Heterosis, the catapult effect and establishment success of a colonizing bird

The Poultry Red Mite, Dermanyssus gallinae, travels far but not frequently, and takes up permanent residence on farms

Management of Dermanyssus gallinae, a cosmopolitan hematophagous mite responsible for damage in layer poultry farming, is hampered by a lack of knowledge of its spatio-temporal population dynamics. Previous studies have shown that the circulation of this pest between farms is of strictly anthropogenic origin, that a mitochondrial haplogroup has been expanding on European farms since the beginning of the 21st century and that its local population growth may be particularly rapid. To refine our understanding of how D. gallinae spreads within and among farms, we characterized the genetic structure of mite populations at different spatial scales and sought to identify the main factors interrupting gene flow between poultry houses and between mitochondrial haplogroups. To this end, we selected and validated the first set of nuclear microsatellite markers for D. gallinae and sequenced a region of the CO1-encoding mitochondrial gene in a subsample of microsatellite-genotyped mites. We also tested certain conditions required for effective contamination of a poultry house through field experimentation, and conducted a survey of practices during poultry transfers. Our results confirm the role of poultry transport in the dissemination of mite populations, but the frequency of effective contamination after the introduction of contaminated material into poultry houses seems lower than expected. The high persistence of mites on farms, even during periods when poultry houses are empty and cleaned, and the very large number of nodes in the logistic network (large number of companies supplying pullets or transporting animals) undoubtedly explain the very high prevalence on farms. Substantial genetic diversity was measured in farm populations, probably as a result of the mite's known haplodiploid mode of sexual reproduction, coupled with the dense logistic network. The possibility of the occasional occurrence of asexual reproduction in this sexually reproducing mite was also revealed in our analyses, which could explain the extreme aggressiveness of its demographic dynamics under certain conditions.

Environmentally independent selection for hybrids between divergent freshwater stickleback lineages in semi-natural ponds

Hybridization following secondary contact of genetically divergent populations can influence the range expansion of invasive species, though specific outcomes depend on the environmental dependence of hybrid fitness. Here, using two genetically and ecologically divergent threespine stickleback lineages that differ in their history of freshwater colonization, we estimate fitness variation of parental lineages and hybrids in semi-natural freshwater ponds with contrasting histories of nutrient loading. In our experiment, we found that fish from the older freshwater lineage (Lake Geneva) and hybrids outperformed fish from the younger freshwater lineage (Lake Constance) in terms of both growth and survival, regardless of the environmental context of our ponds. Across all ponds, hybrids exhibited the highest survival. Although wild-caught adult populations differed in their functional and defence morphology, it is unclear which of these traits underlie the fitness differences observed among juveniles in our experiment. Overall, our work suggests that when hybrid fitness is insensitive to environmental conditions, as observed here, introgression may promote population expansion into unoccupied habitats and accelerate invasion success.

Microsatellites and mitochondrial evidence of multiple introductions of the invasive raccoon Procyon lotor in France

Raccoons (Procyon lotor) are worldwide invaders, due to deliberate or accidental releases, and their impacts exceed hundred of billions of Euros in Europe only. In France, raccoons have currently established three separate, expanding populations. Identifying the current spatial genetic structure, dispersal events and phylogeography of these populations is needed to infer the invasion history and identify management units. We used wild and captive individuals sampled in France and Belgium to characterize the genetic diversity and current population genetic structure of French raccoon populations and identify potential genetic connectivity with the Belgium population using both mitochondrial DNA and microsatellite loci. Results confirm that French populations are the result of at least three independent introductions. While the three populations display low genetic diversity and sign of recent bottleneck, they are still expanding, suggesting that in addition to their ecological plasticity, the remaining genetic diversity is sufficient to successfully adapt to their new environment and allow a quick colonization. Particular attention must be given to the North-Eastern population, which shows genetic admixture with the Belgium population, as admixed individuals may exhibit hybrid vigor facilitating their expansion. The comparison of captive and wild individuals did not allow to identify a potential captive origin of the wild populations. The current regulation in France allowing captivity in zoos without enforcement to tighten the biosecurity of detention facilities might dampen any management measure as few introduced founders might be enough to create new populations.

