Local adaptation along an environmental cline in a species with an inversion polymorphism

Polymorphic inversions are ubiquitous across the animal kingdom and are frequently associated with clines in inversion frequencies across environmental gradients. Such clines are thought to result from selection favouring local adaptation; however, empirical tests are scarce. The seaweed fly Coelopa frigida has an α/β inversion polymorphism, and previous work demonstrated that the α inversion frequency declines from the North Sea to the Baltic Sea and is correlated with changes in tidal range, salinity, algal composition and wrackbed stability. Here, we explicitly test the hypothesis that populations of C. frigida along this cline are locally adapted by conducting a reciprocal transplant experiment of four populations along this cline to quantify survival. We found that survival varied significantly across treatments and detected a significant Location x Substrate interaction, indicating local adaptation. Survival models showed that flies from locations at both extremes had highest survival on their native substrates, demonstrating that local adaptation is present at the extremes of the cline. Survival at the two intermediate locations was, however, not elevated at the native substrates, suggesting that gene flow in intermediate habitats may override selection. Together, our results support the notion that population extremes of species with polymorphic inversions are often locally adapted, even when spatially close, consistent with the growing view that inversions can have direct and strong effects on the fitness of species.     

Predicting the spread of an invasive plant: combining experiments and ecological niche model

Rapid evolutionary adjustments to novel environments may contribute to the successful spread of invasive species, and can lead to niche shifts making range dynamics unpredictable. These effects might be intensified by artificial selection in the course of breeding efforts, since many successful plant invaders were deliberately introduced and cultivated as ornamentals. We hypothesized that the invasion success of Buddleja davidii, the ornamental butterfly bush, is facilitated by local adaptation to minimum temperatures and thus, exhibits unpredictable range dynamics. To assess the potential effects of adaptive evolution and artificial selection on the spread of B. davidii, we combined a common garden experiment investigating local adaptation to frost, with ecological niche modelling of the species’ native and invasive ranges. We expected that populations naturalized in sub‐continental climate are less susceptible to frost than populations from oceanic climate, and that the invasive range does not match predictions based on climatic data from the native range. Indeed, we revealed significant variation among invasive B. davidii populations in frost resistance. However, frost hardiness was not related to geographic location or climatic variables of the populations’ home site, suggesting that invasive B. davidii populations are not locally adapted to minimum temperatures. This is in line with results of our ecological niche model that did not detect a niche shift between the species’ native range in China, and its invasive range in Europe and North America. Furthermore, our niche model showed that the potential invasive range of B. davidii is still not completely occupied. Together with the frost resistance data obtained in our experiment, the results indicate that climatic conditions are currently not limiting the further spread of the species in Europe and North America.     

Performance of native grape phylloxera on host plants within and among terrestrial islands in Arizona, USA

In Arizona, USA, the canyon grape, Vitis arizonica Englemann, and grape phylloxera (Daktulosphaira vitifoliae Fitch, Homoptera, Phylloxeridae) are distributed among mountain ranges that are surrounded by expanses of desert lacking Vitis habitat, thus forming a system of terrestrial islands. Both herbivore and host populations may have diverged genetically among mountain ranges under the influence of restricted gene flow and variable selection among sites. Herbivore adaptation to local hosts would be expected to ensue, with the potential to promote divergence, both in traits under selection and by further reducing the probability of interisland colonization. To test the hypothesis that phylloxera are adapted to local hosts, demographic components of fitness of field-collected native grape phylloxera were measured in the greenhouse on vines of V. arizonica that were categorized as either natal, neighboring, and or isolated hosts. There was no evidence for greater adaptation to natal or neighboring hosts but there were significant interactions between herbivore and host treatments in one experiment. There was genetic variation for gall formation among six clones tested. Though a failure to detect local adaptation could have resulted from low statistical power, benign experimental conditions, or inadequate genetic variation, the divergence of isolated grape populations is suggested to have been insufficient to promote local adaptation in grape phylloxera at the level of isolated mountain ranges. It is further suggested that, within populations, adaptation to individual host plants could be forestalled by selection for ’general purpose genotypes’ through wind-borne displacement of colonizers into the unpredictable environment of a heterogeneous array of hosts. In addition, short-term extinction/colonization dynamics could promote gene flow such that time is insufficient for adaptive mutations or gene combination to arise. Received: 26 December 1998 / Accepted: 24 May 1999     

