Local adaptation within a hybrid species

Ecological divergence among populations may be strongly influenced by their genetic background. For instance, genetic admixture through introgressive hybridization or hybrid speciation is likely to affect the genetic variation and evolvability of phenotypic traits. We studied geographic variation in two beak dimensions and three other phenotypic traits of the Italian sparrow (Passer italiae), a young hybrid species formed through interbreeding between house sparrows (P. domesticus) and Spanish sparrows (P. hispaniolensis). We found that beak morphology was strongly influenced by precipitation regimes and that it appeared to be the target of divergent selection within Italian sparrows. Interestingly, however, the degree of parental genetic contribution in the hybrid species had no effect on phenotypic beak variation. Moreover, beak height divergence may mediate genetic differentiation between populations, consistent with isolation-by-adaptation within this hybrid species. The study illustrates how hybrid species may be relatively unconstrained by their admixed genetic background, allowing them to adapt rapidly to environmental variation.     

Hybridization in contact zone between temperate European pine species

Hybridization studies are important to advance our understanding of the interspecific gene flow and its evolutionary consequences in closely related species. Hybridization and admixture patterns were assessed in a contact zone and reference populations of European pine species using sequence data from 26 nuclear genes and a species-diagnostic cpDNA marker. Reference populations formed three distinct genetic clusters comprising Pinus sylvestris, Pinus mugo/Pinus uliginosa, and Pinus uncinata. Evidence of population structure was found only in P. uliginosa. Based on phenotypic characteristics and molecular data, we identified five groups of individuals in the contact zone in Poland, comprising forms of the parental species and intermediates that were most probably the result of interspecific crosses. A combination of nuclear gene sequence data and a diagnostic organelle marker were used to show that hybridization is frequent in the contact zone and results in hybrid trees with distinct phenotypic identity. The influence of selection in maintaining hybrid phenotypes in environments unsuited to parental species was inferred from nucleotide polymorphism data. A lack of admixture in reference populations suggests that hybridization has not occurred during post-glacial migration and so the contact zone represents a distinct, active example of ongoing evolution. Pine populations in this zone will be a valuable system for studying the genetic basis of hybrid advantage in environmental conditions untypical of pure parental species.     

Morphological Variation in Wild Marmosets (Callithrix penicillata and C. geoffroyi) and Their Hybrids

Evolutionary theory and observation predict wider phenotypic variation in hybrids than parental species. Emergent phenotypic novelty in hybrids may in turn drive new adaptations or speciation by breaking parental phenotypic constraints. Primate hybridization is often documented through genetic evidence, but knowledge about the primate hybrid phenotype remains limited due to a small number of available studies on hybrid primate morphology. Here, we examine pelage and morphometric variation in two Brazilian marmoset species (Callithrix penicillata and C. geoffroyi) and their hybrids. Hybrids were sampled in an anthropogenic hybrid zone in the municipality of Viçosa, Minas Gerais state, Brazil. We analyzed hybrid facial and body pelage color variation, and compared 13 morphometric measures between hybrids and parental species. Five different hybrid facial morphotypes were observed, varying from intermediate to parental-like. Hybrid facial morphotypes were biased towards C. penicillata, suggesting that the pelage of this species may be dominant to that of C. geoffroyi in this context, and indicating that mate preference, and therefore gene flow/introgression, may be biased towards C. penicillata within the hybrid zone. Hybrid morphometric features were on average intermediate to parental species traits, but transgressive hybrids were also observed, suggesting that morphometric variation for the studied traits is consistent with Rieseberg’s complementary allele model. Finally, we observed a decoupling of facial patterning and size/shape in hybrids, relative to parent phenotypes, suggesting that an important factor driving phenotypic novelty within the Viçosa marmoset hybrid zone might be the loosening of evolutionary constraints on phenotypic trait integration.     