Prevalence and Adaptive Impact of Introgression

Alleles that introgress between species can influence the evolutionary and ecological fate of species exposed to novel environments. Hybrid offspring of different species are often unfit, and yet it has long been argued that introgression can be a potent force in evolution, especially in plants. Over the last two decades, genomic data have increasingly provided evidence that introgression is a critically important source of genetic variation and that this additional variation can be useful in adaptive evolution of both animals and plants. Here, we review factors that influence the probability that foreign genetic variants provide long-term benefits (so-called adaptive introgression) and discuss their potential benefits. We find that introgression plays an important role in adaptive evolution, particularly when a species is far from its fitness optimum, such as when they expand their range or are subject to changing environments.

Extensive human-mediated jump dispersal within and across the native and introduced ranges of the invasive termite Reticulitermes flavipes

As native ranges are often geographically structured, invasive species originating from a single source population only carry a fraction of the genetic diversity present in their native range. The invasion process is thus often associated with a drastic loss of genetic diversity resulting from a founder event. However, the fraction of diversity brought to the invasive range may vary under different invasion histories, increasing with the size of the propagule, the number of reintroduction events, and/or the total genetic diversity represented by the various source populations in a multiple-introduction scenario. In this study, we generated a SNP data set for the invasive termite Reticulitermes flavipes from 23 native populations in the eastern United States and six introduced populations throughout the world. Using population genetic analyses and approximate Bayesian computation random forest, we investigated its worldwide invasion history. We found a complex invasion pathway with multiple events out of the native range and bridgehead introductions from the introduced population in France. Our data suggest that extensive long-distance jump dispersal appears common in both the native and introduced ranges of this species, probably through human transportation. Overall, our results show that similar to multiple introduction events into the invasive range, admixture in the native range prior to invasion can potentially favour invasion success by increasing the genetic diversity that is later transferred to the introduced range.

Evidence of ecological niche shift in Rhododendron ponticum (L.) in Britain: Hybridization as a possible cause of rapid niche expansion

Biological invasions threaten global biodiversity and natural resources. Anticipating future invasions is central to strategies for combating the spread of invasive species. Ecological niche models are thus increasingly used to predict potential distribution of invasive species. In this study, we compare ecological niches of Rhododendron ponticum in its native (Iberian Peninsula) and invasive (Britain) ranges. Here, we test the conservation of ecological niche between invasive and native populations of R. ponticum using principal component analysis, niche dynamics analysis, and MaxEnt-based reciprocal niche modeling. We show that niche overlap between native and invasive populations is very low, leading us to the conclusion that the two niches are not equivalent and are dissimilar. We conclude that R. ponticum occupies novel environmental conditions in Britain. However, the evidence of niche shift presented in this study should be treated with caution because of nonanalogue climatic conditions between native and invasive ranges and a small population size in the native range. We then frame our results in the context of contradicting genetic evidence on possible hybridization of this invasive species in Britain. We argue that the existing contradictory studies on whether hybridization caused niche shift in R. ponticum are not sufficient to prove or disprove this hypothesis. However, we present a series of theoretical arguments which indicate that hybridization is a likely cause of the observed niche expansion of R. ponticum in Britain.

