Introgression between invasive and native blue mussels (genus Mytilus) in the central California hybrid zone

The ecological and genetic factors determining the extent of introgression between species in secondary contact zones remain poorly understood. Here, we investigate the relative importance of isolating barriers and the demographic expansion of invasive Mytilus galloprovincialis on the magnitude and the direction of introgression with the native Mytilus trossulus in a hybrid zone in central California. We use double‐digest restriction‐site‐associated DNA sequencing (ddRADseq) to genotype 1337 randomly selected single nucleotide polymorphisms and accurately distinguish early and advanced generation hybrids for the first time in the central California Mytilus spp. hybrid zone. Weak levels of introgression were observed in both directions but were slightly more prevalent from the native M. trossulus into the invasive M. galloprovincialis. Few early and advanced backcrossed individuals were observed across the hybrid zone confirming the presence of strong barriers to interbreeding. Heterogeneous patterns of admixture across the zone of contact were consistent with the colonization history of M. galloprovincialis with more extensive introgression in northern localities furthest away from the putative site of introduction in southern California. These observations reinforce the importance of dynamic spatial and demographic expansions in determining patterns of introgression between close congeners, even in those with high dispersal potential and well‐developed reproductive barriers. Our results suggest that the threat posed by invasive M. galloprovincialis is more ecological than genetic as it has displaced, and continues to displace the native M. trossulus from much of central and southern California.     

Can alternative mating tactics facilitate introgression across a hybrid zone by circumventing female choice?

Reproductive barriers and divergence in species’ mate recognition systems underlie major models of speciation. However, hybridization between divergent species is common, and classic mechanisms to explain permeable reproductive barriers rarely consider how an individual may attain reproductive success. Alternative mating tactics (AMTs) exist in various forms across animal taxa. Such tactics may allow poorer quality individuals to gain mating opportunities and facilitate introgression either through asymmetrical positive selection or by circumventing female choice altogether in areas of secondary contact. One such tactic is satellite behaviour in frogs, where silent males perch near advertisers in an attempt to intercept females. To test whether such satellite male tactics are context‐dependent and favoured by hybrids, we genotyped and quantified the morphology of 80 male spring peeper (Pseudacris crucifer) individuals involved in caller–satellite associations from a secondary contact zone between two intraspecific mitochondrial lineages. Irrespective of population, satellite behaviour was best predicted by size but not body condition. Within the contact zone, pure individuals showed a significantly greater probability of being active callers, whereas hybrids of one lineage were more likely to adopt the satellite tactic. We suggest that satellite behaviour in P. crucifer promotes introgression, breaks down reproductive isolating barriers and contributes to asymmetrical introgression in this secondary contact zone. AMTs may thus be an underexplored but important alternative to oft‐discussed causes of genetic discordance found in hybrid zones.     

Human disturbance causes the formation of a hybrid swarm between two naturally sympatric fish species

Most evidence for hybrid swarm formation stemming from anthropogenic habitat disturbance comes from the breakdown of reproductive isolation between incipient species, or introgression between allopatric species following secondary contact. Human impacts on hybridization between divergent species that naturally occur in sympatry have received considerably less attention. Theory predicts that reinforcement should act to preserve reproductive isolation under such circumstances, potentially making reproductive barriers resistant to human habitat alteration. Using 15 microsatellites, we examined hybridization between sympatric populations of alewife (Alosa pseudoharengus) and blueback herring (A. aestivalis) to test whether the frequency of hybridization and pattern of introgression have been impacted by the construction of a dam that isolated formerly anadromous populations of both species in a landlocked freshwater reservoir. The frequency of hybridization and pattern of introgression differed markedly between anadromous and landlocked populations. The rangewide frequency of hybridization among anadromous populations was generally 0–8%, whereas all landlocked individuals were hybrids. Although neutral introgression was observed among anadromous hybrids, directional introgression leading to increased prevalence of alewife genotypes was detected among landlocked hybrids. We demonstrate that habitat alteration can lead to hybrid swarm formation between divergent species that naturally occur sympatrically, and provide empirical evidence that reinforcement does not always sustain reproductive isolation under such circumstances.     