Genetic and phenotypic variation across a hybrid zone between ecologically divergent tree squirrels (Tamiasciurus)

A hybrid zone along an environmental gradient should contain a clinal pattern of genetic and phenotypic variation. This occurs because divergent selection in the two parental habitats is typically strong enough to overcome the homogenizing effects of gene flow across the environmental transition. We studied hybridization between two parapatric tree squirrels (Tamiasciurus spp.) across a forest gradient over which the two species vary in coloration, cranial morphology and body size. We sampled 397 individuals at 29 locations across a 600‐km transect to seek genetic evidence for hybridization; upon confirming hybridization, we examined levels of genetic admixture in relation to maintenance of phenotypic divergence despite potentially homogenizing gene flow. Applying population assignment analyses to microsatellite data, we found that Tamiasciurus douglasii and T. hudsonicus form two distinct genetic clusters but also hybridize, mostly within transitional forest habitat. Overall, based on this nuclear analysis, 48% of the specimens were characterized as T. douglasii, 9% as hybrids and 43% as T. hudsonicus. Hybrids appeared to be reproductively viable, as evidenced by the presence of later‐generation hybrid genotypes. Observed clines in ecologically important phenotypic traits—fur coloration and cranial morphology—were sharper than the cline of putatively neutral mtDNA, which suggests that divergent selection may maintain phenotypic distinctiveness. The relatively recent divergence of these two species (probably late Pleistocene), apparent lack of prezygotic isolating mechanisms and geographic coincidence of cline centres for both genetic and phenotypic variation suggest that environmental factors play a large role in maintaining the distinctiveness of these two species across the hybrid zone.     

GENETIC DISTANCE BETWEEN SPECIES PREDICTS NOVEL TRAIT EXPRESSION IN THEIR HYBRIDS

Interspecific hybridization can generate transgressive hybrid phenotypes with extreme trait values exceeding the combined range of the parental species. Such variation can enlarge the working surface for natural selection, and may facilitate the evolution of novel adaptations where ecological opportunity exists. The number of quantitative trait loci fixed for different alleles in different species should increase with time since speciation. If transgression is caused by complementary gene action or epistasis, hybrids between more distant species should be more likely to display transgressive phenotypes. To test this prediction we collected data on transgression frequency from the literature, estimated genetic distances between the hybridizing species from gene sequences, and calculated the relationship between the two using phylogenetically controlled methods. We also tested if parental phenotypic divergence affected the occurrence of transgression. We found a highly significant positive correlation between transgression frequency and genetic distance in eudicot plants explaining 43% of the variance in transgression frequency. In total, 36% of the measured traits were transgressive. The predicted effect of time since speciation on transgressive segregation was unconfounded by the potentially conflicting effects of phenotypic differentiation between species. Our analysis demonstrates that the potential impact hybridization may have on phenotypic evolution is predictable from the genetic distance between species.     

No species barrier by call in an avian hybrid zone between California and Gambel's quail (Callipepla californica and C. gambelii)

Acoustic signals sometimes act as premating isolating barriers between animal species, but we know little about the circumstances that dictate the presence and strength of these barriers. Among insects, barriers to backcrossing are strengthened by acoustic signals that are under genetic control. Hybrid signals tend to be intermediate to parental signals, and signals are recognized only by like‐types, which results in reinforced species boundaries. This is not typically the case in avian taxa. Instead, acoustic signal transmission is controlled by some combination of genes and learning, and perhaps as a consequence of this variation, vocalizations play a diversity of roles in avian hybrid zones. I used California and Gambel's quail (Callipepla californica and C. gambelii), hybridizing birds that do not learn to vocalize, to explore whether genetically determined vocalizations function as a species barrier. Using spectral analysis, I measured temporal features of calls of uniquely colour‐banded quail that were recorded across one area of the California and Gambel's quail hybrid zone. Species discrimination is known to occur under captive conditions, though its basis is unexplored. Here I show that differences in the calls of parental species are likely great enough to permit species discrimination. Hybrid call components were intermediate to those of the parental species and covaried with genetic traits, as assessed with seven highly polymorphic microsatellite loci. Contrary to expectation, males as frequently called in response to unlike‐ as like‐type females who had initiated antiphonal calling, which is a courtship call between a female and a male. Furthermore, paired males and females did not share like‐type assembly calls, nor was there a correlation between the female's genetic or plumage traits and her mate's advertisement call. Based on these results, I conclude that California and Gambel's quail recognize each other and hybrids as potential mates and backcrossing occurs frequently. Thus, compatible mating signals could contribute to increased mixing of gene pools and slow the rate of speciation. I suggest that selection to respond to wide signal variation within species and imprinting on calls of mixed‐species coveys may cause mating signal compatibility between classes within the area of hybridization. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 86 , 253–264.     

