Evaluating the within-host fitness effects of mutations fixed during virus adaptation to different ecotypes of a new host

The existence of genetic variation for resistance in host populations is assumed to be essential to the spread of an emerging virus. Models predict that the rate of spread slows down with the increasing frequency and higher diversity of resistance alleles in the host population. We have been using the experimental pathosystem Arabidopsis thaliana—tobacco etch potyvirus (TEV) to explore the interplay between genetic variation in host''s susceptibility and virus diversity. We have recently shown that TEV populations evolving in A. thaliana ecotypes that differ in susceptibility to infection gained within-host fitness, virulence and infectivity in a manner compatible with a gene-for-gene model of host–parasite interactions: hard-to-infect ecotypes were infected by generalist viruses, whereas easy-to-infect ecotypes were infected by every virus. We characterized the genomes of the evolved viruses and found cases of host-driven convergent mutations. To gain further insights in the mechanistic basis of this gene-for-gene model, we have generated all viral mutations individually as well as in specific combinations and tested their within-host fitness effects across ecotypes. Most of these mutations were deleterious or neutral in their local ecotype and only a very reduced number had a host-specific beneficial effect. We conclude that most of the mutations fixed during the evolution experiment were so by drift or by selective sweeps along with the selected driver mutation. In addition, we evaluated the ruggedness of the underlying adaptive fitness landscape and found that mutational effects were mostly multiplicative, with few cases of significant epistasis.     

EXPERIMENTAL EVOLUTION OF AN EMERGING PLANT VIRUS IN HOST GENOTYPES THAT DIFFER IN THEIR SUSCEPTIBILITY TO INFECTION

This study evaluates the extent to which genetic differences among host individuals from the same species condition the evolution of a plant RNA virus. We performed a threefold replicated evolution experiment in which Tobacco etch potyvirus isolate At17b (TEV‐At17b), adapted to Arabidopsis thaliana ecotype Ler‐0, was serially passaged in five genetically heterogeneous ecotypes of A. thaliana. After 15 passages we found that evolved viruses improved their fitness, showed higher infectivity and stronger virulence in their local host ecotypes. The genome of evolved lineages was sequenced and putative adaptive mutations identified. Host‐driven convergent mutations have been identified. Evidences supported selection for increased translational efficiency. Next, we sought for the specificity of virus adaptation by infecting all five ecotypes with all 15 evolved virus populations. We found that some ecotypes were more permissive to infection than others, and that some evolved virus isolates were more specialist/generalist than others. The bipartite network linking ecotypes with evolved viruses was significantly nested but not modular, suggesting that hard‐to‐infect ecotypes were infected by generalist viruses whereas easy‐to‐infect ecotypes were infected by all viruses, as predicted by a gene‐for‐gene model of infection.     

Spatial patterns of neutral and functional genetic variations reveal patterns of local adaptation in raccoon (Procyon lotor) populations exposed to raccoon rabies

Local adaptation is necessary for population survival and depends on the interplay between responses to selective forces and demographic processes that introduce or retain adaptive and maladaptive attributes. Host–parasite systems are dynamic, varying in space and time, where both host and parasites must adapt to their ever‐changing environment in order to survive. We investigated patterns of local adaptation in raccoon populations with varying temporal exposure to the raccoon rabies virus (RRV). RRV infects approximately 85% of the population when epizootic and has been presumed to be completely lethal once contracted; however, disease challenge experiments and varying spatial patterns of RRV spread suggest some level of immunity may exist. We first assessed patterns of local adaptation in raccoon populations along the eastern seaboard of North America by contrasting spatial patterns of neutral (microsatellite loci) and functional, major histocompatibility complex (MHC) genetic diversity and structure. We explored variation of MHC allele frequencies in the light of temporal population exposure to RRV (0–60 years) and specific RRV strains in infected raccoons. Our results revealed high levels of MHC variation (66 DRB exon 2 alleles) and pronounced genetic structure relative to neutral microsatellite loci, indicative of local adaptation. We found a positive association linking MHC genetic diversity and temporal RRV exposure, but no association with susceptibility and resistance to RRV strains. These results have implications for landscape epidemiology studies seeking to predict the spread of RRV and present an example of how population demographics influence the degree to which populations adapt to local selective pressures.     

