Evolutionary origins of invasive populations

Phylogeography of the highly invasive sugar beet nematode, Heterodera schachtii (Schmidt, 1871), based on microsatellites

Plant-parasitic nematodes (PPNs) threaten crop production worldwide. Yet few studies have examined their intraspecific genetic diversity or patterns of invasion, critical data for managing the spread of these cryptic pests. The sugar beet nematode Heterodera schachtii, a global invader that parasitizes over 200 plant species, represents a model for addressing important questions about the invasion genetics of PPNs. Here, a phylogeographic study using 15 microsatellite markers was conducted on 231 H. schachtii individuals sampled from four continents, and invasion history was reconstructed through an approximate Bayesian computation approach, with emphasis on the origin of newly discovered populations in Korea. Multiple analyses confirmed the existence of cryptic lineages within this species, with the Korean populations comprising one group (group 1) and the populations from Europe, Australia, North America, and western Asia comprising another (group 2). No multilocus genotypes were shared between the two groups, and large genetic distance was inferred between them. Population subdivision was also revealed among the populations of group 2 in both population comparison and STRUCTURE analyses, mostly due to different divergent times between invasive and source populations. The Korean populations showed substantial genetic homogeneity and likely originated from a single invasion event. However, none of the other studied populations were implicated as the source. Further studies with additional populations are needed to better describe the distribution of the potential source population for the East Asian lineage.

Invasion triple trouble: environmental fluctuations, fluctuation-adapted invaders and fluctuation-mal-adapted communities all govern invasion success

BACKGROUND

It has been suggested that climate change will lead to increased environmental fluctuations, which will undoubtedly have evolutionary consequences for all biota. For instance, fluctuations can directly increase the risk of invasions of alien species into new areas, as these species have repeatedly been proposed to benefit from disturbances. At the same time increased environmental fluctuations may also select for better invaders. However, selection by fluctuations may also influence the resistance of communities to invasions, which has rarely been tested. We tested eco-evolutionary dynamics of invasion with bacterial clones, evolved either in constant or fluctuating temperatures, and conducted experimental invasions in both conditions.

RESULTS

We found clear evidence that ecological fluctuations, as well as adaptation to fluctuations by both the invader and community, all affected invasions, but played different roles at different stages of invasion. Ecological fluctuations clearly promoted invasions, especially into fluctuation mal-adapted communities. The evolutionary background of the invader played a smaller role.

CONCLUSIONS

Our results indicate that climate change associated disturbances can directly increase the risk of invasions by altering ecological conditions during invasions, as well as via the evolution of both the invader and communities. Our experiment provides novel information on the complex consequences of climate change on invasions in general, and also charts risk factors associated with the spread of environmentally growing opportunistic pathogens.

Potential limits to the benefits of admixture during biological invasion

Species introductions often bring together genetically divergent source populations, resulting in genetic admixture. This geographic reshuffling of diversity has the potential to generate favourable new genetic combinations, facilitating the establishment and invasive spread of introduced populations. Observational support for the superior performance of admixed introductions has been mixed, however, and the broad importance of admixture to invasion questioned. Under most underlying mechanisms, admixture's benefits should be expected to increase with greater divergence among and lower genetic diversity within source populations, though these effects have not been quantified in invaders. We experimentally crossed source populations differing in divergence in the invasive plant Centaurea solstitialis. Crosses resulted in many positive (heterotic) interactions, but fitness benefits declined and were ultimately negative at high source divergence, with patterns suggesting cytonuclear epistasis. We explored the literature to assess whether such negative epistatic interactions might be impeding admixture at high source population divergence. Admixed introductions reported for plants came from sources with a wide range of genetic variation, but were disproportionately absent where there was high genetic divergence among native populations. We conclude that while admixture is common in species introductions and often happens under conditions expected to be beneficial to invaders, these conditions may be constrained by predictable negative genetic interactions, potentially explaining conflicting evidence for admixture's benefits to invasion.

The impact of transposable elements in adaptive evolution

The growing knowledge about the influence of transposable elements (TEs) on (a) long-term genome and transcriptome evolution; (b) genomic, transcriptomic and epigenetic variation within populations; and (c) patterns of somatic genetic differences in individuals continues to spur the interest of evolutionary biologists in the role of TEs in adaptive evolution. As TEs can trigger a broad range of molecular variation in a population with potentially severe fitness and phenotypic consequences for individuals, different mechanisms evolved to keep TE activity in check, allowing for a dynamic interplay between the host, its TEs and the environment in evolution. Here, we review evidence for adaptive phenotypic changes associated with TEs and the basic molecular mechanisms by which the underlying genetic changes arise: (a) domestication, (b) exaptation, (c) host gene regulation, (d) TE-mediated formation of intronless gene copies-so-called retrogenes and (e) overall increased genome plasticity. Furthermore, we review and discuss how the stress-dependent incapacitation of defence mechanisms against the activity of TEs might facilitate adaptive responses to environmental challenges and how such mechanisms might be particularly relevant in species frequently facing novel environments, such as invasive, pathogenic or parasitic species.