Complex patterns of differentiation and gene flow underly the divergence of aposematic phenotypes in Oophaga poison frogs

Hybridization and introgression can have complex consequences for both species evolution and conservation. Here, we investigated the origin and characteristics of a putative hybrid zone between two South American poison dart frog species, Oophaga anchicayensis and the critically endangered Oophaga lehmanni, which are heavily sought after on the illegal pet market. Using a combination of phenotypic (49 traits) and genomic (ddRADseq) data, we found that the putative hybrids are morphologically distinct from their parental species and confirmed genomic signatures of admixture in these populations. Several lines of evidence (hybrid indices, interspecific hybrid heterozygosity, genomic clines, comparisons with simulated hybrids and demographic modelling) support the conclusion that these populations are not comprised of early‐generation hybrids and thus, they probably did not arise as a result of illegal translocations associated with wildlife trafficking. Instead, they probably represent an independent lineage which has persisted through isolation and has only relatively recently re‐established gene flow with both parental species. Furthermore, we detected signals of differential introgression from parental species into these hybrid populations which suggest relaxed stabilizing selection on these aposematic colour morphs, potentially via context‐dependent female choice. These populations thus provide a fascinating window into the role of hybridization, isolation and female choice in the diversification of South American poison dart frogs. In addition, our results underline the importance of landscape conservation measures to protect, not only known localities of nominal species, but also the phenotypic and genomic variation harbored by admixed lineages which represent crucial repositories for the impressive diversity in this system.     

Changes in the structure of the baboon hybrid zone in the Awash National Park,Ethiopia

The zone of hybridization between anubis and hamadryas baboons in the Awash National Park, Ethiopia, evidently changed significantly in the early 1970s. When first studied, by Nagel in 1968, the distribution of adult male phenotypes suggested that the hybrid zone was a narrow, static, genetic sink. In contrast, studies during the 1970s found evidence for a broad hybrid zone intergrading with parental populations and probably expanding. A more detailed investigation of data collected in 1973, presented here, shows that between 1968 and 1973 the distribution of phenotypes changed to a more clinal pattern because of the maturation and successful integration of an extensive cohort of hybrid males. We present a provisional model that accounts for the changes in terms of climatic oscillations, with the hybrid zone tending to expand during periods of climatic instability.     

Influences of gene flow on adaptive speciation in the Dubautia arborea--D. ciliolata complex

Mechanisms of reproductive isolation during plant speciation are often unclear because distinct species often experience high levels of gene flow and hybridization. Adaptive radiations such as the Hawaiian silversword alliance (HSA) provide unique opportunities to study the interactions of selection, gene flow and isolating mechanisms during the speciation process. We examined patterns of phenotypic and genetic differentiation in Dubautia arborea and Dubautia ciliolata, two parapatric HSA taxa that show marked morphological divergence but evidence of weak molecular differentiation, in order to estimate genome-wide differentiation and gene flow patterns. We scored 166 amplified fragment length polymorphism markers in a set of 89 plants from two populations each of D. arborea and D. ciliolata and phenotypically D. arborea-like and D. ciliolata-like plants from a natural hybrid zone. Analyses of population subdivision showed low levels of differentiation between the two species (F(ST) = 0.089) and evidence that the phenotypically parental hybrid zone plants were largely of parental species rather than of hybrid origin. A Bayesian analysis of population ancestry identified a number of plants with admixed D. arborea and D. ciliolata ancestry, even in nonhybrid-zone populations. These results suggest that genome-wide low levels of differentiation between D. arborea and D. ciliolata are in part due to gene flow, and favour models of genic speciation and collective evolution in which gene flow has different effects on selected loci vs. nonselected genomic regions. We discuss ecological and climatic factors that may have shaped patterns of differentiation in this species complex.     

