrid males from both cross directions showed disrupted X chromosome expression during prophase of meiosis I consistent with a loss of meiotic sex chromosome inactivation (MSCI) and Prdm9-associated sterility, but that the degree of disruption was greater in mice with an M. m. musculus X chromosome consistent with previous studies. During postmeiotic development, gene expression on the X chromosome was only disrupted in one cross direction, suggesting that misexpression at this later stage was genotype-specific and not a simple downstream consequence of MSCI disruption which was observed in both reciprocal crosses. Instead, disrupted postmeiotic expression may depend on the magnitude of earlier disrupted MSCI, or the disruption of particular X-linked genes or gene networks. Alternatively, only hybrids with a potential deficit of Sly copies, a Y-linked ampliconic gene family, showed overexpression in postmeiotic cells, consistent with a previously proposed model of antagonistic coevolution between the X- and Y-linked ampliconic genes contributing to disrupted expression late in spermatogenesis. The relative contributions of these two regulatory mechanisms and their impact on sterility phenotypes await further study. Our results further support the hypothesis that X-linked hybrid sterility in house mice has a variable genetic basis, and that genotype-specific disruption of gene regulation contributes to overexpression of the X chromosome at different stages of development. Overall, these findings underscore the critical role of epigenetic regulation of the X chromosome during spermatogenesis and suggest that these processes are prone to disruption in hybrids.

Molecular evidence and ecological niche modeling reveal an extensive hybrid zone among three Bursera species (section Bullockia)

The genus Bursera, includes ~100 shrub and trees species in tropical dry forests with its center of diversification and endemism in Mexico. Morphologically intermediate individuals have commonly been observed in Mexican Bursera in areas where closely related species coexist. These individuals are assumed to result from interspecific hybridization, but no molecular evidence has supported their hybrid origins. This study aimed to investigate the existence of interspecific hybridization among three Mexican Bursera species (Bullockia section: B. cuneata, B. palmeri and B. bipinnata) from nine populations based on DNA sequences (three nuclear and four chloroplast regions) and ecological niche modeling for three past and two future scenario projections. Results from the only two polymorphic nuclear regions (PEPC, ETS) supported the hybrid origin of morphologically intermediate individuals and revealed that B. cuneata and B. bipinnata are the parental species that are genetically closer to the putative hybrids. Ecological niche modeling accurately predicted the occurrence of putative hybrid populations and showed a potential hybrid zone extending in a larger area (74,000 km2) than previously thought. Paleo-reconstructions showed a potential hybrid zone existing from the Last Glacial Maximum (~ 21 kya) that has increased since the late Holocene to the present. Future ecological niche projections show an increment of suitability of the potential hybrid zone for 2050 and 2070 relative to the present. Hybrid zone changes responded mostly to an increase in elevational ranges. Our study provides the first insight of an extensive hybrid zone among three Mexican Bursera species based on molecular data and ecological niche modeling.

Hierarchical genetic structure and implications for conservation of the world's largest salmonid, Hucho taimen

Population genetic analyses can evaluate how evolutionary processes shape diversity and inform conservation and management of imperiled species. Taimen (Hucho taimen), the world’s largest freshwater salmonid, is threatened, endangered, or extirpated across much of its range due to anthropogenic activity including overfishing and habitat degradation. We generated genetic data using high throughput sequencing of reduced representation libraries for taimen from multiple drainages in Mongolia and Russia. Nucleotide diversity estimates were within the range documented in other salmonids, suggesting moderate diversity despite widespread population declines. Similar to other recent studies, our analyses revealed pronounced differentiation among the Arctic (Selenge) and Pacific (Amur and Tugur) drainages, suggesting historical isolation among these systems. However, we found evidence for finer-scale structure within the Pacific drainages, including unexpected differentiation between tributaries and the mainstem of the Tugur River. Differentiation across the Amur and Tugur basins together with coalescent-based demographic modeling suggests the ancestors of Tugur tributary taimen likely diverged in the eastern Amur basin, prior to eventual colonization of the Tugur basin. Our results suggest the potential for differentiation of taimen at different geographic scales, and suggest more thorough geographic and genomic sampling may be needed to inform conservation and management of this iconic salmonid.