Pesticide durability and the evolution of resistance: A novel application of survival analysis

BACKGROUND

Arthropod pests are widely perceived to evolve resistance to insecticides at different rates. Although widespread "successful" species are assumed to evolve quickly and minor pests slowly, few studies have utilized published data on resistance events to test for differences among species. Using 532 records from the Arthropod Pesticide Resistance Database covering 20 species, we applied a survival analysis to model the number of generations from insecticide introduction to the first report of arthropod resistance, providing one of the most comprehensive analyses of this question to date. Our approach tested: 1) whether successful pests evolve resistance faster than close relatives, 2) whether species differ significantly in the time to demonstrate resistance, and 3) whether different insecticide classes differ in durability (length of time an insecticide is used before resistance arises).

RESULTS

We found that species differed significantly in the amount of time it took for resistance to be reported. Overall, the median duration between the introduction of an insecticide and the first report of resistance was 66 generations (95% c.i. 60-78 generations), and highly-resistant arthropods did not evolve resistance faster than their relatives. Insecticide durability did not differ by the mode of action or year of introduction.

CONCLUSION

Arthropod species significantly varied in how rapidly they evolve resistance to new insecticides, regardless of their chemistry. Visualization of the history of insecticide resistance provides information to be used for understanding how pesticide resistance evolved and how it can best be managed. © 2018 Society of Chemical Industry.

A mosaic of phenotypic variation in giant ragweed (Ambrosia trifida): Local- and continental-scale patterns in a range-expanding agricultural weed

Spatial patterns of trait variation across a species' range have implications for population success and evolutionary change potential, particularly in range-expanding and weedy species that encounter distinct selective pressures at large and small spatial scales simultaneously. We investigated intraspecific trait variation in a common garden experiment with giant ragweed (Ambrosia trifida), a highly variable agricultural weed with an expanding geographic range and broad ecological amplitude. Our study included paired populations from agricultural and natural riparian habitats in each of seven regions ranging east to west from the core of the species' distribution in central Ohio to southeastern Minnesota, which is nearer the current invasion front. We observed trait variation across both large- and small-scale putative selective gradients. At large scales, giant ragweed populations from the westernmost locations were nearly four times more fecund and had a nearly 50% increase in reproductive allocation compared to populations from the core. The degree of surface texture on fruits also declined from east to west. Greater fecundity in the west represents a putative trade-off between fruit size and fruit number across the study region, although no such trade-off was found across individual plants. This pattern may effectively result in greater propagule pressure closer to the invasion front. At smaller spatial scales, plants from agricultural populations emerged later and were smaller than plants from riparian populations. However, because plants from agricultural populations allocated more biomass to reproduction, total fecundity did not differ across habitats. Our emergence data are consistent with previous observations showing delayed emergence in agricultural compared to natural populations; thus evolutionary change may be predictable as giant ragweed continues spreading into agricultural fields throughout North America. These shifts in life-history strategy apparently bear no fecundity cost, suggesting that giant ragweed's success can be attributed at least in part to its substantial adaptive potential.

Selection for life-history traits to maximize population growth in an invasive marine species

Species establishing outside their natural range, negatively impacting local ecosystems, are of increasing global concern. They often display life-history features characteristic for r-selected populations with fast growth and high reproduction rates to achieve positive population growth rates (r) in invaded habitats. Here, we demonstrate substantially earlier maturation at a 2 orders of magnitude lower body mass at first reproduction in invasive compared to native populations of the comb jelly Mnemiopsis leidyi. Empirical results are corroborated by a theoretical model for competing life-history traits that predicts maturation at the smallest possible size to optimize r, while individual lifetime reproductive success (R₀ ), optimized in native populations, is near constant over a large range of intermediate maturation sizes. We suggest that high variability in reproductive tactics in native populations is an underappreciated determinant of invasiveness, acting as substrate upon which selection can act during the invasion process.

Local pre-adaptation to disturbance and inbreeding-environment interactions affect colonisation abilities of diploid and tetraploid Centaurea stoebe