Genotype-by-environment interaction and the fitness of plant hybrids in the wild

Natural hybrid zones between related species illustrate processes that contribute to genetic differentiation and species formation. A common viewpoint is that hybrids are essentially unfit, but they exist in a stable tension zone where selection against them is balanced by gene flow between the parent species. An alternative idea is that selection depends on the environment, for example, by favoring opposite traits in the two parental habitats or favoring hybrids within a bounded region. To determine whether selection of hybrids is environment dependent, we crossed plants of naturally hybridizing Ipomopsis aggregata and I. tenuituba in the Colorado Rocky Mountains and reciprocally planted the seed offspring into a suite of natural environments across the hybrid zone. All types of crosses produced similar numbers and weights of seeds. However, survival of the offspring after 5 years differed markedly among cross types. On average, the F1 hybrids had survival and growth rates as high as the average for their parents. But hybrid survival depended strongly on the direction of a cross, that is, on which species served as the maternal parent. This fitness difference between reciprocal hybrids appeared only in the parental environments, suggesting cytonuclear gene interactions that are environment specific. These results indicate that complex genotype-by-environment interactions can contribute to the evolutionary outcome of hybridization.     

Variation in contact zone dynamics between two species of topminnows, Fundulus notatus and F. olivaceus, across isolated drainage systems

Spatially variable selection pressure within heterogeneous environments can result in the evolution of specialist phenotypes that facilitate co-occurrence of closely related species and limit genetic exchange. If divergent selection pressures maintain reproductive isolation, hybridization is expected to correlate with the strength of underlying ecological gradients and the traits shaped by adaptive processes. We sampled ten replicate topminnow (Fundulus olivaceus and Fundulus notatus) hybrid zones in isolated drainages throughout central and southern North America. In all drainages, species were distributed in an upstream–downstream manner with contact zones localized at confluences featuring abrupt shifts from tributary to river habitat. In two drainages, the typical up and downstream positions of species were reversed. Phenotype differences between the species reflect predicted selection differences along stream gradients. Downstream populations (lower food availability and greater predator pressure) generally showed larger investment in reproduction (higher gonadal somatic index), smaller body size and lower somatic condition compared to upstream populations. Phenotypic differences between the species in the two reversed drainages were consistent with convergence of life history traits in the respective habitats. Phenotypes of individuals of hybrid origin (F₁ hybrids or backcrosses) were not significantly different from the average of the two parental forms, though there were trends towards reduced fitness. The prevalence of hybridization among drainages ranged from no hybrids in two drainages to near random mating. The strongest correlates of hybridization rate among replicate hybrid zones were similarity in body shape and the homogeneity of habitat through tributary-river confluences. The two reversed orientation hybrid zones also exhibited high prevalence of hybrids suggesting that phenotypic convergence could lead to increased hybridization.     

Hybrid Zones Between Two European Oaks: a Plant Community Approach

Phenomena of hybridization can affect the ecology and evolution of the species involved in the process, as well as their communities. Although numerous papers focus upon the problem of taxonomy, few of these have attempted to study hybrid zones in relation to the analysis of their communities. On the Iberian Peninsula, hybridization phenomena among different oak species are frequent. It is, however, between Quercus faginea Lam. and Quercus pubescens Willd. where the most noteworthy hybridization phenomenon occurs. In this respect, we are familiar with the existence of different introgression levels but we are unaware of whether these hybrids are the transitory result of the interspecific genetic flow or whether these are maintained by means of extrinsic selection processes. Study of plant communities’ flora and environment might shed light upon this issue. Comparison between plant communities dominated by one of the parental species and those dominated by individuals of hybrid origin might enable us to establish the presence or absence of an environment that is potentially selective in favour of the hybrids. Thus the possible existence of extrinsic selection. Furthermore, this information will help us to understand plant community distribution in an area␣that is difficult to interpret. To this purpose, we used multivariate ordination techniques (DCA and CCA) based upon a total of 395 floristic releves covering the whole range of the parental species on the Iberian Peninsula and upon climatic and edaphic variables for each of these releves. We also compared the groups obtained in relation to floristic similarity (Jaccard index), richness and diversity (Shannon–Weaver index). Forests associated with Quercus pubescens are related to heavy summer precipitation, whereas Quercus faginea forests correspond to lower values of this variable and higher ones for continentality. Between both formations, there is a broad hybrid zone, with diffused borders that are related to an environmental gradient of Mediterranean influence. In this region, two types of forest communities were distinguished, which enabled us to divide the hybrid zones into two territories. Our results allowed us to locate the hybrid zone in an ecotone. The differentiation between habitats appears to indicate models of ecological selection. These models require, by definition, the presence of an environmental gradient between the parental zones. We are, however, aware of the need for future experiments in order to establish whether the hybrids are better adapted than the parental species. Only with availability of all this information can intrinsic selection be rejected.     