Demographic history shapes genomic ancestry in hybrid zones

Demographic factors such as migration rate and population size can impede or facilitate speciation. In hybrid zones, reproductive boundaries between species are tested and demography mediates the opportunity for admixture between lineages that are partially isolated. Genomic ancestry is a powerful tool for revealing the history of admixed populations, but models and methods based on local ancestry are rarely applied to structured hybrid zones. To understand the effects of demography on ancestry in hybrids zones, we performed individual-based simulations under a stepping-stone model, treating migration rate, deme size, and hybrid zone age as parameters. We find that the number of ancestry junctions (the transition points between genomic regions with different ancestries) and heterogenicity (the genomic proportion heterozygous for ancestry) are often closely connected to demographic history. Reducing deme size reduces junction number and heterogenicity. Elevating migration rate increases heterogenicity, but migration affects junction number in more complex ways. We highlight the junction frequency spectrum as a novel and informative summary of ancestry that responds to demographic history. A substantial proportion of junctions are expected to fix when migration is limited or deme size is small, changing the shape of the spectrum. Our findings suggest that genomic patterns of ancestry could be used to infer demographic history in hybrid zones.

Multigenerational backcrossing and introgression between two woodrat species at an abrupt ecological transition

When organisms experience secondary contact after allopatric divergence, genomic regions can introgress differentially depending on their relationships with adaptation, reproductive isolation, recombination, and drift. Analyses of genome-wide patterns of divergence and introgression could provide insight into the outcomes of hybridization and the potential relationship between allopatric divergence and reproductive isolation. Here, we generate population genetic data (26,262 SNPs; 353 individuals) using a reduced-representation sequencing approach to quantify patterns of ancestry, differentiation, and introgression between a pair of ecologically distinct mammals-the desert woodrat (N. lepida) and Bryant's woodrat (N. bryanti)-that hybridize at a sharp ecotone in southern California. Individual ancestry estimates confirmed that hybrids were rare in this bimodal hybrid zone, and entirely consisted of a few F₁ individuals and a broad range of multigenerational backcrosses. Genomic cline analyses indicated more than half of loci had elevated introgression from one genomic background into the other. However, introgression was not associated with relative or absolute measures of divergence, and loci with extreme values for both were not typically found near detoxification enzymes previously implicated in dietary specialization for woodrats. The decoupling of differentiation and introgression suggests that processes other than adaptation, such as drift, may underlie the extreme clines at this contact zone.

The interaction of resource use and gene flow on the phenotypic divergence of benthic and pelagic morphs of Icelandic Arctic charr (Salvelinus alpinus)

Conceptual models of adaptive divergence and ecological speciation in sympatry predict differential resource use, phenotype-environment correlations, and reduced gene flow among diverging phenotypes. While these predictions have been assessed in past studies, connections among them have rarely been assessed collectively. We examined relationships among phenotypic, ecological, and genetic variation in Arctic charr (Salvelinus alpinus) from six Icelandic localities that have undergone varying degrees of divergence into sympatric benthic and pelagic morphs. We characterized morphological variation with geometric morphometrics, tested for differential resource use between morphs using stable isotopes, and inferred the amount of gene flow from single nucleotide polymorphisms. Analysis of stable isotopic signatures indicated that sympatric morphs showed similar difference in resource use across populations, likely arising from the common utilization of niche space within each population. Carbon isotopic signature was also a significant predictor of individual variation in body shape and size, suggesting that variation in benthic and pelagic resource use is associated with phenotypic variation. The estimated percentage of hybrids between sympatric morphs varied across populations (from 0% to 15.6%) but the majority of fish had genotypes (ancestry coefficients) characteristic of pure morphs. Despite evidence of reduced gene flow between sympatric morphs, we did not detect the expected negative relationship between divergence in resource use and gene flow. Three lakes showed the expected pattern, but morphs in the fourth showed no detectable hybridization and had relatively low differences in resource use between them. This coupled with the finding that resource use and genetic differentiation had differential effects on body shape variation across populations suggests that reproductive isolation maintains phenotypic divergence between benthic and pelagic morphs when the effects of resource use are relatively low. Our ability to assess relationships between phenotype, ecology, and genetics deepens our understanding of the processes underlying adaptive divergence in sympatry.