Primary colonisation in invasive ranges most commonly occurs in disturbed habitats, where anthropogenic disturbance may cause physical damage to plants. The tolerance to such damage may differ between cytotypes and among populations as a result of differing population histories (adaptive differentiation between ruderal verus natural habitats). Moreover, founder populations often experience inbreeding depression, the effects of which may increase through physical damage due to inbreeding-environment interactions. We aimed to understand how such colonisation processes differ between diploid and tetraploid Centaurea stoebe populations, with a view to understanding why only tetraploids are invasive. We conducted a clipping experiment (frequency: zero, once or twice in the growing season) on inbred versus outbred offspring originating from 37 C. stoebe populations of varying cytotype, range and habitat type (natural versus ruderal). Aboveground biomass was harvested at the end of the vegetation period, while re-sprouting success was recorded in the following spring. Clipping reduced re-sprouting success and biomass, which was significantly more pronounced in natural than in ruderal populations. Inbreeding depression was not detected under benign conditions, but became increasingly apparent in biomass when plants were clipped. The effects of clipping and inbreeding did not differ between cytotypes. Adaptive differentiation in disturbance tolerance was higher among populations than between cytotypes, which highlights the potential of pre-adaptation in ruderal populations during early colonisation on anthropogenically disturbed sites. While the consequences of inbreeding increased through clipping-mediated stress, they were comparable between cytotypes, and consequently do not contribute to understanding the cytotype shift in the invasive range.

A molecular phylogenetic appraisal of the acanthostomines Acanthostomum and Timoniella and their position within Cryptogonimidae (Trematoda: Opisthorchioidea)

The phylogenetic position of three taxa from two trematode genera, belonging to the subfamily Acanthostominae (Opisthorchioidea: Cryptogonimidae), were analysed using partial 28S ribosomal DNA (Domains 1-2) and internal transcribed spacers (ITS1-5.8S-ITS2). Bayesian inference and Maximum likelihood analyses of combined 28S rDNA and ITS1 + 5.8S + ITS2 sequences indicated the monophyly of the genus Acanthostomum (A. cf. americanum and A. burminis) and paraphyly of the Acanthostominae. These phylogenetic relationships were consistent in analyses of 28S alone and concatenated 28S + ITS1 + 5.8S + ITS2 sequences analyses. Based on molecular phylogenetic analyses, the subfamily Acanthostominae is therefore a paraphyletic taxon, in contrast with previous classifications based on morphological data. Phylogenetic patterns of host specificity inferred from adult stages of other cryptogonimid taxa are also well supported. However, analyses using additional genera and species are necessary to support the phylogenetic inferences from this study. Our molecular phylogenetic reconstruction linked two larval stages of A. cf. americanum cercariae and metacercariae. Here, we present the evolutionary and ecological implications of parasitic infections in freshwater and brackish environments.

Naturalization of European plants on other continents: The role of donor habitats

The success of European plant species as aliens worldwide is thought to reflect their association with human-disturbed environments. However, an explicit test including all human-made, seminatural and natural habitat types of Europe, and their contributions as donor habitats of naturalized species to the rest of the globe, has been missing. Here we combine two databases, the European Vegetation Checklist and the Global Naturalized Alien Flora, to assess how human influence in European habitats affects the probability of naturalization of their plant species on other continents. A total of 9,875 native European vascular plant species were assigned to 39 European habitat types; of these, 2,550 species have become naturalized somewhere in the world. Species that occur in both human-made habitats and seminatural or natural habitats in Europe have the highest probability of naturalization (64.7% and 64.5% of them have naturalized). Species associated only with human-made or seminatural habitats still have a significantly higher probability of becoming naturalized (41.7% and 28.6%, respectively) than species confined to natural habitats (19.4%). Species associated with arable land and human settlements were recorded as naturalized in the largest number of regions worldwide. Our findings highlight that plant species' association with native-range habitats disturbed by human activities, combined with broad habitat range, play an important role in shaping global patterns of plant invasions.

Evolving mutation rate advances the invasion speed of a sexual species

Background

Many species are shifting their ranges in response to global climate change. Range expansions are known to have profound effects on the genetic composition of populations. The evolution of dispersal during range expansion increases invasion speed, provided that a species can adapt sufficiently fast to novel local conditions. Genetic diversity at the expanding range border is however depleted due to iterated founder effects. The surprising ability of colonizing species to adapt to novel conditions while being subjected to genetic bottlenecks is termed ‘the genetic paradox of invasive species’. Mutational processes have been argued to provide an explanation for this paradox. Mutation rates can evolve, under conditions that favor an increased rate of adaptation, by hitchhiking on beneficial mutations through induced linkage disequilibrium. Here we argue that spatial sorting, iterated founder events, and population structure benefit the build-up and maintenance of such linkage disequilibrium. We investigate if the evolution of mutation rates could play a role in explaining the ‘genetic paradox of invasive species’ for a sexually reproducing species colonizing a landscape of gradually changing conditions.

Results

We use an individual-based model to show the evolutionary increase of mutation rates in sexual populations during range expansion, in coevolution with the dispersal probability. The observed evolution of mutation rate is adaptive and clearly advances invasion speed both through its effect on the evolution of dispersal probability, and the evolution of local adaptation. This also occurs under a variable temperature gradient, and under the assumption of 90% lethal mutations.