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

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

Concordance of genetic and phenotypic characters across a sapsucker hybrid zone

Hybridization has presented a challenge for taxonomists and conservation biologists, since hybridizing forms could be stable evolutionary entities or ephemeral forms that are blending together. However, hybrid zones also provide a unique opportunity for evolutionary biologists who study the interaction between gene flow and reproductive isolation in speciation. Three forms of woodpeckers (sapsuckers; genus Sphyrapicus) in North America that are mostly geographically separated but hybridize with each other where they come into contact present a remarkable system for the study of hybridization. We provide the first comprehensive analysis of phenotypic and genetic variation across a hybrid zone between two of these forms, the red‐breasted Sphyrapicus ruber and yellow‐bellied S. varius sapsuckers. The objective was to infer whether selection maintains the differences between forms. Our analysis of eight morphometric and 20 plumage traits, and two molecular markers showed clear differences between the forms and roughly concordant clinal variation across a narrow hybrid zone. Thirty percent of sampled birds in the hybrid zone had mixed west/east genotypes at the genetic markers examined. The center of the genetic cline was located 20 km west of the crest of the Rocky Mountains. The width of the zone was 122 km, narrower than would be expected under neutral blending given reasonable estimates of the age of the zone and individual dispersal distances. Heterozygote deficit and cytonuclear disequilibrium at the centre of the hybrid zone suggested nonrandom mating or limited hybridization. Given these patterns and lack of evidence for habitat segregation we conclude that this hybrid zone is maintained by selection, most likely in the form of hybrid inferiority. This study provides an illustrative example of extensive hybridization between stable entities, providing additional evidence against the historical practice of treating hybridizing forms as members of the same species.     

A Natural Homoploid Hybrid between Centaurea horrida and Centaurea filiformis (Asteraceae) as Revealed by Morphological and Genetic Traits

Studies over the last two decades demonstrate that hybridization has played an integral role in the evolution of several sections of the genus Centaurea. Nevertheless, natural hybridization between narrow Mediterranean endemic Centaurea species has not been documented as yet. A population of fertile Centaurea individuals exhibiting intermediate morphological traits between two Sardinian narrow endemics, C. horrida and C. filiformis, was identified at the Tavolara Islet (Sardinia, Italy). Intermediate leaf length and head width characterized this population, suggesting its hybrid origin. The putative hybrid population was structured (i.e., composed of seedlings, saplings and adult individuals) and had a relatively high levels of seed production. The number of chromosomes was identical to that of the proposed progenitors (2n?=?18). Genotyping at five microsatellite loci showed that the putative hybrid possessed several alleles in common with the proposed parental species and intermediate values of genetic differentiation, as indicated by both F _ST and R _ST, between C. horrida and C. filiformis. We therefore conclude that the studied intermediate population is of hybrid origin, and discuss possible mechanisms of its reproductive isolation from the parental species, potential re-introgression, and evolutionary implications of this hybridization.     

Hybridization increases canopy arthropod diversity in the <Emphasis Type="Italic">Quercus affinis</Emphasis> × <Emphasis Type="Italic">Quercus laurina</Emphasis> complex

Understanding the factors that influence the diversity and composition of arthropod communities is a major topic in ecology. Canopy arthropod communities are a major constituent of biodiversity and show great variation in time and space according to different factors. Recently, genetic variation within tree species has attracted attention as a significant factor determining the diversity and composition of canopy arthropod communities. A major source of genetic and phenotypic novelty in plant species is interspecific hybridization, and therefore it is of interest to evaluate how this process affects the communities of associated organisms. In this study, we used microsatellite markers and geometric morphometry of leaf shape to analyze genetic and morphological variation in 45 individuals in a local hybrid zone between the oaks Quercus affinis and Q. laurina in Mexico. Individual trees were assigned to one of the parental species or to the hybrid category. The percentage of leaf area removed by herbivores was quantified in each individual and the canopies of five individuals of each categeory (two parental species and hybrids) was fogged with insecticide to assess the diversity and composition of arthropod communities. Results indicated that hybrid trees experience higher levels of herbivory than parental species and also sustain a higher abundance and richness of canopy arthropods. In general, our study supports the “hybrid susceptibility hypothesis” that predicts a higher incidence of associated arthropods on hybrid plants than in their parental species as result of the disruption of co-adapted gene complexes associated to resistance traits.     