Characterizing patterns of introgressive hybridization between two species of Tyrannus following concurrent range expansion

Hybridization between two species typically occurs when allopatric or ecologically dissimilar species expand into areas of secondary contact or habitat transitions. However, as species' ranges shift rapidly in response to environmental change, the potential for novel types of ephemeral hybrid zones exists. Here, we document and describe the occurrence, prevalence and symmetry of a previously undocumented hybrid zone involving two sympatric, ecologically similar sister species that have been expanding their ranges eastward in the central USA: Scissor‐tailed Flycatchers Tyrannus forficatus and Western Kingbirds Tyrannus verticalis. We identified cases of hybridization and introgression using analyses of eight microsatellite DNA loci and a single mitochondrial gene. We also evaluated short‐term reproductive consequences of hybridization for both species by surveying for both species and potential hybrids at the periphery of their ranges in northeastern Arkansas and western Tennessee, USA. Genetic data revealed bi‐directional backcrossing at the periphery of the species’ ranges, including a cryptic hybrid. We also analysed DNA of putative ‘pure’ individuals from other parts of their ranges and detected two cryptic admixed individuals, suggesting backcrossed individuals from the periphery may be dispersing to breed or that hybridization events have occurred in the core. Finally, our results suggest that there are no short‐term reproductive consequences of hybridization for the two species. In total, hybrid zones that occur at the edges of expanding, sympatric ranges may be ephemeral; we suggest they play an important role in introgression and may have long‐standing impacts for sympatric sister species. Exploring the extent of hybrid zones such as this for other range‐expanding taxa will elucidate whether this type of hybrid zone is unique or a common occurrence.     

Maintenance of species boundaries in a Neotropical radiation of Begonia

A major goal of evolutionary biology is to determine the mechanisms generating biodiversity. In Begonia, one of the largest plant genera (1900+ species), it has been postulated that the high number of endemic species is a by‐product of low gene flow among populations, which predisposes the group to speciation. However, this model of divergence requires that reproductive barriers accumulate rapidly among diverging species that overlap in their geographic ranges, otherwise speciation will be opposed by homogenizing gene flow in zones of secondary contact. Here, we test the outcomes of secondary contact in Begonia by genotyping multiple sympatric sites with 12 nuclear and seven plastid loci. We show that three sites of secondary contact between B. heracleifolia and B. nelumbiifolia are highly structured, mostly containing parental genotypes, with few F1 hybrids. A sympatric site between B. heracleifolia and B. sericoneura contains a higher proportion of F1s, but little evidence of introgression. The lack of later‐generation hybrids contrasts with that documented in many other plant taxa, where introgression is extensive. Our results, in conjunction with previous genetic work, show that Begonia demonstrate properties making them exceptionally prone to speciation, at multiple stages along the divergence continuum. Not only are populations weakly connected by gene flow, promoting allopatric speciation, but species often show strong reproductive barriers in secondary contact. Whether similar mechanisms contribute to diversification in other large genera remains to be tested.     

Asymmetry in reproductive isolation and its effect on directional mitochondrial introgression in the parapatric ground beetles <Emphasis Type="Italic">Carabus yamato</Emphasis> and <Emphasis Type="Italic">C. albrechti</Emphasis>

Speciation studies seek to clarify the origin of reproductive isolation, the various mechanisms working from mate recognition through postzygotic stages. Asymmetric effects of isolating barriers can result in asymmetrical gene introgression during interspecific hybridization. The flightless ground beetles Carabus yamato and C. albrechti are distributed parapatrically in Japan, showing repeated asymmetrical introgression of mitochondria from C. albrechti to C. yamato. This pattern suggests that reproductive isolation between these species is strong, but incomplete and asymmetric (i.e., weaker for the cross between a C. albrechti female and a C. yamato male). To test this hypothesis, we conducted interspecific mating experiments in the laboratory. The estimates of total reproductive isolation, which occurred mainly at the premating and postmating/prezygotic stages, were high (isolation index = 0.964 for C. yamato female × C. albrechti male and 0.886 for the reciprocal cross), supporting the hypothesis of strong, but incomplete isolation. However, the observed difference between the reciprocal crosses was not sufficiently large to conclude that it caused directional introgression of mitochondria. Instead, we found asymmetry in individual isolating barriers in the postmating/prezygotic stages that coincided with the prediction, perhaps resulting from morphological mismatch of heterospecific genitalia. Although this asymmetry was compensated for by an inverse asymmetry of isolation in the postzygotic stage, the contribution of these individual barriers to total isolation may change for our expectation when considering females mating with multiple heterospecific males.     