Conclusions

In this study we show novel consequences of the particular genetic properties of populations under spatial disequilibrium, i.e. the coevolution of dispersal probability and mutation rate, even in a sexual species and under realistic spatial gradients, resulting in faster invasions. The evolution of mutation rates can therefore be added to the list of possible explanations for the ‘genetic paradox of invasive species’. We conclude that range expansions and the evolution of mutation rates are in a positive feedback loop, with possibly far-reaching ecological consequences concerning invasiveness and the adaptability of species to novel environmental conditions.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-017-0998-8) contains supplementary material, which is available to authorized users.

Host-selected mutations converging on a global regulator drive an adaptive leap towards symbiosis in bacteria

Host immune and physical barriers protect against pathogens but also impede the establishment of essential symbiotic partnerships. To reveal mechanisms by which beneficial organisms adapt to circumvent host defenses, we experimentally evolved ecologically distinct bioluminescent Vibrio fischeri by colonization and growth within the light organs of the squid Euprymna scolopes. Serial squid passaging of bacteria produced eight distinct mutations in the binK sensor kinase gene, which conferred an exceptional selective advantage that could be demonstrated through both empirical and theoretical analysis. Squid-adaptive binK alleles promoted colonization and immune evasion that were mediated by cell-associated matrices including symbiotic polysaccharide (Syp) and cellulose. binK variation also altered quorum sensing, raising the threshold for luminescence induction. Preexisting coordinated regulation of symbiosis traits by BinK presented an efficient solution where altered BinK function was the key to unlock multiple colonization barriers. These results identify a genetic basis for microbial adaptability and underscore the importance of hosts as selective agents that shape emergent symbiont populations.

Invasions and extinctions through the looking glass of evolutionary ecology

Invasive and endangered species reflect opposite ends of a spectrum of ecological success, yet they experience many similar eco-evolutionary challenges including demographic bottlenecks, hybridization and novel environments. Despite these similarities, important differences exist. Demographic bottlenecks are more transient in invasive species, which (i) maintains ecologically relevant genetic variation, (ii) reduces mutation load, and (iii) increases the efficiency of natural selection relative to genetic drift. Endangered species are less likely to benefit from admixture, which offsets mutation load but also reduces fitness when populations are locally adapted. Invading species generally experience more benign environments with fewer natural enemies, which increases fitness directly and also indirectly by masking inbreeding depression. Adaptive phenotypic plasticity can maintain fitness in novel environments but is more likely to evolve in invasive species encountering variable habitats and to be compromised by demographic factors in endangered species. Placed in an eco-evolutionary context, these differences affect the breadth of the ecological niche, which arises as an emergent property of antagonistic selection and genetic constraints. Comparative studies of invasions and extinctions that apply an eco-evolutionary perspective could provide new insights into the environmental and genetic basis of ecological success in novel environments and improve efforts to preserve global biodiversity.This article is part of the themed issue 'Human influences on evolution, and the ecological and societal consequences'.

Environmental fluctuations do not select for increased variation or population-based resistance in Escherichia coli

Little is known about the mechanisms that enable organisms to cope with unpredictable environments. To address this issue, we used replicate populations of Escherichia coli selected under complex, randomly changing environments. Under four novel stresses that had no known correlation with the selection environments, individual cells of the selected populations had significantly lower lag and greater yield compared to the controls. More importantly, there were no outliers in terms of growth, thus ruling out the evolution of population-based resistance. We also assayed the standing phenotypic variation of the selected populations, in terms of their growth on 94 different substrates. Contrary to expectations, there was no increase in the standing variation of the selected populations, nor was there any significant divergence from the ancestors. This suggested that the greater fitness in novel environments is brought about by selection at the level of the individuals, which restricts the suite of traits that can potentially evolve through this mechanism. Given that day-to-day climatic variability of the world is rising, these results have potential public health implications. Our results also underline the need for a very different kind of theoretical approach to study the effects of fluctuating environments.

On the maintenance of genetic variation and adaptation to environmental change: considerations from population genomics in fishes

The first goal of this paper was to overview modern approaches to local adaptation, with a focus on the use of population genomics data to detect signals of natural selection in fishes. Several mechanisms are discussed that may enhance the maintenance of genetic variation and evolutionary potential, which have been overlooked and should be considered in future theoretical development and predictive models: the prevalence of soft sweeps, polygenic basis of adaptation, balancing selection and transient polymorphisms, parallel evolution, as well as epigenetic variation. Research on fish population genomics has provided ample evidence for local adaptation at the genome level. Pervasive adaptive evolution, however, seems to almost never involve the fixation of beneficial alleles. Instead, adaptation apparently proceeds most commonly by soft sweeps entailing shifts in frequencies of alleles being shared between differentially adapted populations. One obvious factor contributing to the maintenance of standing genetic variation in the face of selective pressures is that adaptive phenotypic traits are most often highly polygenic, and consequently the response to selection should derive mostly from allelic co-variances among causative loci rather than pronounced allele frequency changes. Balancing selection in its various forms may also play an important role in maintaining adaptive genetic variation and the evolutionary potential of species to cope with environmental change. A large body of literature on fishes also shows that repeated evolution of adaptive phenotypes is a ubiquitous evolutionary phenomenon that seems to occur most often via different genetic solutions, further adding to the potential options of species to cope with a changing environment. Moreover, a paradox is emerging from recent fish studies whereby populations of highly reduced effective population sizes and impoverished genetic diversity can apparently retain their adaptive potential in some circumstances. Although more empirical support is needed, several recent studies suggest that epigenetic variation could account for this apparent paradox. Therefore, epigenetic variation should be fully integrated with considerations pertaining to role of soft sweeps, polygenic and balancing selection, as well as repeated adaptation involving different genetic basis towards improving models predicting the evolutionary potential of species to cope with a changing world.