Investigating the morphological and genetic divergence of arctic char (Salvelinus alpinus) populations in lakes of arctic Alaska

Polymorphism facilitates coexistence of divergent morphs (e.g., phenotypes) of the same species by minimizing intraspecific competition, especially when resources are limiting. Arctic char (Salvelinus sp.) are a Holarctic fish often forming morphologically, and sometimes genetically, divergent morphs. In this study, we assessed the morphological and genetic diversity and divergence of 263 individuals from seven populations of arctic char with varying length-frequency distributions across two distinct groups of lakes in northern Alaska. Despite close geographic proximity, each lake group occurs on landscapes with different glacial ages and surface water connectivity, and thus was likely colonized by fishes at different times. Across lakes, a continuum of physical (e.g., lake area, maximum depth) and biological characteristics (e.g., primary productivity, fish density) exists, likely contributing to characteristics of present-day char populations. Although some lakes exhibit bimodal size distributions, using model-based clustering of morphometric traits corrected for allometry, we did not detect morphological differences within and across char populations. Genomic analyses using 15,934 SNPs obtained from genotyping by sequencing demonstrated differences among lake groups related to historical biogeography, but within lake groups and within individual lakes, genetic differentiation was not related to total body length. We used PERMANOVA to identify environmental and biological factors related to observed char size structure. Significant predictors included water transparency (i.e., a primary productivity proxy), char density (fish·ha-1), and lake group. Larger char occurred in lakes with greater primary production and lower char densities, suggesting less intraspecific competition and resource limitation. Thus, char populations in more productive and connected lakes may prove more stable to environmental changes, relative to food-limited and closed lakes, if lake productivity increases concomitantly. Our findings provide some of the first descriptions of genomic characteristics of char populations in arctic Alaska, and offer important consideration for the persistence of these populations for subsistence and conservation.

Spontaneous hybridization and introgression between walleye (Sander vitreus) and sauger (Sander canadensis) in two large reservoirs: Insights from genotyping by sequencing

Anthropogenic activities may facilitate undesirable hybridization and genomic introgression between fish species. Walleye (Sander vitreus) and sauger (Sander canadensis) are economically valuable freshwater species that can spontaneously hybridize in areas of sympatry. Levels of genomic introgression between walleye and sauger may be increased by modifications to waterbodies (e.g., reservoir development) and inadvertent propagation of hybrids in stocking programs. We used genotyping by sequencing (GBS) to examine 217 fish from two large reservoirs with mixed populations of walleye and sauger in Saskatchewan, Canada (Lake Diefenbaker, Tobin Lake). Analyses with 20,038 (r90) and 478 (r100) single nucleotide polymorphisms clearly resolved walleye and sauger, and classified hybrids with high confidence. F1, F2, and multigeneration hybrids were detected in Lake Diefenbaker, indicating potentially high levels of genomic introgression. In contrast, only F1 hybrids were detected in Tobin Lake. Field classification of fish was unreliable; 7% of fish were misidentified based on broad species categories. Important for activities such as brood stock selection, 12 of 173 (7%) fish field identified as pure walleye, and one of 24 (4%) identified as pure sauger were actually hybrids. In addition, two of 15 (13%) field-identified hybrids were actually pure walleye or sauger. We conclude that hybridization and introgression are occurring in Saskatchewan reservoirs and that caution is warranted when using these populations in stocking programs. GBS offers a powerful and flexible tool for examining hybridization without preidentification of informative loci, eliminating some of the key challenges associated with other marker types.