Chromosomal rearrangements directly cause underdominant F1 pollen sterility in Mimulus lewisii–Mimulus cardinalis hybrids

Chromosomal rearrangements can contribute to the evolution of postzygotic reproductive isolation directly, by disrupting meiosis in F₁ hybrids, or indirectly, by suppressing recombination among genic incompatibilities. Because direct effects of rearrangements on fertility imply fitness costs during their spread, understanding the mechanism of F₁ hybrid sterility is integral to reconstructing the role(s) of rearrangements in speciation. In hybrids between monkeyflowers Mimulus cardinalis and Mimulus lewisii, rearrangements contain all quantitative trait loci (QTLs) for both premating barriers and pollen sterility, suggesting that they may have facilitated speciation in this model system. We used artificial chromosome doubling and comparative mapping to test whether heterozygous rearrangements directly cause underdominant male sterility in M. lewisii–M. cardinalis hybrids. Consistent with a direct chromosomal basis for hybrid sterility, synthetic tetraploid F₁s showed highly restored fertility (83.4% pollen fertility) relative to diploids F₁s (36.0%). Additional mapping with Mimulus parishii–M. cardinalis and M. parishii–M. lewisii hybrids demonstrated that underdominant male sterility is caused by one M. lewisii specific and one M. cardinalis specific reciprocal translocation, but that inversions had no direct effects on fertility. We discuss the importance of translocations as causes of reproductive isolation, and consider models for how underdominant rearrangements spread and fix despite intrinsic fitness costs.     

Current and historical hybridization with differential introgression among three species of cyprinid fishes (genus <Emphasis Type="Italic">Cyprinella</Emphasis>)

Hybridization is common among freshwater fishes, particular among the Cyprinidae. We used two mitochondrial genes and one nuclear gene to characterize hybridization among two species pairs of Cyprinella in southwestern North America. Genalogical patterns revealed that C. lutrensis and C. venusta are currently hybridizing in several localities producing apparent F₁, F₂ and backcross generations, yet there was no evidence for introgression outside of local hybrid zones. Alternatively, mitochondrial haplotypes from C. lutrensis appear to have introgressed into a C. lepida population in the Nueces River completely replacing the native C. lepida haplotype. There was no evidence of introgression of nuclear DNA and there does not appear to be ongoing hybridization. The population of C. lepida from the nearby Frio River exhibits no evidence of hybridization with C. lutrensis. Thus, contact between C. lutrensis and C. venusta results in the formation of localized hybrid swarms, while contact between C. lutrensis and C. lepida has resulted in complete mitochondrial introgression in the Nueces River or no apparent hybridization in the Frio River. The three different outcomes of contact between these species illustrate the variable nature of interspecific reproductive interactions and provide an excellent system in which to better understand the factors influencing hybridization among freshwater fishes.     

Genetic differentiation and habitat connectivity across towhee hybrid zones in Mexico

Hybridization can either reinforce or erode species boundaries; therefore, hybrid zones offer a natural experimental setting in which to assess the dynamics of reproductive isolation. Secondary contact zones, in particular, present a partial separation of the original divergence mechanisms and the subsequent genomic architecture of reproductive isolation (or lack thereof). The spatial context of secondary contact and its consequent effect on dispersal play vital roles on the contact’s outcome. In a hybrid complex between two towhee species in Mexico, Pipilo maculatus and Pipilo ocai, two major hybrid gradients provide natural replicates for comparison. However, genetic analyses demonstrate significant divergence between geographically separate parental populations of each species and divergence of populations within each hybrid zone. The two hybrid transects (Teziutlán and Transvolcanic) are distinct and evidence suggests allelic introgression both across the species boundary and between the two transects. Habitat corridors for dispersal represent functional connectivity hotspots where the two transects meet. Both habitat connectivity and genetic differentiation between geographically disparate parental populations appear to influence the dynamics of gene flow across the hybrid gradients. In southern sympatric populations (Mt. Orizaba and Oaxaca) where morphological evidence for hybridization is scarce, opposing species’ alleles appear to traverse through the hybrid zones rather than arising from cryptic local hybridization. These results illustrate the importance of environmentally mediated gene flow in the context of secondary contact as an important force influencing evolutionary trajectory.     