Resolving the genetic paradox of invasions: Preadapted genomes and postintroduction hybridization of bigheaded carps in the Mississippi River Basin

The genetic paradox of biological invasions is complex and multifaceted. In particular, the relative role of disparate propagule sources and genetic adaptation through postintroduction hybridization has remained largely unexplored. To add resolution to this paradox, we investigate the genetic architecture responsible for the invasion of two invasive Asian carp species, bighead carp (Hypophthalmichthys nobilis) and silver carp (H. molitrix) (bigheaded carps) that experience extensive hybridization in the Mississippi River Basin (MRB). We sequenced the genomes of bighead and silver carps (~1.08G bp and ~1.15G bp, respectively) and their hybrids collected from the MRB. We found moderate-to-high heterozygosity in bighead (0.0021) and silver (0.0036) carps, detected significantly higher dN/dS ratios of single-copy orthologous genes in bigheaded carps versus 10 other species of fish, and identified genes in both species potentially associated with environmental adaptation and other invasion-related traits. Additionally, we observed a high genomic similarity (96.3% in all syntenic blocks) between bighead and silver carps and over 90% embryonic viability in their experimentally induced hybrids. Our results suggest intrinsic genomic features of bigheaded carps, likely associated with life history traits that presumably evolved within their native ranges, might have facilitated their initial establishment of invasion, whereas ex-situ interspecific hybridization between the carps might have promoted their range expansion. This study reveals an alternative mechanism that could resolve one of the genetic paradoxes in biological invasions and provides invaluable genomic resources for applied research involving bigheaded carps.

Phylogeography of Lionfishes (Pterois) Indicate Taxonomic Over Splitting and Hybrid Origin of the Invasive Pterois volitans

The lionfish is an iconic marine fish, and recently renowned for a disastrous introduction into the West Atlantic. Genetic surveys of the putative invaders (Pterois volitans and Pterois miles) in their natural Indo-Pacific range can illuminate both topics. Previous research indicated that P. volitans and P. miles are sister species that hybridize in the invasive range, but hybridization in the native range is unknown. Here, we apply mtDNA COI and 2 nuclear introns (S7 RP1 and Gpd2) from 229 lionfish including the 2 invaders and 2 closely-related taxa (44 P. miles, 91 P. volitans, 31 Pterois lunulata, and 63 Pterois russelii) from 10 locations in their native ranges. Genetic data are supplemented with key morphological characters: dorsal, anal, and pectoral fin ray counts. We observed 2 lineages (d = 4.07%, 0.89%, and 2.75% at COI, S7 RP1, and Gpd2, respectively) among the 4 putative species: an Indian Ocean lineage represented by P. miles, and a Pacific Ocean lineage represented by P. lunulata and P. russelii. All specimens of the invasive P. volitans appear to be hybrids between the Indian Ocean P. miles and a Pacific lineage encompassing P. lunulata/russelii, a conclusion supported by both genetics and morphology. The divergences between Indian and Pacific forms are within the range of species-level partitions in fishes, and we recommend retention of the names P. miles and P. russelii for Indian and Pacific forms. The hybrid origin of the Atlantic invasion invokes the possibility of heterosis as a contributing factor to invasion success.

Genetic admixture accelerates invasion via provisioning rapid adaptive evolution

Genetic admixture, the intraspecific hybridization among divergent introduced sources, can immediately facilitate colonization via hybrid vigor and profoundly enhance invasion via contributing novel genetic variation to adaption. As hybrid vigor is short-lived, provisioning adaptation is anticipated to be the dominant and long-term profit of genetic admixture, but the evidence for this is rare. We employed the 30 years' geographic-scale invasion of the salt marsh grass, Spartina alterniflora, as an evolutionary experiment and evaluated the consequences of genetic admixture by combining the reciprocal transplant experiment with quantitative and population genetic surveys. Consistent with the documentation, we found that the invasive populations in China had multiple origins from the southern Atlantic coast and the Gulf of Mexico in the US. Interbreeding among these multiple sources generated a "hybrid swarm" that spread throughout the coast of China. In the northern and mid-latitude China, natural selection greatly enhanced fecundity, plant height and shoot regeneration compared to the native populations. Furthermore, genetic admixture appeared to have broken the negative correlation between plant height and shoot regeneration, which was genetically-based in the native range, and have facilitated the evolution of super competitive genotypes in the invasive range. In contrast to the evolved northern and mid-latitude populations, the southern invasive populations showed slight increase of plant height and shoot regeneration compared to the native populations, possibly reflecting the heterotic effect of the intraspecific hybridization. Therefore, our study suggests a critical role of genetic admixture in accelerating the geographic invasion via provisioning rapid adaptive evolution.