Genotype and sex-based host variation in behaviour and susceptibility drives population disease dynamics

Host heterogeneity in pathogen transmission is widespread and presents a major hurdle to predicting and minimizing disease outbreaks. Using Drosophila melanogaster infected with Drosophila C virus as a model system, we integrated experimental measurements of social aggregation, virus shedding, and disease-induced mortality from different genetic lines and sexes into a disease modelling framework. The experimentally measured host heterogeneity produced substantial differences in simulated disease outbreaks, providing evidence for genetic and sex-specific effects on disease dynamics at a population level. While this was true for homogeneous populations of single sex/genetic line, the genetic background or sex of the index case did not alter outbreak dynamics in simulated, heterogeneous populations. Finally, to explore the relative effects of social aggregation, viral shedding and mortality, we compared simulations where we allowed these traits to vary, as measured experimentally, to simulations where we constrained variation in these traits to the population mean. In this context, variation in infectiousness, followed by social aggregation, was the most influential component of transmission. Overall, we show that host heterogeneity in three host traits dramatically affects population-level transmission, but the relative impact of this variation depends on both the susceptible population diversity and the distribution of population-level variation.     

The pvr1 locus in Capsicum encodes a translation initiation factor eIF4E that interacts with Tobacco etch virus VPg

Mutations in the eIF4E homolog encoded at the pvr1 locus in Capsicum result in broad-spectrum potyvirus resistance attributed to the pvr1 resistance allele, a gene widely deployed in agriculture for more than 50 years. We show that two other resistance genes, previously known to be eIF4E with narrower resistance spectra, pvr2(1) and pvr2(2), are alleles at the pvr1 locus. Based on these data and current nomenclature guidelines, we have re-designated these alleles, pvr1(1) and pvr1(2), respectively. Point mutations in pvr1, pvr1(1), and pvr1(2) grouped to similar regions of eIF4E and were predicted by protein homology models to cause conformational shifts in the encoded proteins. The avirulence determinant in this potyvirus system has previously been identified as VPg, therefore yeast two-hybrid and GST pull-down assays were carried out with proteins encoded by the pvr1 alleles and VPg from two different strains of Tobacco etch virus (TEV) that differentially infected Capsicum lines carrying these genes. While the protein encoded by the susceptible allele pvr1+ interacted strongly, proteins translated from all three resistance alleles (pvr1, pvr1(1), and pvr1(2)) failed to bind VPg from either strain of TEV. This failure to bind correlated with resistance or reduced susceptibility, suggesting that interruption of the interaction between VPg and this eIF4E paralog may be necessary, but is not sufficient for potyvirus resistance in vivo. Among the three resistance alleles, only the pvr1 gene product failed to bind m7-GTP cap-analog columns, suggesting that disrupted cap binding is not required for potyvirus resistance.     

Spatio‐temporal analysis of Nova virus,a divergent hantavirus circulating in the European mole in Belgium

Over the last decade, the recognized host range of hantaviruses has expanded considerably with the discovery of distinct hantaviruses in shrews, moles and bats. Unfortunately, in‐depth studies of these viruses have been limited. Here we describe a comprehensive analysis of the spatial distribution, genetic diversity and evolution of Nova virus, a hantavirus that has the European mole as its natural host. Our analysis demonstrated that Nova virus has a high prevalence and widespread distribution in Belgium. While Nova virus displayed relatively high nucleotide diversity in Belgium, amino acid changes were limited. The nucleocapsid protein was subjected to strong purifying selection, reflecting the strict evolutionary constraints placed upon Nova virus by its host. Spatio‐temporal analysis using Bayesian evolutionary inference techniques demonstrated that Nova virus had efficiently spread in the European mole population in Belgium, forming two distinct clades, representing east and west of Belgium. The influence of landscape barriers, in the form of the main waterways, on the dispersal velocity of Nova virus was assessed using an analytical framework for comparing Bayesian viral phylogenies with environmental landscape data. We demonstrated that waterways did not act as an environmental resistance factor slowing down Nova virus diffusion in the mole population. With this study, we provide information about the spatial diffusion of Nova virus and contribute sequence information that can be applied in further functional studies.     