Model-based genotype and ancestry estimation for potential hybrids with mixed-ploidy

Non-random mating among individuals can lead to spatial clustering of genetically similar individuals and population stratification. This deviation from panmixia is commonly observed in natural populations. Consequently, individuals can have parentage in single populations or involving hybridization between differentiated populations. Accounting for this mixture and structure is important when mapping the genetics of traits and learning about the formative evolutionary processes that shape genetic variation among individuals and populations. Stratified genetic relatedness among individuals is commonly quantified using estimates of ancestry that are derived from a statistical model. Development of these models for polyploid and mixed-ploidy individuals and populations has lagged behind those for diploids. Here, we extend and test a hierarchical Bayesian model, called entropy, which can use low-depth sequence data to estimate genotype and ancestry parameters in autopolyploid and mixed-ploidy individuals (including sex chromosomes and autosomes within individuals). Our analysis of simulated data illustrated the trade-off between sequencing depth and genome coverage and found lower error associated with low-depth sequencing across a larger fraction of the genome than with high-depth sequencing across a smaller fraction of the genome. The model has high accuracy and sensitivity as verified with simulated data and through analysis of admixture among populations of diploid and tetraploid Arabidopsis arenosa.

Riverscape properties contribute to the origin and structure of a hybrid zone in a Neotropical freshwater fish

Understanding the structure of hybrid zones provides valuable insights about species boundaries and speciation, such as the evolution of barriers to gene flow and the strength of selection. In river networks, studying evolutionary processes in hybrid zones can be especially challenging, given the influence of past and current river properties along with biological species-specific traits. Here, we suggest that a natural hybrid zone between two divergent lineages of the sexually dimorphic Neotropical fish Nematocharax venustus was probably established by secondary contact as a result of a river capture event between the Contas and Pardo river basins. This putative river capture is supported by hydrogeological evidence of elbows of capture, wind gaps and geological faults. The morphological (colour pattern) and genetic (mtDNA and RADseq) variation reveal a clinal transition between parental lineages along the main river, with predominance of F2 hybrids at the centre of the hybrid zone, absence of early generation backcrosses and different levels of hybridization in the tributaries. We highlight that different sources of information are crucial for understanding how the riverscape spatial history influences the connectivity between and within river systems and, consequently, the dynamics of gene flow between freshwater lineages/species.

Leveraging eDNA to expand the study of hybrid zones

Hybrid zones are important windows into ecological and evolutionary processes. Our understanding of the significance and prevalence of hybridization in nature has expanded with the generation and analysis of genome‐spanning data sets. That said, most hybridization research still has restricted temporal and spatial resolution, which limits our ability to draw broad conclusions about evolutionary and conservation related outcomes. Here, we argue that rapidly advancing environmental DNA (eDNA) methodology could be adopted for studies of hybrid zones to increase temporal sampling (contemporary and historical), refine and geographically expand sampling density, and collect data for taxa that are difficult to directly sample. Genomic data in the environment offer the potential for near real‐time biological tracking of hybrid zones, and eDNA provides broad, but as yet untapped, potential to address eco‐evolutionary questions.

Reticulate evolution as a management challenge: Patterns of admixture with phylogenetic distance in endemic fishes of western North America

Admixture in natural populations is a long-standing management challenge, with population genomic approaches offering means for adjudication. We now more clearly understand the permeability of species boundaries and the potential of admixture for promoting adaptive evolution. These issues particularly resonate in western North America, where tectonism and aridity have fragmented and reshuffled rivers over millennia, in turn promoting reticulation among endemic fishes, a situation compounded by anthropogenic habitat modifications and non-native introductions. The melding of historic and contemporary admixture has both confused and stymied management. We underscore this situation with a case study that quantifies basin-wide admixture among a group of native and introduced fishes by employing double-digest restriction site-associated DNA (ddRAD) sequencing. Our approach: (a) quantifies the admixed history of 343 suckers (10 species of Catostomidae) across the Colorado River Basin; (b) gauges admixture within the context of phylogenetic distance and "ecological specialization"; and (c) extrapolates potential drivers of introgression across hybrid crosses that involve endemic as well as invasive species. Our study extends across an entire freshwater basin and expands previous studies more limited in scope both geographically and taxonomically. Our results detected admixture involving all 10 species, with habitat alterations not only accelerating the breakdown of reproductive isolation, but also promoting introgression. Hybridization occurred across the genus despite phylogenetic distance, whereas introgression was only detected within subgenera, implicating phylogenetic distance and/or ecological specialization as drivers of reproductive isolation. Understanding the extent of admixture and reproductive isolation across multiple species serves to disentangle their reticulate evolutionary histories and provides a broadscale perspective for basin-wide conservation and management.