Hybridization and speciation

Hybridization has many and varied impacts on the process of speciation. Hybridization may slow or reverse differentiation by allowing gene flow and recombination. It may accelerate speciation via adaptive introgression or cause near‐instantaneous speciation by allopolyploidization. It may have multiple effects at different stages and in different spatial contexts within a single speciation event. We offer a perspective on the context and evolutionary significance of hybridization during speciation, highlighting issues of current interest and debate. In secondary contact zones, it is uncertain if barriers to gene flow will be strengthened or broken down due to recombination and gene flow. Theory and empirical evidence suggest the latter is more likely, except within and around strongly selected genomic regions. Hybridization may contribute to speciation through the formation of new hybrid taxa, whereas introgression of a few loci may promote adaptive divergence and so facilitate speciation. Gene regulatory networks, epigenetic effects and the evolution of selfish genetic material in the genome suggest that the Dobzhansky–Muller model of hybrid incompatibilities requires a broader interpretation. Finally, although the incidence of reinforcement remains uncertain, this and other interactions in areas of sympatry may have knock‐on effects on speciation both within and outside regions of hybridization.     

Structure and dynamics of hybrid zones at different stages of speciation in the common vole (Microtus arvalis)

The genetic structure and dynamics of hybrid zones provide crucial information for understanding the processes and mechanisms of evolutionary divergence and speciation. In general, higher levels of evolutionary divergence between taxa are more likely to be associated with reproductive isolation and may result in suppressed or strongly restricted hybridization. In this study, we examined two secondary contact zones between three deep evolutionary lineages in the common vole (Microtus arvalis). Differences in divergence times between the lineages can shed light on different stages of reproductive isolation and thus provide information on the ongoing speciation process in M. arvalis. We examined more than 800 individuals for mitochondrial (mtDNA), Y‐chromosome and autosomal markers and used assignment and cline analysis methods to characterize the extent and direction of gene flow in the contact zones. Introgression of both autosomal and mtDNA markers in a relatively broad area of admixture indicates selectively neutral hybridization between the least‐divergent lineages (Central and Eastern) without evidence for partial reproductive isolation. In contrast, a very narrow area of hybridization, shifts in marker clines and the quasi‐absence of Y‐chromosome introgression support a moving hybrid zone and unidirectional selection against male hybrids between the lineages with older divergence (Central and Western). Data from a replicate transect further support non‐neutral processes in this hybrid zone and also suggest a role for landscape history in the movement and shaping of geneflow profiles.     

Variable levels of introgression between the endangered Podarcis carbonelli and highly divergent congeneric species

Recent empirical studies have demonstrated that speciation with gene flow is more common than previously thought. From a conservation perspective, the potential negative effects of hybridization raise concerns on the genetic integrity of endangered species. However, introgressive hybridization has also been growingly recognized as a source of diversity and new advantageous alleles. Carbonell’s wall lizard (Podarcis carbonelli) is an endangered species whose distribution overlaps with four other congeneric species. Our goal here was to determine whether P. carbonelli is completely reproductively isolated from its congeners and to evaluate the relevance of hybridization and interspecific gene flow for developing a conservation plan. We used restriction site associated DNA (RAD) sequencing to discover SNPs in samples from four contact zones between P. carbonelli and four other species. Principal component analysis, multilocus genotype assignment and interspecific heterozygosity suggest incomplete reproductive isolation and ongoing gene flow between species. However, hybridization dynamics vary across all pairs, suggesting complex interactions between multiple intrinsic and extrinsic barriers. Despite seemingly ubiquitous interspecific gene flow, we found evidence of strong reproductive isolation across most contact zones. Instead, indirect effects of hybridization like waste of reproductive effort in small isolated populations may be more problematic. Our results highlight the need to further evaluate the consequences of introgression for P. carbonelli, both on a geographic and genomic level and included in a comprehensive and urgently needed conservation plan. Besides, those findings will add important insights on the potential effects of hybridization and introgression for endangered species.Subject terms: Population genetics, Speciation     

Hybridization in a warmer world

Climate change is profoundly affecting the evolutionary trajectory of individual species and ecological communities, in part through the creation of novel species assemblages. How climate change will influence competitive interactions has been an active area of research. Far less attention, however, has been given to altered reproductive interactions. Yet, reproductive interactions between formerly isolated species are inevitable as populations shift geographically and temporally as a result of climate change, potentially resulting in introgression, speciation, or even extinction. The susceptibility of hybridization rates to anthropogenic disturbance was first recognized in the 1930s. To date, work on anthropogenically mediated hybridization has focused primarily on either physical habitat disturbance or species invasion. Here, I review recent literature on hybridization to identify how ecological responses to climate change will increase the likelihood of hybridization via the dissolution of species barriers maintained by habitat, time, or behavior. Using this literature, I identify several cases where novel hybrid zones have recently formed, likely as a result of changing climate. Future research should focus on identifying areas and taxonomic groups where reproductive species interactions are most likely to be influenced by climate change. Furthermore, a better understanding of the evolutionary consequences of climate‐mediated secondary contact is urgently needed. Paradoxically, hybridization is both a major conservation concern and an important source of novel genetic and phenotypic variation. Hybridization may therefore both contribute to increasing rates of extinction and stimulate the creation of novel phenotypes that will speed adaptation to novel climates. Predicting which result will occur following secondary contact will be an important contribution to conservation for many species.     