Widespread hybridization in the introduced hog deer population of Victoria, Australia, and its implications for conservation

In Australia, many species have been introduced that have since undergone drastic declines in their native range. One species of note is the hog deer (Axis porcinus) which was introduced in the 1860s to Victoria, Australia, and has since become endangered in its native range throughout South-East Asia. There is increased interest in using non-native populations as a source for genetic rescue; however, considerations need to be made of the genetic suitability of the non-native population. Three mitochondrial markers and two nuclear markers were sequenced to assess the genetic variation of the Victorian population of hog deer, which identified that the Victorian population has hybrid origins with the closely related chital (Axis axis), a species that is no longer present in the wild in Victoria. In addition, the mitochondrial D-loop region within the Victorian hog deer is monomorphic, demonstrating that mitochondrial genetic diversity is very low within this population. This study is the first to report of long-term persistence of hog deer and chital hybrids in a wild setting, and the continual survival of this population suggests that hybrids of these two species are fertile. Despite the newly discovered hybrid status in Victorian hog deer, this population may still be beneficial for future translocations within the native range. However, more in-depth analysis of genetic diversity within the Victorian hog deer population and investigation of hybridization rates within the native range are necessary before translocations are attempted.

Founder effects on trans-generational dynamics of closed inbreeding lineages of the predatory mite Phytoseiulus persimilis

Both close inbreeding and distant outbreeding may reduce fitness below the level of individuals with intermediate parental relatedness. In the haplodiploid plant-inhabiting predatory mite Phytoseiulus persimilis, which is patchily distributed within and among host plants, fitness is indeed reduced in the short term, i.e. by a single generation of inbreeding. However, in the medium to long term (multiple generations), distant out-breeding should provide for favorable demographic founder effects in isolated populations. We tested this prediction in isolated experimental lineages founded by females mated to a sibling (close inbreeding), a male from the same population (intermediate relatedness) or a male from another population (distant outbreeding) and monitored lineage growth and persistence over four generations. Cross-generationally, lineages founded by distantly outbred females performed the best, i.e. produced the most descendants. However, this was solely due to superior performance from the F₂ generation onwards, whereas in the F₁ generation, lineages founded by females mated to males from their own population (intermediate relatedness) performed the best, as predicted from short-term in- and out-breeding depression effects. At the genetic level, this result was most likely due to distantly outbred founders introducing higher allelic variability and lower homozygosity levels, counterbalancing inbreeding depression, which inevitably occurs in isolated lineages, from the F₂ generation onwards.

The effects of agent hybridization on the efficacy of biological control of tansy ragwort at high elevations

The success rate of weed biological control programs is difficult to evaluate and the factors affecting it remain poorly understood. One aspect which is still unclear is whether releases of multiple, genetically distinct populations of a biological control agent increase the likelihood of success, either by independent colonization of different environmental niches or by hybridization that may increase the agent's fitness and adaptive ability. Since hybridization is often invoked to explain the success of unintentionally introduced exotic species, hybridization among biocontrol agents may be similarly important in shaping the effectiveness of biological control programs. In this study, we first evaluated intraspecific hybridization among populations of a weed biological control agent, the ragwort flea beetle, Longitarsus jacobaeae. These insects were introduced as part of a classical biological control program from Italy and Switzerland. We genotyped 204 individuals from 15 field sites collected in northwest Montana, and an additional 52 individuals that served as references for Italian and Swiss populations. Bayesian analysis of population structure assigned seven populations as pure Swiss and one population as pure Italian, while intraspecific hybrid individuals were detected in seven populations at frequencies of 5%-69%. Subsequently, we conducted a 2-year exclusion experiment using six sites with Swiss beetles and three with hybrid beetles to evaluate the impact of biological control. We found that biological control by Swiss beetles and by hybrid beetles is effective, increasing mortality of the target plant, Jacobaea vulgaris, by 42% and 45%, and reducing fecundity of surviving plants by 44% and 72%, respectively. Beetle densities were higher and mortality of larger plants was higher at sites with hybrids present. These results suggest that hybridization of ragwort flea beetles at high-elevation sites may improve biological control of tansy ragwort and that intraspecific hybridization of agents could benefit biological control programs.