Phenotypic Plasticity Promotes Balanced Polymorphism in Periodic Environments by a Genomic Storage Effect

Phenotypic plasticity is known to evolve in perturbed habitats, where it alleviates the deleterious effects of selection. But the effects of plasticity on levels of genetic polymorphism, an important precursor to adaptation in temporally varying environments, are unclear. Here we develop a haploid, two-locus population-genetic model to describe the interplay between a plasticity modifier locus and a target locus subject to periodically varying selection. We find that the interplay between these two loci can produce a "genomic storage effect" that promotes balanced polymorphism over a large range of parameters, in the absence of all other conditions known to maintain genetic variation. The genomic storage effect arises as recombination allows alleles at the two loci to escape more harmful genetic backgrounds and associate in haplotypes that persist until environmental conditions change. Using both Monte Carlo simulations and analytical approximations we quantify the strength of the genomic storage effect across a range of selection pressures, recombination rates, plasticity modifier effect sizes, and environmental periods.

Unequal contribution of native South African phylogeographic lineages to the invasion of the African clawed frog, Xenopus laevis, in Europe

Due to both deliberate and accidental introductions, invasive African Clawed Frog (Xenopus laevis) populations have become established worldwide. In this study, we investigate the geographic origins of invasive X. laevis populations in France and Portugal using the phylogeographic structure of X. laevis in its native South African range. In total, 80 individuals from the whole area known to be invaded in France and Portugal were analysed for two mitochondrial and three nuclear genes, allowing a comparison with 185 specimens from the native range. Our results show that native phylogeographic lineages have contributed differently to invasive European X. laevis populations. In Portugal, genetic and historical data suggest a single colonization event involving a small number of individuals from the south-western Cape region in South Africa. In contrast, French invasive X. laevis encompass two distinct native phylogeographic lineages, i.e., one from the south-western Cape region and one from the northern regions of South Africa. The French X. laevis population is the first example of a X. laevis invasion involving multiple lineages. Moreover, the lack of population structure based on nuclear DNA suggests a potential role for admixture within the invasive French population.

Evolutionary mechanisms of habitat invasions, using the copepod Eurytemora affinis as a model system

The study of the copepod Eurytemora affinis has provided unprecedented insights into mechanisms of invasive success. In this invited review, I summarize a subset of work from my laboratory to highlight key insights gained from studying E. affinis as a model system. Invasive species with brackish origins are overrepresented in freshwater habitats. The copepod E. affinis is an example of such a brackish invader, and has invaded freshwater habitats multiple times independently in recent years. These invasions were accompanied by the evolution of physiological tolerance and plasticity, increased body fluid regulation, and evolutionary shifts in ion transporter (V‐type H⁺ATPase, Na⁺, K⁺‐ATPase) activity and expression. These evolutionary changes occurred in parallel across independent invasions in nature and in laboratory selection experiments. Selection appears to act on standing genetic variation during invasions, and maintenance of this variation is likely facilitated through ‘beneficial reversal of dominance’ in salinity tolerance across habitats. Expression of critical ion transporters is localized in newly discovered Crusalis leg organs. Increased freshwater tolerance is accompanied by costs to development time and greater requirements for food. High‐food concentration increases low‐salinity tolerance, allowing saline populations to invade freshwater habitats. Mechanisms observed here likely have relevance for other taxa undergoing fundamental niche expansions.