Local adaptation at range edges: comparing elevation and latitudinal gradients

Local adaptation at range edges influences species’ distributions and how they respond to environmental change. However, the factors that affect adaptation, including gene flow and local selection pressures, are likely to vary across different types of range edge. We performed a reciprocal transplant experiment to investigate local adaptation in populations of Plantago lanceolata and P. major from central locations in their European range and from their latitudinal and elevation range edges (in northern Scandinavia and Swiss Alps, respectively). We also characterized patterns of genetic diversity and differentiation in populations using molecular markers. Range‐centre plants of P. major were adapted to conditions at the range centre, but performed similarly to range‐edge plants when grown at the range edges. There was no evidence for local adaptation when comparing central and edge populations of P. lanceolata. However, plants of both species from high elevation were locally adapted when compared with plants from high latitude, although the reverse was not true. This asymmetry was associated with greater genetic diversity and less genetic differentiation over the elevation gradient than over the latitudinal gradient. Our results suggest that adaptation in some range‐edge populations could increase their performance following climate change. However, responses are likely to differ along elevation and latitudinal gradients, with adaptation more likely at high‐elevation. Furthermore, based upon these results, we suggest that gene flow is unlikely to constrain adaptation in range‐edge populations of these species.     

Host plant use of Phyllotreta nemorum: do coadapted gene complexes play a role?

The view of (insect) populations as assemblages of local subpopulations connected by gene flow is gaining ground. In such structured populations, local adaptation may occur. In phytophagous insects, one way in which local adaptation has been demonstrated is by performing reciprocal transplant experiments where performance of insects on native and novel host plants are compared. Trade-offs are assumed to be responsible for a negative correlation in performance on alternative host plants. Due to mixed results of these experiments, the importance of trade-offs in host plant use of phytophagous insects has been under discussion. Here we propose that another genetic mechanism, the evolution of coadapted gene complexes, might also be associated with local adaptation. In this case, however, transplant experiments might not reveal any local adaptation until hybridization takes place. We review the results we have obtained in our work on the host plant use of the flea beetle Phyllotreta nemorum L. (Coleoptera: Chrysomelidae: Alticinae), and propose a hypothesis involving coadapted genes to explain the distribution of genes that render P. nemorum resistant to defences of one of its host plants, Barbarea vulgaris R. Br. (Cruciferae).     

Patterns of introduction and adaptation during the invasion of Aegilops triuncialis (Poaceae) into Californian serpentine soils

Multiple introductions can play a prominent role in explaining the success of biological invasions. One often cited mechanism is that multiple introductions of invasive species prevent genetic bottlenecks by parallel introductions of several distinct genotypes that, in turn, provide heritable variation necessary for local adaptation. Here, we show that the invasion of Aegilops triuncialis into California, USA, involved multiple introductions that may have facilitated invasion into serpentine habitats. Using microsatellite markers, we compared the polymorphism and genetic structure of populations of Ae. triuncialis invading serpentine soils in California to that of accessions from its native range. In a glasshouse study, we also compared phenotypic variation in phenological and fitness traits between invasive and native populations grown on loam soil and under serpentine edaphic conditions. Molecular analysis of invasive populations revealed that Californian populations cluster into three independent introductions (i.e. invasive lineages). Our glasshouse common garden experiment found that all Californian populations exhibited higher fitness under serpentine conditions. However, the three invasive lineages appear to represent independent pathways of adaptation to serpentine soil. Our results suggest that the rapid invasion of serpentine habitats in California may have been facilitated by the existence of colonizing Eurasian genotypes pre‐adapted to serpentine soils.     

No evidence for local adaptation and an epigenetic underpinning in native and non‐native ruderal plant species in Germany

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Effects of competition with Bromus tectorum on early establishment of Poa secunda accessions: can seed source impact restoration success?