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.

Sex-specific effects of outbreeding on offspring quality in pike (Esox lucius)

Intraspecific genetic admixture occurs when previously separated populations within a species start interbreeding, and it can have either positive, negative, or neutral effects on reproductive performance. As there currently is no reliable predictor for the outcome of admixture, an increased knowledge about admixture effects in different species and populations is important to increase the understanding about what determines the response to admixture. We tested for effects of admixture on F1 offspring quality in three subpopulations of pike (Esox lucius). Gametes were collected in the field, and eggs from each female were experimentally fertilized with milt from a male from each population (one "pure" and two "admixed" treatments). Three offspring quality measures (hatching success, fry survival, and fry length) were determined and compared between (a) pure and admixed population combinations and (b) the sex-specific treatments within each admixed population combination (based on the origin of the male and female, respectively). The results suggested that although there were no overall effects of admixture on offspring quality, the consequences for a given population combination could be sex-specific and thus differ depending on which of the parents originated from one or the other population. All offspring quality traits were influenced by both maternal ID and paternal ID. Sex- and individual-specific effects can have implications for dispersal behavior and gene flow between natural populations, and are important to consider in conservation efforts.

Sleeping with the enemy: introgressive hybridization in two invasive centrarchids

Introgressive hybridization between Micropterus dolomieu and Micropterus salmoides was assessed in their invaded South African range using nine microsatellite markers and two mtDNA gene regions. Although M. dolomieu and M. salmoides are distantly related, indicated by the large uncorrected pairwise distances observed between the two species, mitochondrial introgression and unidirectional admixture was detected.

Population genetic signatures of a climate change driven marine range extension

Shifts in species distribution, or 'range shifts', are one of the most commonly documented responses to ocean warming, with important consequences for the function and structure of ecosystems, and for socio-economic activities. Understanding the genetic signatures of range shifts can help build our knowledge of the capacity of species to establish and persist in colonised areas. Here, seven microsatellite loci were used to examine the population connectivity, genetic structure and diversity of Octopus tetricus, which has extended its distribution several hundred kilometres polewards associated with the southwards extension of the warm East Australian Current along south-eastern Australia. The historical distribution and the range extension zones had significant genetic differences but levels of genetic diversity were comparable. The population in the range extension zone was sub-structured, contained relatively high levels of self-recruitment and was sourced by migrants from along the entire geographic distribution. Genetic bottlenecks and changes in population size were detected throughout the range extension axis. Persistent gene flow from throughout the historical zone and moderate genetic diversity may buffer the genetic bottlenecks and favour the range extension of O. tetricus. These characteristics may aid adaptation, establishment, and long-term persistence of the population in the range extension zone.

Dissecting the null model for biological invasions: A meta-analysis of the propagule pressure effect

A consistent determinant of the establishment success of alien species appears to be the number of individuals that are introduced to found a population (propagule pressure), yet variation in the form of this relationship has been largely unexplored. Here, we present the first quantitative systematic review of this form, using Bayesian meta-analytical methods. The relationship between propagule pressure and establishment success has been evaluated for a broad range of taxa and life histories, including invertebrates, herbaceous plants and long-lived trees, and terrestrial and aquatic vertebrates. We found a positive mean effect of propagule pressure on establishment success to be a feature of every hypothesis we tested. However, establishment success most critically depended on propagule pressures in the range of 10–100 individuals. Heterogeneity in effect size was associated primarily with different analytical approaches, with some evidence of larger effect sizes in animal rather than plant introductions. Conversely, no variation was accounted for in any analysis by the scale of study (field to global) or methodology (observational, experimental, or proxy) used. Our analyses reveal remarkable consistency in the form of the relationship between propagule pressure and alien population establishment success.