Phenotypic Plasticity in Reproductive Traits of the Perennial Shrub Ulex europaeus in Response to Shading: A Multi-Year Monitoring of Cultivated Clones

Phenotypic plasticity may be advantageous for plants to be able to rapidly cope with new and changing environments associated with climate change or during biological invasions. This is especially true for perennial plants, as they may need a longer period to respond genetically to selective pressures than annuals, and also because they are more likely to experience environmental changes during their lifespan. However, few studies have explored the plasticity of the reproductive life history traits of woody perennial species. This study focuses on a woody shrub, Ulex europaeus (common gorse), and on the response of its reproductive traits to one important environmental factor, shading. The study was performed on clones originating from western France (within the native range of this invasive species) and grown for seven years. We compared traits of plants grown in a shade treatment (with two successive shade levels) vs. full natural light. The traits monitored included flowering onset, pod production and seed predation. All traits studied responded to shading, exhibiting various levels of plasticity. In particular, dense shade induced a radical but reversible decrease in flower and pod production, while moderate shade had little effect on reproductive traits. The magnitude of the response to dense shade depended on the genotype, showing a genetically based polymorphism of plasticity. The level of plasticity also showed substantial variations between years, and the effect of environmental variations was cumulative over time. This suggests that plasticity can influence the lifetime fitness of U. Europaeus and is involved in the capacity of the species to grow under contrasting environmental conditions.

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.

Escherichia coli populations in unpredictably fluctuating environments evolve to face novel stresses through enhanced efflux activity

There is considerable understanding about how laboratory populations respond to predictable (constant or deteriorating environment) selection for single environmental variables such as temperature or pH. However, such insights may not apply when selection environments comprise multiple variables that fluctuate unpredictably, as is common in nature. To address this issue, we grew replicate laboratory populations of Escherichia coli in nutrient broth whose pH and concentrations of salt (NaCl) and hydrogen peroxide (H2 O2 ) were randomly changed daily. After ~170 generations, the fitness of the selected populations had not increased in any of the three selection environments. However, these selected populations had significantly greater fitness in four novel environments which have no known fitness-correlation with tolerance to pH, NaCl or H2 O2 . Interestingly, contrary to expectations, hypermutators did not evolve. Instead, the selected populations evolved an increased ability for energy-dependent efflux activity that might enable them to throw out toxins, including antibiotics, from the cell at a faster rate. This provides an alternate mechanism for how evolvability can evolve in bacteria and potentially lead to broad-spectrum antibiotic resistance, even in the absence of prior antibiotic exposure. Given that environmental variability is increasing in nature, this might have serious consequences for public health.

The devil is in the details: genetic variation in introduced populations and its contributions to invasion

The influence of genetic variation on invasion success has captivated researchers since the start of the field of invasion genetics 50 years ago. We review the history of work on this question and conclude that genetic variation-as surveyed with molecular markers-appears to shape invasion rarely. Instead, there is a significant disconnect between marker assays and ecologically relevant genetic variation in introductions. We argue that the potential for adaptation to facilitate invasion will be shaped by the details of genotypes affecting phenotypes, and we highlight three areas in which we see opportunities to make powerful new insights. (i) The genetic architecture of adaptive variation. Traits shaped by large-effect alleles may be strongly impacted by founder events yet more likely to respond to selection when genetic drift is strong. Large-effect loci may be especially relevant for traits involved in biotic interactions. (ii) Cryptic genetic variation exposed during invasion. Introductions have strong potential to uncover masked variation due to alterations in genetic and ecological environments. (iii) Genetic interactions during admixture of multiple source populations. As divergence among sources increases, positive followed by increasingly negative effects of admixture should be expected. Although generally hypothesized to be beneficial during invasion, admixture is most often reported among sources of intermediate divergence, supporting the possibility that incompatibilities among divergent source populations might be limiting their introgression. Finally, we note that these details of invasion genetics can be coupled with comparative demographic analyses to link genetic changes to the evolution of invasiveness itself.

Evidence for rapid ecological range expansion in a newly invasive plant

Little is known about how an introduced species may expand its ecological range, i.e. the set of local environmental conditions in which it can successfully establish populations. Delimiting this range of conditions is a methodological challenge, because it is impossible to sample all potential field locations for any species in a given region. Developing approaches to track ecological range over time could substantially contribute to understanding invasion dynamics. In this study, we use a previously established sampling strategy to document apparent changes across a 15-year time interval in the ecological range of the Asian annual Polygonum cespitosum Blume in northeastern North America, where the species has recently become invasive. Using a structured sample drawn from a large set of field populations, we determined the range of light, soil moisture and soil nutrient conditions that the species currently occupies in this region and the proportional distribution of individuals in differing types of microsite, and compared them with field measurements that were similarly determined 15 years earlier. Although in 1994 the species was absent from both high-light and flooded habitats, in 2009 P. cespitosum occurred in open as well as shaded habitats, across a wide range of moisture conditions. In 2009 the species also occupied a greater proportion of high-light microsites within field sites than in 1994. These findings suggest an expanded ecological range that, intriguingly, is consistent with the recent evolution in North American P. cespitosum populations of adaptive plasticity in response to high light. Possible non-evolutionary explanations for the change in field distribution are also considered.