Gene flow and species delimitation in fishes of Western North America: Flannelmouth (Catostomus latipinnis) and Bluehead sucker (C. Pantosteus discobolus)

The delimitation of species boundaries, particularly those obscured by reticulation, is a critical step in contemporary biodiversity assessment. It is especially relevant for conservation and management of indigenous fishes in western North America, represented herein by two species with dissimilar life histories codistributed in the highly modified Colorado River (i.e., flannelmouth sucker, Catostomus latipinnis; bluehead sucker, C. (Pantosteus) discobolus). To quantify phylogenomic patterns and examine proposed taxonomic revisions, we first employed double-digest restriction site-associated DNA sequencing (ddRAD), yielding 39,755 unlinked SNPs across 139 samples. These were subsequently evaluated with multiple analytical approaches and by contrasting life history data. Three phylogenetic methods and a Bayesian assignment test highlighted similar phylogenomic patterns in each, but with considerable difference in presumed times of divergence. Three lineages were detected in bluehead sucker, supporting elevation of C. (P.) virescens to species status and recognizing C. (P.) discobolus yarrowi (Zuni bluehead sucker) as a discrete entity. Admixture in the latter necessitated a reevaluation of its contemporary and historic distributions, underscoring how biodiversity identification can be confounded by complex evolutionary histories. In addition, we defined three separate flannelmouth sucker lineages as ESUs (evolutionarily significant units), given limited phenotypic and genetic differentiation, contemporary isolation, and lack of concordance (per the genealogical concordance component of the phylogenetic species concept). Introgression was diagnosed in both species, with the Little Colorado and Virgin rivers in particular. Our diagnostic methods, and the agreement of our SNPs with previous morphological, enzymatic, and mitochondrial work, allowed us to partition complex evolutionary histories into requisite components, such as isolation versus secondary contact.

Recent hybrids recapitulate ancient hybrid outcomes

Genomic outcomes of hybridization depend on selection and recombination in hybrids. Whether these processes have similar effects on hybrid genome composition in contemporary hybrid zones versus ancient hybrid lineages is unknown. Here we show that patterns of introgression in a contemporary hybrid zone in Lycaeides butterflies predict patterns of ancestry in geographically adjacent, older hybrid populations. We find a particularly striking lack of ancestry from one of the hybridizing taxa, Lycaeides melissa, on the Z chromosome in both the old and contemporary hybrids. The same pattern of reduced L. melissa ancestry on the Z chromosome is seen in two other ancient hybrid lineages. More generally, we find that patterns of ancestry in old or ancient hybrids are remarkably predictable from contemporary hybrids, which suggests selection and recombination affect hybrid genomes in a similar way across disparate time scales and during distinct stages of speciation and species breakdown.

Genomic regions underlying metabolic and neuronal signaling pathways are temporally consistent in a moving avian hybrid zone

The study of hybrid zones can provide insight into the genetic basis of species differences that are relevant for the maintenance of reproductive isolation. Hybrid zones can also provide insight into climate change, species distributions, and evolution. The hybrid zone between black-capped chickadees (Poecile atricapillus) and Carolina chickadees (Poecile carolinensis) is shifting northward in response to increasing winter temperatures but is not increasing in width. This pattern indicates strong selection against chickadees with admixed genomes. Using high-resolution genomic data, we identified regions of the genomes that are outliers in both time points and do not introgress between the species; these regions may be involved in the maintenance of reproductive isolation. Genes involved in metabolic regulation processes were overrepresented in this dataset. Several gene ontology categories were also temporally consistent-including glutamate signaling, synaptic transmission, and catabolic processes-but the nucleotide variants leading to this pattern were not. Our results support recent findings that hybrids between black-capped and Carolina chickadees have higher basal metabolic rates than either parental species and suffer spatial memory and problem-solving deficits. Metabolic breakdown, as well as spatial memory and problem-solving, in hybrid chickadees may act as strong postzygotic isolation mechanisms in this moving hybrid zone.