Alien species are a major contributor to human-induced global environmental change. The probability of whether or not an alien species will successfully establish in a novel environment is often related to the number of times a species is introduced and the number of individuals that are introduced each time, collectively termed ‘propagule pressure’. Despite this evidence, we don’t yet know whether this is a universal characteristic of species invasions, and the role of propagule pressure continues to be questioned. Here, we present a quantitative meta-analysis of the relationship between propagule pressure and establishment success across a broad range of species and geographies. We found that propagule pressure was consistently and positively associated with the establishment success of alien species. We conclude that propagule pressure is indeed the most consistent and strongest determinant of alien species establishment. No other factors suggested to explain establishment success can claim such universal support. Our results underpin a clear policy and management target for slowing invasion rates by reducing propagule pressure—ideally to single figures or zero—regardless of any other feature of the invasion.

Effects of admixture in native and invasive populations of Lythrum salicaria

Intraspecific hybridization between diverged populations can enhance fitness via various genetic mechanisms. The benefits of such admixture have been proposed to be particularly relevant in biological invasions, when invasive populations originating from different source populations are found sympatrically. However, it remains poorly understood if admixture is an important contributor to plant invasive success and how admixture effects compare between invasive and native ranges. Here, we used experimental crosses in Lythrum salicaria, a species with well-established history of multiple introductions to Eastern North America, to quantify and compare admixture effects in native European and invasive North American populations. We observed heterosis in between-population crosses both in native and invasive ranges. However, invasive-range heterosis was restricted to crosses between two different Eastern and Western invasion fronts, whereas heterosis was absent in geographically distant crosses within a single large invasion front. Our results suggest that multiple introductions have led to already-admixed invasion fronts, such that experimental crosses do not further increase performance, but that contact between different invasion fronts further enhances fitness after admixture. Thus, intra-continental movement of invasive plants in their introduced range has the potential to boost invasiveness even in well-established and successfully spreading invasive species.

Hybridization as a facilitator of species range expansion

Explaining the evolution of species geographical ranges is fundamental to understanding how biodiversity is distributed and maintained. The solution to this classic problem in ecology and evolution remains elusive: we still do not fully know how species geographical ranges evolve and what factors fuel range expansions. Resolving this problem is now more crucial than ever with increasing biodiversity loss, global change and movement of species by humans. Here, we describe and evaluate the hypothesis that hybridization between species can contribute to species range expansion. We discuss how such a process can occur and the empirical data that are needed to test this hypothesis. We also examine how species can expand into new environments via hybridization with a resident species, and yet remain distinct species. Generally, hybridization may play an underappreciated role in influencing the evolution of species ranges. Whether-and to what extent-hybridization has such an effect requires further study across more diverse taxa.

Beyond propagule pressure: importance of selection during the transport stage of biological invasions

Biological invasions are largely considered to be a "numbers game", wherein the larger the introduction effort, the greater the probability that an introduced population will become established. However, conditions during transport - an early stage of the invasion - can be particularly harsh, thereby greatly reducing the size of a population available to establish in a new region. Some successful non-indigenous species are more tolerant of environmental and anthropogenic stressors than related native species, possibly stemming from selection (ie survival of only pre-adapted individuals for particular environmental conditions) during the invasion process. By reviewing current literature concerning population genetics and consequences of selection on population fitness, we propose that selection acting on transported populations can facilitate local adaptation, which may result in a greater likelihood of invasion than predicted by propagule pressure alone. Specifically, we suggest that detailed surveys should be conducted to determine interactions between molecular mechanisms and demographic factors, given that current management strategies may underestimate invasion risk.