Traits allowing some ant species to nest syntopically with the fire ant Solenopsis saevissima in its native range

Supercolonies of the red fire ant Solenopsis saevissima (Smith) develop in disturbed environments and likely alter the ant community in the native range of the species. For example, in French Guiana only 8 ant species were repeatedly noted as nesting in close vicinity to its mounds. Here, we verified if a shared set of biological, ecological, and behavioral traits might explain how these 8 species are able to nest in the presence of S. saevissima. We did not find this to be the case. We did find, however, that all of them are able to live in disturbed habitats. It is likely that over the course of evolution each of these species acquired the capacity to live syntopically with S. saevissima through its own set of traits, where colony size (4 species develop large colonies), cuticular compounds which do not trigger aggressiveness (6 species) and submissive behaviors (4 species) complement each other.

Emergence of long-term balanced polymorphism under cyclic selection of spatially variable magnitude

A fundamental question in evolutionary biology is what promotes genetic variation at nonneutral loci, a major precursor to adaptation in changing environments. In particular, balanced polymorphism under realistic evolutionary models of temporally varying environments in finite natural populations remains to be demonstrated. Here, we propose a novel mechanism of balancing selection under temporally varying fitnesses. Using forward-in-time computer simulations and mathematical analysis, we show that cyclic selection that spatially varies in magnitude, such as along an environmental gradient, can lead to elevated levels of nonneutral genetic polymorphism in finite populations. Balanced polymorphism is more likely with an increase in gene flow, magnitude and period of fitness oscillations, and spatial heterogeneity. This polymorphism-promoting effect is robust to small systematic fitness differences between competing alleles or to random environmental perturbation. Furthermore, we demonstrate analytically that protected polymorphism arises as spatially heterogeneous cyclic fitness oscillations generate a type of storage effect that leads to negative frequency dependent selection. Our findings imply that spatially variable cyclic environments can promote elevated levels of nonneutral genetic variation in natural populations.

Experimental evolution in fluctuating environments: tolerance measurements at constant temperatures incorrectly predict the ability to tolerate fluctuating temperatures

The ability to predict the consequences of fluctuating environments on species distribution and extinction often relies on determining the tolerances of species or genotypes in different constant environments (i.e. determining tolerance curves). However, very little is known about the suitability of measurements made in constant environments to predict the level of adaptation to rapidly fluctuating environments. To explore this question, we used bacterial clones adapted to constant or fluctuating temperatures and found that measurements across a range of constant temperatures did not indicate any adaptation to fluctuating temperatures. However, adaptation to fluctuating temperatures was only apparent if growth was measured during thermal fluctuation. Thus, tolerance curves based on measurements in constant environments can be misleading in predicting the ability to tolerate fast environmental fluctuations. Such complications could lead to false estimates of the genetic merits of genotypes and extinction risks of species due to climate change-induced thermal fluctuations.

Genetics, adaptation, and invasion in harsh environments

We analyze mathematical models to examine how the genetic basis of fitness affects the persistence of a population suddenly encountering a harsh environment where it would go extinct without evolution. The results are relevant for novel introductions and for an established population whose existence is threatened by a sudden change in the environment. The models span a range of genetic assumptions, including identical loci that contribute to absolute fitness, a two-locus quantitative genetic model with nonidentical loci, and a model with major and minor genes affecting a quantitative trait. We find as a general (though not universal) pattern that prospects for persistence narrow as more loci contribute to fitness, in effect because selection per locus is increasingly weakened with more loci, which can even overwhelm any initial enhancement of fitness that adding loci might provide. When loci contribute unequally to fitness, genes of small effect can significantly reduce extinction risk. Indeed, major and minor genes can interact synergistically to reduce the time needed to evolve growth. Such interactions can also increase vulnerability to extinction, depending not just on how genes interact but also on the initial genetic structure of the introduced, or newly invaded, population.

Comparison of whole mitochondrial genome sequences from two clades of the invasive ascidian, Didemnum vexillum

The mitochondria are the main source of cellular energy production and have an important role in development, fertility, and thermal limitations. Adaptive mitochondrial DNA mutations have the potential to be of great importance in determining aspects of the life history of an organism. Phylogenetic analyses of the globally invasive marine ascidian Didemnum vexillum using the mitochondrial cytochrome c oxidase 1 (COX1) coding region, revealed two distinct clades. Representatives of one clade (denoted by 'B') are geographically restricted to D. vexillum's native region (north-west Pacific Ocean, including Japan), whereas members of the other clade (denoted by 'A') have been introduced and become invasive in temperate coastal areas around the world. Persistence of clade B's restricted distribution may reflect it being inherently less invasive than clade A. To investigate this we sought to determine if the two clades differ significantly in other mitochondrial genes of functional significance, specifically, alterations in amino acids encoded in mitochondrial enzyme subunits. Differences in functional mitochondrial genes could indicate an increased ability for clade A colonies to tolerate a wider range of environmental temperature. Full mitochondrial genomic sequences from D. vexillum clades A and B were obtained and they predict significant sequence differences in genes encoding for enzymes involved in oxidative phosphorylation. Diversity levels were relatively high and showed divergence across almost all genes, with p-distance values between the two clades indicating recent divergence. Both clades showed an excess of rare variants, which is consistent with balancing selection or a recent population expansion. Results presented here will inform future research focusing on examining the functional properties of the corresponding mitochondrial respiration enzymes, of A and B clade enzymes. By comparing closely related taxa that have differing distributions it is possible to identify genes and phenotypes suited to particular environments. The examination of mitochondrial genotypes, and associated enzyme functioning, across populations may aid in our understanding of thermal tolerance and environmental adaptation.