Dengue and dengue haemorrhagic fever: implications of host genetics

Little is known of the role of human leucocyte antigen (HLA) alleles or non-HLA alleles in determining resistance, susceptibility or the severity of acute viral infections. Dengue fever (DF) and dengue haemorrhagic fever (DHF) are suitable models for immunogenetic studies, yet only superficial efforts have been made to study dengue disease to date. DF and DHF can be caused by both primary and secondary infection by any of the four serotypes of the dengue virus. Differences in host susceptibility to infectious disease and disease severity cannot be attributed solely to the virus virulence. Variations in immune response, often associated with polymorphism in the human genome, can now be detected. Data on the influence of human genes in DF and DHF are discussed here in relation to (1) associations between HLA polymorphism and dengue disease susceptibility or resistance, (2) protective alleles influencing progression to severe disease, (3) alleles restricting CD4(+) and CD8(+) T lymphocytes, and (4) non-HLA genetic factors that may contribute to DHF evolution. Recent discoveries regarding genetic associations in other viral infections may provide clues to understanding the development of end-stage complications in dengue disease. The scanty positive data presented here indicate a need for detailed genetic studies in different ethnic groups in different countries during the acute phase of DF and DHF on a larger number of patients.     

Nosocomial Plasmodium falciparum infections confirmed by molecular typing in Medellín, Colombia

Background

Malaria is a serious, sometimes fatal, disease caused by Plasmodium infection of human red blood cells. The host-parasite co-evolutionary processes are well understood by the association of coding variations such as G6PD, Duffy blood group receptor, HLA, and beta-globin gene variants with malaria resistance. The profound genetic diversity in host is attributed to polymorphic microsatellites loci. The microsatellite alleles in bacterial species are known to have aided their survival in fatal environmental conditions. The fascinating question is whether microsatellites are genomic cushion in the human genome to combat disease stress and has cause-effect relationships with infections.

Presentation of the hypothesis

It is hypothesized that repeat units or alleles of microsatellites TH01 and D5S818, located in close proximity to beta-globin gene and immune regulatory region in human play a role in malaria predisposition. Association of alleles at aforesaid microsatellites with malaria infection was analysed. To overrule the false association in unrecognized population stratification, structure analysis and AMOVA were performed among the sampled groups.

Testing of hypothesis

Associations of microsatellite alleles with malaria infection were verified using recombination rate, Chi-square, and powerful likelihood tests. Further investigation of population genetic structure, and AMOVA was done to rule out the confounding effects of population stratification in interpretation of association studies.

Implication of the hypothesis

Lower recombination rate (θ) between microsatellites and genes implicated in host fitness; positive association between alleles -13 (D5S818), 9 (TH01) and strong susceptibility to Plasmodium falciparum; and alleles-12 (D5S818) and 6 (TH01) rendering resistance to human host were evident. The interesting fact emerging from the study was that while predisposition to malaria was a prehistoric attribute, among TH01 alleles; evolution of resistant allele-6 was a recent phenomenon, which could conceivably be driven by infection related selective forces. The host's microsatellite allelic associations with malaria infection were valid in the light of low genetic variance between sampled groups and no population stratification.     

Resistance to a nuclear polyhedrosis virus in the light-brown apple moth Epiphyas postvittana (Lepidoptera: Tortricidae)

Measurements were made of the relative susceptibility to a nuclear polyhedrosis virus of three populations of light-brown apple moth Epiphyas postvittana: a resistant laboratory strain (CAN), a susceptible laboratory strain (BAR), and a field population. CAN was found to be 50 times more resistant than BAR and 160 times more resistant than the field line. Experiments on hybrid crosses of resistant and susceptible strains showed that resistance is genetically determined. This serves as a warning of the possible selection of virus-resistant strains of insect pests, where viral insecticides are being used in the field.     

Antibiotic Treatment Regimes as a Driver of the Global Population Dynamics of a Major Gonorrhea Lineage