The role of hybridization and introgression in maintaining species integrity and cohesion in naturally isolated inselberg bromeliad populations

• Hybridization is a widespread phenomenon present in numerous lineages across the tree of life. Its evolutionary consequences range from effects on the origin and maintenance, to the loss of biodiversity.
• We studied genetic diversity and intra‐ and interspecific gene flow between two sympatric populations of closely‐related species, Pitcairnia flammea and P. corcovadensis (Bromeliaceae), which are adapted to naturally fragmented Neotropical inselbergs, based on nuclear and plastidial DNA.
• Our main results indicate a strong reproductive isolation barrier, although low levels of interspecific gene flow were observed in both sympatric populations. The low rates of intraspecific gene flow observed for both P. corcovadensis and P. flammea populations corroborate the increasing body of evidence that inselberg bromeliad species are maintained as discrete evolutionary units despite the presence of low genetic connectivity. Nuclear patterns of genetic diversity and gene flow revealed that hybridization and introgression might not cause species extinction via genetic assimilation of the rare P. corcovadensis.
• In the face of reduced intraspecific gene exchange, hybridization and introgression may be important aspects of the Pitcairnia diversification process, with a positive evolutionary impact at the bromeliad community level, and thus contribute to increasing and maintaining genetic diversity in local isolated inselberg populations.

     

Hybridization and introgression during density‐dependent range expansion: European wildcats as a case study

Interbreeding between historically allopatric species with incomplete reproductive barriers may result when species expand their range. The genetic consequences of such hybridization depend critically on the dynamics of the range expansion. Hybridization models during range expansion have been developed but assume dispersal to be independent from neighboring population densities. However, organisms may disperse because they are attracted by conspecifics or because they prefer depopulated areas. Here, through spatially explicit simulations, we assess the effect of various density‐dependent dispersal modes on the introgression between two species. We find huge introgression from the local species into the invasive one with all dispersal modes investigated, even when the hybridization rate is relatively low. This represents a general expectation for neutral genes even if the dispersal modes differ in colonization times and amount of introgression. Invasive individuals attracted by conspecifics need more time to colonize the whole area and are more introgressed by local genes, whereas the opposite is found for solitary individuals. We applied our approach to a recent expansion of European wildcats in the Jura Mountains and the hybridization with domestic cats. We show that the simulations explained better the observed level of introgression at nuclear, mtDNA, and Y chromosome markers, when using solitary dispersal for wildcats instead of random or gregarious dispersal, in accordance with ecological knowledge. Thus, use of density‐dependent dispersal models increases the predictive power of the approach.     

Highly variable reproductive isolation among pairs of Catostomus species

Hybridization between diverged taxa tests the strength of reproductive isolation and can therefore reveal mechanisms of reproductive isolation. However, it remains unclear how consistent reproductive isolation is across species' ranges and to what extent reproductive isolation might remain polymorphic as species diverge. To address these questions, we compared outcomes of hybridization across species pairs of Catostomus fishes in three rivers in the Upper Colorado River basin, where an introduced species, C. commersoni, hybridizes with at least two native species, C. discobolus and C. latipinnis. We observed substantial heterogeneity in outcomes of hybridization, both between species pairs and across geographically separate rivers within each species pair. We also observed hybridization of additional related species with our focal species, suggesting that reproductive isolation in this group involves interactions of multiple evolutionary and ecological factors. These findings suggest that a better understanding of the determinants of variation in reproductive isolation is needed and that studies of reproductive isolation in hybrids should consider how the dynamics and mechanisms of reproductive isolation vary over ecological space and over evolutionary time. Our results also have implications for the conservation and management of native catostomids in the Colorado River basin. Heterogeneity in outcomes of hybridization suggests that the threat posed by hybridization and genetic introgression to the persistence of native species probably varies with extent of reproductive isolation, both across rivers and across species pairs.     