Hybridization drives genetic erosion in sympatric desert fishes of western North America

Many species have evolved or currently coexist in sympatry due to differential adaptation in a heterogeneous environment. However, anthropogenic habitat modifications can either disrupt reproductive barriers or obscure environmental conditions which underlie fitness gradients. In this study, we evaluated the potential for an anthropogenically-mediated shift in reproductive boundaries that separate two historically sympatric fish species (Gila cypha and G. robusta) endemic to the Colorado River Basin using ddRAD sequencing of 368 individuals. We first examined the integrity of reproductive isolation while in sympatry and allopatry, then characterized hybrid ancestries using genealogical assignment tests. We tested for localized erosion of reproductive isolation by comparing site-wise genomic clines against global patterns and identified a breakdown in the drainage-wide pattern of selection against interspecific heterozygotes. This, in turn, allowed for the formation of a hybrid swarm in one tributary, and asymmetric introgression where species co-occur. We also detected a weak but significant relationship between genetic purity and degree of consumptive water removal, suggesting a role for anthropogenic habitat modifications in undermining species boundaries or expanding historically limited introgression. In addition, results from basin-wide genomic clines suggested that hybrids and parental forms are adaptively nonequivalent. If so, then a failure to manage for hybridization will exacerbate the long-term extinction risk in parental populations. These results reinforce the role of anthropogenic habitat modification in promoting interspecific introgression in sympatric species by relaxing divergent selection. This, in turn, underscores a broader role for hybridization in decreasing global biodiversity within rapidly deteriorating environments.

Coevolution of male and female mate choice can destabilize reproductive isolation

Sexual interactions play an important role in the evolution of reproductive isolation, with important consequences for speciation. Theoretical studies have focused on the evolution of mate preferences in each sex separately. However, mounting empirical evidence suggests that premating isolation often involves mutual mate choice. Here, using a population genetic model, we investigate how female and male mate choice coevolve under a phenotype matching rule and how this affects reproductive isolation. We show that the evolution of female preferences increases the mating success of males with reciprocal preferences, favouring mutual mate choice. However, the evolution of male preferences weakens indirect selection on female preferences and, with weak genetic drift, the coevolution of female and male mate choice leads to periodic episodes of random mating with increased hybridization (deterministic 'preference cycling' triggered by stochasticity). Thus, counterintuitively, the process of establishing premating isolation proves rather fragile if both male and female mate choice contribute to assortative mating.

Variable hybridization outcomes in trout are predicted by historical fish stocking and environmental context

Hybridization can profoundly affect the genomic composition and phenotypes of closely related species, and provides an opportunity to identify mechanisms that maintain reproductive isolation between species. Recent evidence suggests that hybridization outcomes within a species pair can vary across locations. However, we still do not know how variable outcomes of hybridization are across geographic replicates, and what mechanisms drive that variation. In this study, we described hybridization outcomes across 27 locations in the North Fork Shoshone River basin (Wyoming, USA) where native Yellowstone cutthroat trout and introduced rainbow trout co-occur. We used genomic data and hierarchical Bayesian models to precisely identify ancestry of hybrid individuals. Hybridization outcomes varied across locations. In some locations, only rainbow trout and advanced backcrossed hybrids towards rainbow trout were present, while trout in other locations had a broader range of ancestry, including both parental species and first-generation hybrids. Later-generation intermediate hybrids were rare relative to backcrossed hybrids and rainbow trout individuals. Using an individual-based simulation, we found that outcomes of hybridization in the North Fork Shoshone River basin deviate substantially from what we would expect under null expectations of random mating and no selection against hybrids. Since this deviation implies that some mechanisms of reproductive isolation function to maintain parental taxa and a diversity of hybrid types, we then modelled hybridization outcomes as a function of environmental variables and stocking history that are likely to affect prezygotic barriers to hybridization. Variables associated with history of fish stocking were the strongest predictors of hybridization outcomes, followed by environmental variables that might affect overlap in spawning time and location.