The Neisseria gonorrhoeae multilocus sequence type (ST) 1901 is among the lineages most commonly associated with treatment failure. Here, we analyze a global collection of ST-1901 genomes to shed light on the emergence and spread of alleles associated with reduced susceptibility to extended-spectrum cephalosporins (ESCs).The genetic diversity of ST-1901 falls into a minor and a major clade, both of which were inferred to have originated in East Asia. The dispersal of the major clade from Asia happened in two separate waves expanding from ∼1987 and 1996, respectively. Both waves first reached North America, and from there spread to Europe and Oceania, with multiple secondary reintroductions to Asia.The ancestor of the second wave acquired the penA 34.001 allele, which significantly reduces susceptibility to ESCs. Our results suggest that the acquisition of this allele granted the second wave a fitness advantage at a time when ESCs became the key drug class used to treat gonorrhea. Following its establishment globally, the lineage has served as a reservoir for the repeated emergence of clones fully resistant to the ESC ceftriaxone, an essential drug for effective treatment of gonorrhea.We infer that the effective population sizes of both clades went into decline as treatment schemes shifted from fluoroquinolones via ESC monotherapy to dual therapy with ceftriaxone and azithromycin in Europe and the United States. Despite the inferred recent population size decline, the short evolutionary path from the penA 34.001 allele to alleles providing full ceftriaxone resistance is a cause of concern.     

Susceptibility of Italian agile frog populations to an emerging strain of Ranavirus parallels population genetic diversity

Western populations of the Italian agile frog (Rana latastei) experience widespread genetic depletion. Based on population genetic theory, molecular models of immunity and previous empirical studies, population genetic depletion predicts increased susceptibility of populations to emergent pathogens. We experimentally compared susceptibility of R. latastei populations upon exposure to an emerging strain of Ranavirus, frog virus 3 (FV3), using six populations spanning the geographical range and range of population genetic diversity found in nature. Our findings confirm this prediction, suggesting that the loss of genetic diversity accompanying range expansion and population isolation is coincident with increased mortality risk from an emergent pathogen. Loss of heterozygosity and escape from selection imposed by immunologically cross‐reactive pathogens may potentially generate range‐wide variation in disease resistance.     

Insecticide resistance and genetic structure of Aedes aegypti populations from Rio de Janeiro State,Brazil

Vector control largely relies on neurotoxic chemicals, and insecticide resistance (IR) directly threatens their effectiveness. In some cases, specific alleles cause IR, and knowledge of the genetic diversity and gene flow among mosquito populations is crucial to track their arrival, rise, and spread. Here we evaluated Aedes aegypti populations’ susceptibility status, collected in 2016 from six different municipalities of Rio de Janeiro state (RJ), to temephos, pyriproxyfen, malathion, and deltamethrin. We collected eggs of Ae. aegypti in Campos dos Goytacazes (Cgy), Itaperuna (Ipn), Iguaba Grande (Igg), Itaboraí (Ibr), Mangaratiba (Mgr), and Vassouras (Vsr). We followed the World Health Organization (WHO) guidelines and investigated the degree of susceptibility/resistance of mosquitoes to these insecticides. We used the Rockefeller strain as a susceptible positive control. We genotyped the V1016I and F1534C knockdown resistance (kdr) alleles using qPCR TaqMan SNP genotyping assay. Besides, with the use of Ae. aegypti SNP-chip, we performed genomic population analyses by genotyping more than 15,000 biallelic SNPs in mosquitoes from each population. We added previous data from populations from other countries to evaluate the ancestry of RJ populations.All RJ Ae. aegypti populations were susceptible to pyriproxyfen and malathion and highly resistant to deltamethrin. The resistance ratios for temephos was below 3,0 in Cgy, Ibr, and Igg populations, representing the lowest rates since IR monitoring started in this Brazilian region. We found the kdr alleles in high frequencies in all populations, partially justifying the observed resistance to pyrethroid. Population genetics analysis showed that Ae. aegypti revealed potential higher migration among some RJ localities and low genetic structure for most of them. Future population genetic studies, together with IR data in Ae aegypti on a broader scale, can help us predict the gene flow within and among the Brazilian States, allowing us to track the dynamics of arrival and changes in the frequency of IR alleles, and providing critical information to improving vector control program.     

Reversion of a resistance-breaking mutation shows reversion costs and high virus diversity at necrotic local lesions