When landscapes are heavily impacted by biological invasion, local populations of native plant species may no longer be adapted to altered environmental conditions. In these cases, it is useful to investigate alternative sources of germplasm, such as cultivated varieties, for planting at restoration sites. This study compared cultivated and wild (local) varieties of the native perennial bunchgrass, Poa secunda J. Presl, grown with and without the exotic, invasive Bromus tectorum L. in a greenhouse setting. While P. secunda cultivars emerged and grew more rapidly than wild seed sources, this advantage declined in the presence of B. tectorum and cultivated germplasm did not outperform wild accessions in the presence of an invasive species. Given the novel genetic background of cultivars and their potential to alter patterns of dominance in native plant communities, we recommend the use of local or regional wild seed sources when possible to conserve regional patterns of genetic diversity and adaptation. Use of multiple seed sources may increase the potential for capturing vigorous genotypes in the restoration seed mix. In cases where sites are heavily impacted by exotic, invasive species, other control measures will be necessary to improve establishment of native species in grassland restoration programs.     

Genetic variability of invasive species,Fallopia convolvulus (Polygonaceae) in Iran

Abstract

Fallopia (Polygonaceae) as a noxious weed contains 17 species in the world out of which three species occur in Iran with invasive distribution. F. convolvulus growing in wide range of soil types causes significant problems for native ecosystems of river banks. In this study, we have examined genetic variability in F. convolvulus for the first time in Iran. Ten Inter Simple Sequence Repeats (ISSR) markers were used to study the genetic variability on 11 populations of this species. Genetic diversity parameters, genetic distance and gene flow were determined. Genetic variation at inter- and intra-population level was evaluated by different methods. AMOVA and structure analyses revealed high genetic diversity within populations. Mantel test revealed a significant correlation between genetic and geographic distances. Between populations a limited gene flow was observed. It is concluded that local adaptation, low gene exchange and genetic drift can affect genetic diversity of F. convolvulus. Despite self-compatibility of this species, it is proposed that outcrossing may occur because of higher genetic variation among populations of this taxon.     

The common cuckoo Cuculus canorus is not locally adapted to its reed warbler Acrocephalus scirpaceus host

The obligate avian brood parasitic common cuckoo Cuculus canorus comprises different strains of females that specialize on particular host species by laying eggs of a constant type that often mimics those of the host. Whether cuckoos are locally adapted for mimicking populations of the hosts on which they are specialized has never been investigated. In this study, we first explored the possibility of local adaptation in cuckoo egg mimicry over a geographical mosaic of selection exerted by one of its main European hosts, the reed warbler Acrocephalus scirpaceus. Secondly, we investigated whether cuckoos inhabiting reed warbler populations with a broad number of alternative suitable hosts at hand were less locally adapted. Cuckoo eggs showed different degrees of mimicry to different reed warbler populations. However, cuckoo eggs did not match the egg phenotypes of their local host population better than eggs of other host populations, indicating that cuckoos were not locally adapted for mimicry on reed warblers. Interestingly, cuckoos exploiting reed warblers in populations with a relatively larger number of co-occurring cuckoo gentes showed lower than average levels of local adaptation in egg volume. Our results suggest that cuckoo local adaptation might be prevented when different cuckoo populations exploit more or fewer different host species, with gene flow or frequent host switches breaking down local adaptation where many host races co-occur.     

Determinants of hierarchical genetic structure in Atlantic salmon populations: environmental factors vs. anthropogenic influences

Disentangling the effects of natural environmental features and anthropogenic factors on the genetic structure of endangered populations is an important challenge for conservation biology. Here, we investigated the combined influences of major environmental features and stocking with non‐native fish on the genetic structure and local adaptation of Atlantic salmon (Salmo salar) populations. We used 17 microsatellite loci to genotype 975 individuals originating from 34 French rivers. Bayesian analyses revealed a hierarchical genetic structure into five geographically distinct clusters. Coastal distance, geological substrate and river length were strong predictors of population structure. Gene flow was higher among rivers with similar geologies, suggesting local adaptation to geological substrate. The effect of river length was mainly owing to one highly differentiated population that has the farthest spawning grounds off the river mouth (up to 900 km) and the largest fish, suggesting local adaptation to river length. We detected high levels of admixture in stocked populations but also in neighbouring ones, implying large‐scale impacts of stocking through dispersal of non‐native individuals. However, we found relatively few admixed individuals suggesting a lower fitness of stocked fish and/or some reproductive isolation between wild and stocked individuals. When excluding stocked populations, genetic structure increased as did its correlation with environmental factors. This study overall indicates that geological substrate and river length are major environmental factors influencing gene flow and potential local adaptation among Atlantic salmon populations but that stocking with non‐native individuals may ultimately disrupt these natural patterns of gene flow among locally adapted populations.     