Admixture is a driver rather than a passenger in experimental invasions

Genetic admixture propels invasions of Callosobruchus maculatus across experimental landscapes. In Focus: Wagner, N.K., Ochocki, B.M., Crawford, K.M., Compagnoni, A. & Miller, T.E.X. (2017) Genetic mixture of multiple source populations accelerates invasive range expansion. Journal of Animal Ecology, 86, 21-34. In this issue of Journal of Animal Ecology, Wagner et al. (2017) demonstrate that genetic diversity can alter the course of spread of biological invasions. They employ Callosobruchus seed beetles in a clever array of linked habitat patches to compare experimental invasions using individuals from single population sources or from mixes of two, four or six population sources. By taking a model-selection approach, they find that any amount of mixture propels growth rates and spread of introduced populations. This suggests that heterosis alone can alter the course of an invasive range expansion.

The Effect of Parasitism and Interpopulation Hybridization on Aedes albopictus (Diptera: Culicidae) Fitness

Recent research in mosquito population genetics suggests that interpopulation hybridization has likely contributed to the rapid spread of the container-breeding mosquitoes. Here, I used laboratory experiments to investigate whether interpopulation Aedes (Stegomyia) albopictus (Skuse) F1 and F2 hybrids exhibit higher fitness than parental populations, and whether hybrid mosquito performance is related to infection by the coevolved protozoan parasite Ascogregarina taiwanensis (Lien and Levine). Overall, there were significant differences in development time, wing length, and survival between the two parental mosquito populations, but no difference in per capita growth rate r. Hybrid mosquitoes were generally intermediate in phenotype to the parentals, except that F2 females were significantly larger than the midparent average. In addition, As. taiwanensis parasites produced fewest oocysts when they were reared in hosts of hybrid origin. These data suggest that hybridization between previously isolated mosquito populations can result in slight increases in potential mosquito reproductive success, via increased hybrid body size, and via the temporary escape from coevolved parasites. These findings are significant because studies have shown that even slight hybrid vigor can have positive fitness consequences for population persistence. Although this was a laboratory experiment extending only to the F2 generation, many other invasive insects also carry coevolved parasites, and thus the patterns seen in this mosquito system may be broadly relevant.

Isolation mediates persistent founder effects on zooplankton colonisation in new temporary ponds

Understanding the colonisation process in zooplankton is crucial for successful restoration of aquatic ecosystems. Here, we analyzed the clonal and genetic structure of the cyclical parthenogenetic rotifer Brachionus plicatilis by following populations established in new temporary ponds during the first three hydroperiods. Rotifer populations established rapidly after first flooding, although colonisation was ongoing throughout the study. Multilocus genotypes from 7 microsatellite loci suggested that most populations (10 of 14) were founded by few clones. The exception was one of the four populations that persisted throughout the studied hydroperiods, where high genetic diversity in the first hydroperiod suggested colonisation from a historical egg bank, and no increase in allelic diversity was detected with time. In contrast, in another of these four populations, we observed a progressive increase of allelic diversity. This population became less differentiated from the other populations suggesting effective gene flow soon after its foundation. Allelic diversity and richness remained low in the remaining two, more isolated, populations, suggesting little gene flow. Our results highlight the complexity of colonisation dynamics, with evidence for persistent founder effects in some ponds, but not in others, and with early immigration both from external source populations, and from residual, historical diapausing egg banks.