An instance of host range evolution relevant to plant virus disease control is resistance breaking. Resistance breaking can be hindered by across-host fitness trade-offs generated by negative effects of resistance-breaking mutations on the virus fitness in susceptible hosts. Different mutations in pepper mild mottle virus (PMMoV) coat protein result in the breaking in pepper plants of the resistance determined by the L³ resistance allele. Of these, mutation M138N is widespread in PMMoV populations, despite associated fitness penalties in within-host multiplication and survival. The stability of mutation M138N was analysed by serial passaging in L³ resistant plants. Appearance on passaging of necrotic local lesions (NLL), indicating an effective L³ resistance, showed reversion to nonresistance-breaking phenotypes was common. Most revertant genotypes had the mutation N138K, which affects the properties of the virus particle, introducing a penalty of reversion. Hence, the costs of reversion may determine the evolution of resistance-breaking in addition to resistance-breaking costs. The genetic diversity of the virus population in NLL was much higher than in systemically infected tissues, and included mutations reported to break L³ resistance other than M138N. Infectivity assays on pepper genotypes with different L alleles showed high phenotypic diversity in respect to L alleles in NLL, including phenotypes not reported in nature. Thus, high diversity at NLL may potentiate the appearance of genotypes that enable the colonization of new host genotypes or species. Collectively, the results of this study contribute to better understanding the evolutionary dynamics of resistance breaking and host-range expansions.     

Diversity in CRISPR‐based immunity protects susceptible genotypes by restricting phage spread and evolution

Diversity in host resistance often associates with reduced pathogen spread. This may result from ecological and evolutionary processes, likely with feedback between them. Theory and experiments on bacteria–phage interactions have shown that genetic diversity of the bacterial adaptive immune system can limit phage evolution to overcome resistance. Using the CRISPR–Cas bacterial immune system and lytic phage, we engineered a host–pathogen system where each bacterial host genotype could be infected by only one phage genotype. With this model system, we explored how CRISPR diversity impacts the spread of phage when they can overcome a resistance allele, how immune diversity affects the evolution of the phage to increase its host range and if there was feedback between these processes. We show that increasing CRISPR diversity benefits susceptible bacteria via a dilution effect, which limits the spread of the phage. We suggest that this ecological effect impacts the evolution of novel phage genotypes, which then feeds back into phage population dynamics.     

Experimental manipulation of population‐level MHC diversity controls pathogen virulence evolution in Mus musculus

The virulence levels attained by serial passage of pathogens through similar host genotypes are much higher than observed in natural systems; however, it is unknown what keeps natural virulence levels below these empirically demonstrated maximum levels. One hypothesis suggests that host diversity impedes pathogen virulence, because adaptation to one host genotype carries trade‐offs in the ability to replicate and cause disease in other host genotypes. To test this hypothesis, with the simplest level of population diversity within the loci of the major histocompatibility complex (MHC), we serially passaged Friend virus complex (FVC) through two rounds, in hosts with either the same MHC genotypes (pure passage) or hosts with different MHC genotypes (alternated passage). Alternated passages showed a significant overall reduction in viral titre (31%) and virulence (54%) when compared to pure passages. Furthermore, a resistant host genotype initially dominated any effects due to MHC diversity; however, when FVC was allowed to adapt to the resistant host genotype, predicted MHC effects emerged; that is, alternated lines show reduced virulence. These data indicate serial exposure to diverse MHC genotypes is an impediment to pathogen adaptation, suggesting genetic variation at MHC loci is important for limiting virulence in a rapidly evolving pathogen and supports negative frequency‐dependent selection as a force maintaining MHC diversity in host populations.     

Homogenous Population Genetic Structure of the Non-Native Raccoon Dog (Nyctereutes procyonoides) in Europe as a Result of Rapid Population Expansion

The extent of gene flow during the range expansion of non-native species influences the amount of genetic diversity retained in expanding populations. Here, we analyse the population genetic structure of the raccoon dog (Nyctereutes procyonoides) in north-eastern and central Europe. This invasive species is of management concern because it is highly susceptible to fox rabies and an important secondary host of the virus. We hypothesized that the large number of introduced animals and the species’ dispersal capabilities led to high population connectivity and maintenance of genetic diversity throughout the invaded range. We genotyped 332 tissue samples from seven European countries using 16 microsatellite loci. Different algorithms identified three genetic clusters corresponding to Finland, Denmark and a large ‘central’ population that reached from introduction areas in western Russia to northern Germany. Cluster assignments provided evidence of long-distance dispersal. The results of an Approximate Bayesian Computation analysis supported a scenario of equal effective population sizes among different pre-defined populations in the large central cluster. Our results are in line with strong gene flow and secondary admixture between neighbouring demes leading to reduced genetic structuring, probably a result of its fairly rapid population expansion after introduction. The results presented here are remarkable in the sense that we identified a homogenous genetic cluster inhabiting an area stretching over more than 1500km. They are also relevant for disease management, as in the event of a significant rabies outbreak, there is a great risk of a rapid virus spread among raccoon dog populations.     