Local thermal adaptation detected during multiple life stages across populations of Drosophila melanogaster

Thermal adaptation is typically detected by examining the tolerance of a few populations to extreme temperatures within a single life stage. However, the extent to which adaptation occurs among many different populations might depend on the tolerance of multiple life stages and the average temperature range that the population experiences. Here, we examined local adaptation to native temperature conditions in eleven populations of the well‐known cosmopolitan fruit fly, Drosophila melanogaster. These populations were sampled from across the global range of D. melanogaster. We measured traits related to fitness during each life stage to determine whether certain stages are more sensitive to changes in temperature than others. D. melanogaster appeared to show local adaptation to native temperatures during the egg, larval and adult life stages, but not the pupal stage. This suggests that across the entire distribution of D. melanogaster, certain life stages might be locally adapted to native temperatures, whereas other stages might use phenotypic plasticity or tolerance to a wide range of temperatures experienced in the native environment of this species.     

Tetranychus evansi spider mite populations suppress tomato defenses to varying degrees

Plant defense suppression is an offensive strategy of herbivores, in which they manipulate plant physiological processes to increase their performance. Paradoxically, defense suppression does not always benefit the defense‐suppressing herbivores, because lowered plant defenses can also enhance the performance of competing herbivores and can expose herbivores to increased predation. Suppression of plant defense may therefore entail considerable ecological costs depending on the presence of competitors and natural enemies in a community. Hence, we hypothesize that the optimal magnitude of suppression differs among locations. To investigate this, we studied defense suppression across populations of Tetranychus evansi spider mites, a herbivore from South America that is an invasive pest of solanaceous plants including cultivated tomato, Solanum lycopersicum, in other parts of the world. We measured the level of expression of defense marker genes in tomato plants after infestation with mites from eleven different T. evansi populations. These populations were chosen across a range of native (South American) and non‐native (other continents) environments and from different host plant species. We found significant variation at three out of four defense marker genes, demonstrating that T. evansi populations suppress jasmonic acid‐ and salicylic acid‐dependent plant signaling pathways to varying degrees. While we found no indication that this variation in defense suppression was explained by differences in host plant species, invasive populations tended to suppress plant defense to a smaller extent than native populations. This may reflect either the genetic lineage of T. evansi—as all invasive populations we studied belong to one linage and both native populations to another—or the absence of specialized natural enemies in invasive T. evansi populations.     

Forests are not immune to plant invasions: phenotypic plasticity and local adaptation allow <Emphasis Type="Italic">Prunella vulgaris</Emphasis> to colonize a temperate evergreen rainforest

In the South American temperate evergreen rainforest (Valdivian forest), invasive plants are mainly restricted to open sites, being rare in the shaded understory. This is consistent with the notion of closed-canopy forests as communities relatively resistant to plant invasions. However, alien plants able to develop shade tolerance could be a threat to this unique forest. Phenotypic plasticity and local adaptation are two mechanisms enhancing invasiveness. Phenotypic plasticity can promote local adaptation by facilitating the establishment and persistence of invasive species in novel environments. We investigated the role of these processes in the recent colonization of Valdivian forest understory by the perennial alien herb Prunella vulgaris from nearby populations in open sites. Using reciprocal transplants, we found local adaptation between populations. Field data showed that the shade environment selected for taller plants and greater specific leaf areas. We found population differentiation and within-population genetic variation in both mean values and reaction norms to light variation of several ecophysiological traits in common gardens from seeds collected in sun and shade populations. The colonization of the forest resulted in a reduction of plastic responses to light variation, which is consistent with the occurrence of genetic assimilation and suggests that P. vulgaris individuals adapted to the shade have reduced probabilities to return to open sites. All results taken together confirm the potential for rapid evolution of shade tolerance in P. vulgaris and suggest that this alien species may pose a threat to the native understory flora of Valdivian forest.     