QTL analysis of floral traits in Louisiana iris hybrids

The formation of hybrid zones between nascent species is a widespread phenomenon. The evolutionary consequences of hybridization are influenced by numerous factors, including the action of natural selection on quantitative trait variation. Here we examine how the genetic basis of floral traits of two species of Louisiana Irises affects the extent of quantitative trait variation in their hybrids. Quantitative trait locus (QTL) mapping was used to assess the size (magnitude) of phenotypic effects of individual QTL, the degree to which QTL for different floral traits are colocalized, and the occurrence of mixed QTL effects. These aspects of quantitative genetic variation would be expected to influence (1) the number of genetic steps (in terms of QTL substitutions) separating the parental species phenotypes; (2) trait correlations; and (3) the potential for transgressive segregation in hybrid populations. Results indicate that some Louisiana Iris floral trait QTL have large effects and QTL for different traits tend to colocalize. Transgressive variation was observed for six of nine traits, despite the fact that mixed QTL effects influence few traits. Overall, our QTL results imply that the genetic basis of floral morphology and color traits might facilitate the maintenance of phenotypic divergence between Iris fulva and Iris brevicaulis, although a great deal of phenotypic variation was observed among hybrids.     

Natural hybridization and hybrid zones between Quercus crassifolia and Quercus crassipes (Fagaceae) in Mexico: morphological and molecular evidence

Hybrid zones provide interesting systems to study genetic and ecological interaction between different species. The correct identification of hybrids is necessary to understand the evolutionary process involved in hybridization. An oak species complex occurring in Mexico formed by two parental species, Quercus crassifolia H. & B. and Q. crassipes H. & B., and their putative hybrid species, Q. dysophylla, was analyzed with molecular markers (random amplified polymorphic DNA [RAPDs]) and morphological tools in seven hybrid zones (10 trees per taxa in each hybrid zone) and two pure sites for each parental species (20 trees per site). We tested whether geographic proximity of hybrid plants to the allopatric site of a parental species increases its morphological and genetic similarity with its parent. Seventeen morphological traits were measured in 8700 leaves from 290 trees. Total DNA of 250 individuals was analyzed with six diagnostic RAPD primers. Quercus crassifolia differed significantly from Q. crassipes in all the examined characters. Molecular markers and morphological characters were highly coincident and support the hypothesis of hybridization in this complex, although both species remain distinct in mixed stands. Clusters and a hybrid index (for molecular and morphological data) showed that individuals from the same parental species were more similar among themselves than to putative hybrids, indicating occasional hybridization with segregation in hybrid types or backcrossing to parents. Evidence does not indicate a unidirectional pattern of gene flow.     

Genetic architecture and genomic patterns of gene flow between hybridizing species of Picea

Hybrid zones provide an opportunity to study the effects of selection and gene flow in natural settings. We employed nuclear microsatellites (single sequence repeat (SSR)) and candidate gene single-nucleotide polymorphism markers (SNPs) to characterize the genetic architecture and patterns of interspecific gene flow in the Picea glauca × P. engelmannii hybrid zone across a broad latitudinal (40–60 degrees) and elevational (350–3500 m) range in western North America. Our results revealed a wide and complex hybrid zone with broad ancestry levels and low interspecific heterozygosity, shaped by asymmetric advanced-generation introgression, and low reproductive barriers between parental species. The clinal variation based on geographic variables, lack of concordance in clines among loci and the width of the hybrid zone points towards the maintenance of species integrity through environmental selection. Congruency between geographic and genomic clines suggests that loci with narrow clines are under strong selection, favoring either one parental species (directional selection) or their hybrids (overdominance) as a result of strong associations with climatic variables such as precipitation as snow and mean annual temperature. Cline movement due to past demographic events (evidenced by allelic richness and heterozygosity shifts from the average cline center) may explain the asymmetry in introgression and predominance of P. engelmannii found in this study. These results provide insights into the genetic architecture and fine-scale patterns of admixture, and identify loci that may be involved in reproductive barriers between the species.     

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

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