Cytonuclear equilibrium following interspecific introgression in a turtle lacking sex chromosomes

When reproductive barriers break down, interspecific hybridization can lead to gene flow between evolutionarily distinct species. Studying the fate of these introgressing elements can offer valuable insights into the factors contributing to reproductive isolation. We have identified a population of false map turtles (Graptemys pseudogeographica) that hybridized historically with the common map turtle (Graptemys geographica), but were subsequently isolated from interbreeding for several generations by unique geological events. Although many studies conclude that genic interactions involving sex chromosomes impact the introgression of mitochondrial or nuclear genomes, Graptemys turtles have environmental sex determination, and thus introgression can be explored while controlling for the effects of sex‐specific heterogameity. We identified and sequenced a species‐specific mitochondrial control region marker, as well as two nuclear markers (ODC and HNFAL), in turtles from across the ranges of these species. We found both nuclear and mitochondrial introgression in our study population, and present evidence consistent with the proposed time range of reproductive contact and isolation. We also report an absence of cytonuclear or linkage disequilibrium among markers, indicating that some important pre‐ and postzygotic barriers to gene flow that characterize other systems are absent in Graptemys. Finally, we show that Graptemys turtles have a complex molecular evolutionary history, and that leaks in reproductive barriers probably occur frequently. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 405–417.     

Evolutionary dynamics of hybridization and introgression following the recent colonization of Glossy Ibis (Aves: Plegadis falcinellus) into the New World

Geographic range shifts can cause secondary contact and hybridization between closely related species, revealing mechanisms of species formation and integrity. These dynamics typically play out in restricted geographic regions, but highly vagile species may experience major distributional changes resulting in broad areas of contact. The Glossy Ibis (Plegadis falcinellus) is a dispersive waterbird of the Old World and Australia that colonized eastern North America in the early 19th century and came into contact with the native White‐faced Ibis (P. chihi). Putative hybrids between the two species have been observed across North America. To examine the population genomic consequences of this natural invasion, we sequenced 4,616 ultraconserved elements from 66 individuals sampled across the distributions of falcinellus, chihi, and the Puna Ibis (P. ridgwayi) of South America. We found genomic differentiation among the three species. Loci with high sequence divergence were often shared across all pairwise species comparisons, were associated with regions of high nucleotide diversity, and were concentrated on the Z chromosome. We detected signals of genetic admixture between chihi and falcinellus in individuals both near and far from their core area of sympatry. Genomic cline analyses revealed evidence of greater introgression into falcinellus from chihi, but we found little evidence for selection against hybrids. We also found signals of admixture between ridgwayi and South American populations of chihi. Our results indicate vagile species can experience pervasive introgression upon secondary contact, although we suggest these dynamics may be more ephemeral than the stable hybrid zones often observed in less dispersive organisms.     

Is there hybridization between diploid and tetraploid Euphrasia in a secondary contact zone?

Premise

Strong postzygotic reproductive isolating barriers are usually expected to limit the extent of natural hybridization between species with contrasting ploidy. However, genomic sequencing has revealed previously overlooked examples of natural cross-ploidy hybridization in some flowering plant genera, suggesting that the phenomenon may be more common than once thought. We investigated potential cross-ploidy hybridization in British eyebrights (Euphrasia, Orobanchaceae), a group from which 13 putative cross-ploidy hybrid combinations have been reported based on morphology.

Methods

We analyzed a contact zone between diploid Euphrasia rostkoviana and tetraploid E. arctica in Wales. We sequenced part of the internal transcribed spacer (ITS) of nuclear ribosomal DNA and used genotyping by sequencing (GBS) to look for evidence of cross-ploidy hybridization and introgression.

Results

Common variant sites in the ITS region were fixed between diploids and tetraploids, indicating a strong barrier to hybridization. Clustering analyses of 356 single-nucleotide polymorphisms (SNPs) generated using GBS clearly separated samples by ploidy and revealed strong genetic structure (F_ST = 0.44). However, the F_ST distribution across all SNPs was bimodal, indicating potential differential selection on loci between diploids and tetraploids. Demographic inference suggested potential gene flow, limited to around one or fewer migrants per generation.

Conclusions

Our results suggest that recent cross-ploidy hybridization is rare or absent in a site of secondary contact in Euphrasia. While a strong ploidy barrier prevents hybridization over ecological timescales, such hybrids may form in stable populations over evolutionary timescales, potentially allowing cross-ploidy introgression to take place.