Spatiotemporal variation in local adaptation of a specialist insect herbivore to its long‐lived host plant

Local adaptation of interacting species to one another indicates geographically variable reciprocal selection. This process of adaptation is central in the organization and maintenance of genetic variation across populations. Given that the strength of selection and responses to it often vary in time and space, the strength of local adaptation should in theory vary between generations and among populations. However, such spatiotemporal variation has rarely been explicitly demonstrated in nature and local adaptation is commonly considered to be relatively static. We report persistent local adaptation of the short‐lived herbivore Abrostola asclepiadis to its long‐lived host plant Vincetoxicum hirundinaria over three successive generations in two studied populations and considerable temporal variation in local adaptation in six populations supporting the geographic mosaic theory. The observed variation in local adaptation among populations was best explained by geographic distance and population isolation, suggesting that gene flow reduces local adaptation. Changes in herbivore population size did not conclusively explain temporal variation in local adaptation. Our results also imply that short‐term studies are likely to capture only a part of the existing variation in local adaptation.     

Interspecific competition counteracts negative effects of dispersal on adaptation of an arthropod herbivore to a new host

Dispersal and competition have both been suggested to drive variation in adaptability to a new environment, either positively or negatively. A simultaneous experimental test of both mechanisms is however lacking. Here, we experimentally investigate how population dynamics and local adaptation to a new host plant in a model species, the two‐spotted spider mite (Tetranychus urticae), are affected by dispersal from a stock population (no‐adapted) and competition with an already adapted spider mite species (Tetranychus evansi). For the population dynamics, we find that competition generally reduces population size and increases the risk of population extinction. However, these negative effects are counteracted by dispersal. For local adaptation, the roles of competition and dispersal are reversed. Without competition, dispersal exerts a negative effect on adaptation (measured as fecundity) to a novel host and females receiving the highest number of immigrants performed similarly to the stock population females. By contrast, with competition, adding more immigrants did not result in a lower fecundity. Females from populations with competition receiving the highest number of immigrants had a significantly higher fecundity than females from populations without competition (same dispersal treatment) and than the stock population females. We suggest that by exerting a stronger selection on the adapting populations, competition can counteract the migration load effect of dispersal. Interestingly, adaptation to the new host does not significantly reduce performance on the ancestral host, regardless of dispersal rate or competition. Our results highlight that assessments of how species can adapt to changing conditions need to jointly consider connectivity and the community context.     

Effects of Competition and Life History Stage on the Expression of Local Adaptation in Two Native Bunchgrasses

Concerns about the use of genetically appropriate material in restoration often focus on questions of local adaptation. Many reciprocal transplant studies have demonstrated local adaptation in native plant species, but very few have examined how interspecific competition affects the expression of adaptive variation. Our study examined regional scales of adaptation between foothill and coastal populations of two California native bunchgrasses (Elymus glaucus and Nassella pulchra). By combining competitive manipulations with reciprocal transplants, we examined the importance of the vegetation at a site as a selective factor in the process of local adaptation. By monitoring survival and reproduction of reciprocally transplanted populations over the course of 3 years, we also studied the effect of life history stage on the expression of local adaptation. For most of the fitness components we measured, local adaptation was detected and interspecific competition consistently amplified its expression. Expression of local adaptation was especially apparent in the more inbreeding species E. glaucus and suggests that with weaker gene flow, selection may be more effective in creating ecotypes within this species. Local adaptation was detected at all life history stages but was most strongly expressed in traits associated with adult reproduction and the viability of seeds produced by the transplants. Taken together, our results indicate that the importance of local adaptation will become more apparent in the later stages of a restoration project as the plants at a site begin to reproduce and as they experience greater interspecific competition from the maturing vegetation at the site.     