Differences in evolutionary history translate into differences in invasion success of alien mammals in South Africa

Attempts to investigate the drivers of invasion success are generally limited to the biological and evolutionary traits distinguishing native from introduced species. Although alien species introduced to the same recipient environment differ in their invasion intensity – for example, some are “strong invaders”; others are “weak invaders” – the factors underlying the variation in invasion success within alien communities are little explored. In this study, we ask what drives the variation in invasion success of alien mammals in South Africa. First, we tested for taxonomic and phylogenetic signal in invasion intensity. Second, we reconstructed predictive models of the variation in invasion intensity among alien mammals using the generalized linear mixed-effects models. We found that the family Bovidae and the order Artiodactyla contained more “strong invaders” than expected by chance, and that such taxonomic signal did not translate into phylogenetic selectivity. In addition, our study indicates that latitude, gestation length, social group size, and human population density are only marginal determinant of the variation in invasion success. However, we found that evolutionary distinctiveness – a parameter characterising the uniqueness of each alien species – is the most important predictive variable. Our results indicate that the invasive behavior of alien mammals may have been “fingerprinted” in their evolutionary past, and that evolutionary history might capture beyond ecological, biological and life-history traits usually prioritized in predictive modeling of invasion success. These findings have applicability to the management of alien mammals in South Africa.

Testing for beneficial reversal of dominance during salinity shifts in the invasive copepod Eurytemora affinis, and implications for the maintenance of genetic variation

Maintenance of genetic variation at loci under selection has profound implications for adaptation under environmental change. In temporally and spatially varying habitats, non-neutral polymorphism could be maintained by heterozygote advantage across environments (marginal overdominance), which could be greatly increased by beneficial reversal of dominance across conditions. We tested for reversal of dominance and marginal overdominance in salinity tolerance in the saltwater-to-freshwater invading copepod Eurytemora affinis. We compared survival of F1 offspring generated by crossing saline and freshwater inbred lines (between-salinity F1 crosses) relative to within-salinity F1 crosses, across three salinities. We found evidence for both beneficial reversal of dominance and marginal overdominance in salinity tolerance. In support of reversal of dominance, survival of between-salinity F1 crosses was not different from that of freshwater F1 crosses under freshwater conditions and saltwater F1 crosses under saltwater conditions. In support of marginal overdominance, between-salinity F1 crosses exhibited significantly higher survival across salinities relative to both freshwater and saltwater F1 crosses. Our study provides a rare empirical example of complete beneficial reversal of dominance associated with environmental change. This mechanism might be crucial for maintaining genetic variation in salinity tolerance in E. affinis populations, allowing rapid adaptation to salinity changes during habitat invasions.

The dynamics of a metapopulation: changes in life-history traits in resident herring that co-occur with oceanic herring during spawning

Different populations of Atlantic herring are regarded as forming a metapopulation, but we know little about the dynamics of the connectivity and degree of interbreeding between the populations. Based on data from three periods between 1962 and 2011, we identified the presence of two components of herring in a small semi-enclosed coastal marine ecosystem based on different somatic growth patterns and mean vertebrae sum (VS). The two components were interpreted as belonging to a resident herring population and the migratory, oceanic Norwegian spring spawning (NSS) herring population, and they co-occurred during spawning. In the 1960s, resident herring characterized by slow growth and low VS co-occurred with rapid growth, high VS oceanic NSS herring. Similar slow-growing resident and rapid-growing NSS herring were found in the 1970–80s, but both populations now had low VS suggesting similar origins. Finally, in the 2000s both populations showed rapid growth. The changes coincided with the NSS herring going from a state of high abundance and oceanic distribution to a collapse in the late 1960s that resulted in a coastal distribution closer to resident herring populations, before full recovery and resumption of the migratory, oceanic pattern in the 1990s. During all three periods, NSS herring were only present in the local system up to an age of about five years, but the synchronous spawning of the populations supports mixed spawning and interbreeding. During the investigation period both longevity, length at age (growth) and length-at-first maturity increased markedly for the resident herring, which then became more similar to the NSS herring. Genetic and/or cultural factors are believed to be the main causes of the observed changes in life history traits, although some effect of changes in environmental factors cannot be excluded. Our study suggests that relationships among populations in a metapopulation can be highly dynamic.