Genomic and phenotypic consequences of two independent secondary contact zones between allopatric lineages of the anadromous ice goby Leucopsarion petersii

Genetic and phenotypic analyses of independent secondary contact zones between certain pairs of divergent populations offer powerful opportunities to assess whether the consequences vary with different environmental backgrounds. Populations of the ice goby Leucopsarion petersii are distributed throughout the Japanese archipelago and comprise genetically and phenotypically divergent groups in the Japan Sea and the Pacific Ocean. In particular, populations in the Japan Sea have a larger body size and numbers of vertebrae than those in the Pacific Ocean. Herein, we performed integrated analyses of genotypes and phenotypes of two independent secondary contact zones and investigated their consequences. Population genetic analyses revealed asymmetric introgression of the mitochondrial genome of either lineage relative to little admixture of nuclear genomes in both secondary contact zones. On phenotype analyses, vertebral numbers were clearly explained by nuclear genomic ancestry in both secondary contact zones, whereas body size was not, suggesting that a little introgression of nuclear genes regulates body size. Actually, we observed biased introgression of a candidate gene, neuropeptide Y (NPY), which potentially controls body size in the ice goby. Moreover, the body size changes in the introgressed populations possibly affect the introgression patterns of mitochondrial genomes across these zones. Collectively, our results demonstrated that genomic and phenotypic consequences of secondary contact varied in marine variable environments.

Dissection by genomic and plumage variation of a geographically complex hybrid zone between two Australian non-sister parrot species, Platycercus adscitus and Platycercus eximius

The study of hybrid zones advances understanding of the speciation process, and approaches incorporating genomic data are increasingly used to draw significant conclusions about the impact of hybridisation. Despite the progress made, the complex interplay of factors that can lead to substantially variable hybridisation outcomes are still not well understood, and many systems and/or groups remain comparatively poorly studied. Our study aims to broaden the literature on avian hybrid zones, investigating a potentially geographically and temporally complex putative hybrid zone between two native Australian non-sister parrot species, the pale-headed and eastern rosellas (Platycercus adscitus and Platycercus eximius, respectively). We analysed six plumage traits and >1400 RADseq loci and detected hybrid individuals and an unexpectedly complex geographic structure. The hybrid zone is larger than previously described due to either observer bias or its movement over recent decades. It comprises different subregions where genetic and plumage signals of admixture vary markedly in their concordance. Evidence of contemporary hybridisation (later generation and backcrossed individuals) both within and beyond the previously defined zone, when coupled with a lack of F1 hybrids and differential patterns of introgression among potentially diagnostic loci, indicates a lack of post-zygotic barriers to gene flow between species. Despite ongoing gene flow, species boundaries are likely maintained largely by strong pre-mating barriers. These findings are discussed in detail and future avenues for research into this system are proposed, which would be of benefit to the speciation and hybrid zone literature.

The genomic landscape at a late stage of stickleback speciation: High genomic divergence interspersed by small localized regions of introgression

Speciation is a continuous process and analysis of species pairs at different stages of divergence provides insight into how it unfolds. Previous genomic studies on young species pairs have revealed peaks of divergence and heterogeneous genomic differentiation. Yet less known is how localised peaks of differentiation progress to genome-wide divergence during the later stages of speciation in the presence of persistent gene flow. Spanning the speciation continuum, stickleback species pairs are ideal for investigating how genomic divergence builds up during speciation. However, attention has largely focused on young postglacial species pairs, with little knowledge of the genomic signatures of divergence and introgression in older stickleback systems. The Japanese stickleback species pair, composed of the Pacific Ocean three-spined stickleback (Gasterosteus aculeatus) and the Japan Sea stickleback (G. nipponicus), which co-occur in the Japanese islands, is at a late stage of speciation. Divergence likely started well before the end of the last glacial period and crosses between Japan Sea females and Pacific Ocean males result in hybrid male sterility. Here we use coalescent analyses and Approximate Bayesian Computation to show that the two species split approximately 0.68-1 million years ago but that they have continued to exchange genes at a low rate throughout divergence. Population genomic data revealed that, despite gene flow, a high level of genomic differentiation is maintained across the majority of the genome. However, we identified multiple, small regions of introgression, occurring mainly in areas of low recombination rate. Our results demonstrate that a high level of genome-wide divergence can establish in the face of persistent introgression and that gene flow can be localized to small genomic regions at the later stages of speciation with gene flow.