Go forth,evolve and prosper: the genetic basis of adaptive evolution in an invasive species

Invasive species stand accused of a familiar litany of offences, including displacing native species, disrupting ecological processes and causing billions of dollars in ecological damage (Cox 1999 ). Despite these transgressions, invasive species have at least one redeeming virtue – they offer us an unparalleled opportunity to investigate colonization and responses of populations to novel conditions in the invaded habitat (Elton 1958 ; Sakai et al. 2001 ). Invasive species are by definition colonists that have arrived and thrived in a new location. How they are able to thrive is of great interest, especially considering a paradox of invasion (Sax & Brown 2000 ): if many populations are locally adapted (Leimu & Fischer 2008 ), how could species introduced into new locations become so successful? One possibility is that populations adjust to the new conditions through plasticity – increasing production of allelopathic compounds (novel weapons), or taking advantage of new prey, for example. Alternatively, evolution could play a role, with the populations adapting to the novel conditions of the new habitat. There is increasing evidence, based on phenotypic data, for rapid adaptive evolution in invasive species (Franks et al. 2012 ; Colautti & Barrett 2013 ; Sultan et al. 2013 ). Prior studies have also demonstrated genetic changes in introduced populations using neutral markers, which generally do not provide information on adaptation. Thus, the genetic basis of adaptive evolution in invasive species has largely remained unknown. In this issue of Molecular Ecology, Vandepitte et al. ( 2014 ) provide some of the first evidence in invasive populations for molecular genetic changes directly linked to adaptation.     

MULTIVARIATE ADAPTATION BUT NO INCREASE IN COMPETITIVE ABILITY IN INVASIVE GERANIUM CAROLINIANUM L. (GERANIACEAE)

Adaptive evolution can affect the successful establishment of invasive species, but changes in selective pressures, loss of genetic variation in relevant traits, and/or altered trait correlations can make adaptation difficult to predict. We used a common‐garden experiment to assess trait correlations and patterns of adaptation in the invasive plant, Geranium carolinianum, sampled across 20 populations in its native (United States) and invasive (China) ranges. We used multivariate Q_ST – F_ST tests to determine if phenotypic differences between countries are attributable to adaptation. We also compared population‐level variation within each country to assess whether local adaptation resulted in similar multivariate phenotypes in the United States and China. Between countries, most phenotypic differences are indistinguishable from genetic drift, although we detected a signature of adaptation to the colder, drier winters in China. There was no evidence for increases in invasive traits in China. Within countries, strong multivariate adaptation appears to be driven by latitudinal climatic variation in the United States, but not in China. Additionally, adaptive trait combinations as well as their underlying correlations differ between the two countries, indicating that adaptation in invasive populations does not parallel patterns in native populations due to differences in selection pressures, genetic constraints, or both.     

Adaptive plasticity and niche expansion in an invasive thistle

Phenotypic differentiation in size and fecundity between native and invasive populations of a species has been suggested as a causal driver of invasion in plants. Local adaptation to novel environmental conditions through a micro‐evolutionary response to natural selection may lead to phenotypic differentiation and fitness advantages in the invaded range. Local adaptation may occur along a stress tolerance trade‐off, favoring individuals that, in benign conditions, shift resource allocation from stress tolerance to increased vigor and fecundity and, therefore, invasiveness. Alternately, the typically disturbed invaded range may select for a plastic, generalist strategy, making phenotypic plasticity the main driver of invasion success. To distinguish between these hypotheses, we performed a field common garden and tested for genetically based phenotypic differentiation, resource allocation shifts in response to water limitation, and local adaptation to the environmental gradient which describes the source locations for native and invasive populations of diffuse knapweed (Centaurea diffusa). Plants were grown in an experimental field in France (naturalized range) under water addition and limitation conditions. After accounting for phenotypic variation arising from environmental differences among collection locations, we found evidence of genetic variation between the invasive and native populations for most morphological and life‐history traits under study. Invasive C. diffusa populations produced larger, later maturing, and therefore potentially fitter individuals than native populations. Evidence for local adaptation along a resource allocation trade‐off for water limitation tolerance is equivocal. However, native populations do show evidence of local adaptation to an environmental gradient, a relationship which is typically not observed in the invaded range. Broader analysis of the climatic niche inhabited by the species in both ranges suggests that the physiological tolerances of C. diffusa may have expanded in the invaded range. This observation could be due to selection for plastic, “general‐purpose” genotypes with broad environmental tolerances.