The age and evolution of an antiviral resistance mutation in Drosophila melanogaster

What selective processes underlie the evolution of parasites and their hosts? Arms-race models propose that new host-resistance mutations or parasite counter-adaptations arise and sweep to fixation. Frequency-dependent models propose that selection favours pathogens adapted to the most common host genotypes, conferring an advantage to rare host genotypes. Distinguishing between these models is empirically difficult. The maintenance of disease-resistance polymorphisms has been studied in detail in plants, but less so in animals, and rarely in natural populations. We have made a detailed study of genetic variation in host resistance in a natural animal population, Drosophila melanogaster, and its natural pathogen, the sigma virus. We confirm previous findings that a single (albeit complex) mutation in the gene ref(2)P confers resistance against sigma and show that this mutation has increased in frequency under positive selection. Previous studies suggested that ref(2)P polymorphism reflects the progress of a very recent selective sweep, and that in Europe during the 1980s, this was followed by a sweep of a sigma virus strain able to infect flies carrying this mutation. We find that the ref(2)P resistance mutation is considerably older than the recent spread of this viral strain and suggest that—possibly because it is recessive—the initial spread of the resistance mutation was very slow.     

Neutral drift and polymorphism in gene-for-gene systems

Pathogens are a main driving force of the evolution of plants and animals. Being resistant to diseases confers a high selective advantage to hosts, yet many host–pathogen systems show a remarkable degree of polymorphism of host resistance and pathogen virulence. The most common explanation of this phenomenon is that both resistance and virulence genes are costly and that there is selection against those genes when they are unnecessary. Here, we use stochastic multi‐locus simulations to show that the origin and the maintenance of genetic polymorphism in plant–pathogen systems can be explained without costs. In multi‐locus gene‐for‐gene systems, temporal domination of a super pathogen can cause polymorphism in resistance through neutral drift. With an increasing number of susceptible alleles in the host population, pathogen types other than the super race are able to cause infections and invade the population, leading to higher pathogen diversity and in turn to higher host diversity.     

Virus evolution and transmission in an ever more connected world

The frequency and global impact of infectious disease outbreaks, particularly those caused by emerging viruses, demonstrate the need for a better understanding of how spatial ecology and pathogen evolution jointly shape epidemic dynamics. Advances in computational techniques and the increasing availability of genetic and geospatial data are helping to address this problem, particularly when both information sources are combined. Here, we review research at the intersection of evolutionary biology, human geography and epidemiology that is working towards an integrated view of spatial incidence, host mobility and viral genetic diversity. We first discuss how empirical studies have combined viral spatial and genetic data, focusing particularly on the contribution of evolutionary analyses to epidemiology and disease control. Second, we explore the interplay between virus evolution and global dispersal in more depth for two pathogens: human influenza A virus and chikungunya virus. We discuss the opportunities for future research arising from new analyses of human transportation and trade networks, as well as the associated challenges in accessing and sharing relevant spatial and genetic data.     

Multiple Sources of Introduction of North American Arabidopsis thaliana from across Eurasia

Large-scale movement of organisms across their habitable range, or migration, is an important evolutionary process that can shape genetic diversity and influence the adaptive spread of alleles. Although human migrations have been studied in great detail with modern and ancient genomes, recent anthropogenic influence on reducing the biogeographical constraints on the migration of nonnative species has presented opportunities in several study systems to ask the questions about how repeated introductions shape genetic diversity in the introduced range. We present an extensive overview of population structure of North American Arabidopsis thaliana by studying a set of 500 whole-genome sequenced and over 2,800 RAD-seq genotyped individuals in the context of global diversity represented by Afro-Eurasian genomes. We use methods based on haplotype and rare-allele sharing as well as phylogenetic modeling to identify likely sources of introductions of extant N. American A. thaliana from the native range in Africa and Eurasia. We find evidence of admixture among the introduced lineages having increased haplotype diversity and reduced mutational load. We also detect signals of selection in immune-system-related genes that may impart qualitative disease resistance to pathogens of bacterial and oomycete origin. We conclude that multiple introductions to a nonnative range can rapidly enhance the adaptive potential of a colonizing species by increasing haplotypic diversity through admixture. Our results lay the foundation for further investigations into the functional significance of admixture.