Genetic admixture and heterosis may enhance the invasiveness of common ragweed

Biological invasions are often associated with multiple introductions and genetic admixture of previously isolated populations. In addition to enhanced evolutionary potential through increased genetic variation, admixed genotypes may benefit from heterosis, which could contribute to their increased performance and invasiveness. To deepen our understanding of the mechanisms and management strategies for biological invasions, we experimentally studied whether intraspecific admixture causes heterosis in common ragweed (Ambrosia artemisiifolia) by comparing the performance of crosses (F1) between populations relative to crosses within these populations for each range (native, introduced) under different ecologically relevant conditions (control, drought, competition, simulated herbivory). Performance of admixed genotypes was highly variable, ranging from strong heterotic effects to weak outbreeding depression. Moreover, heterosis was not uniformly observed among between-population crosses, but certain native population crosses showed considerable heterosis, especially under simulated herbivory. In contrast, heterosis was largely absent in crosses from the introduced range, possibly implying that these populations were already admixed and benefit little from further mixing. In conclusion, these results support the hypothesis that heterosis may contribute to biological invasions, and indicate the need to minimize new introductions of exotic species, even if they are already present in the introduced range.

Vegetative and Adaptive Traits Predict Different Outcomes for Restoration Using Hybrids

Hybridization has been implicated as a driver of speciation, extinction, and invasiveness, but can also provide resistant breeding stock following epidemics. However, evaluating the appropriateness of hybrids for use in restoration programs is difficult. Past the F1 generation, the proportion of a progenitor's genome can vary widely, as can the combinations of parental genomes. Detailed genetic analysis can reveal this information, but cannot expose phenotypic alterations due to heterosis, transgressive traits, or changes in metabolism or development. In addition, because evolution is often driven by extreme individuals, decisions based on phenotypic averages of hybrid classes may have unintended results. We demonstrate a strategy to evaluate hybrids for use in restoration by visualizing hybrid phenotypes across selected groups of traits relative to both progenitor species. Specifically, we used discriminant analysis to differentiate among butternut (Juglans cinerea L.), black walnut (J. nigra L.), and Japanese walnut (J. ailantifolia Carr. var. cordiformis) using vegetative characters and then with functional adaptive traits associated with seedling performance. When projected onto the progenitor trait space, naturally occurring hybrids (J. × bixbyi Rehd.) between butternut and Japanese walnut showed introgression toward Japanese walnut at vegetative characters but exhibited a hybrid swarm at functional traits. Both results indicate that hybrids have morphological and ecological phenotypes that distinguish them from butternut, demonstrating a lack of ecological equivalency that should not be carried into restoration breeding efforts. Despite these discrepancies, some hybrids were projected into the space occupied by butternut seedlings' 95% confidence ellipse, signifying that some hybrids were similar at the measured traits. Determining how to consistently identify these individuals is imperative for future breeding and species restoration efforts involving hybrids. Discriminant analysis provides a useful technique to visualize past selection mechanisms and current variation in hybrid populations, especially when key ecological traits that distinguish progenitors are unknown. Furthermore, discriminant analysis affords a tool to assess ecological equivalency of hybrid populations and breeding program efforts to select for certain traits and monitor the amount of variability of those traits, relative to progenitors.

Admixture between native and invasive populations may increase invasiveness of Mimulus guttatus

Self-fertilization and admixture of genotypes from different populations can have major fitness consequences in native species. However, few studies have addressed their potential roles in invasive species. Here, we used plants of Mimulus guttatus from seven native North American, three invasive Scottish and four invasive New Zealand populations to address this. We created seeds from self-fertilization, within-population outcrossing, between-population outcrossing within the same range, and outcrossing between the native and invasive ranges. A greenhouse experiment showed that native and invasive plants of M. guttatus suffered to similar degrees from inbreeding depression, in terms of asexual reproduction and biomass production. After outcrossing with plants from other populations, M. guttatus benefited from heterosis, in terms of asexual and sexual reproduction, and biomass production, particularly when plants from native and invasive populations were crossed. This suggests that, when novel genotypes of M. guttatus from the native North American range will be introduced to the invasive ranges, subsequent outcrossing with M. guttatus plants that are already there might further boost invasiveness of this species.

Genetic mixture of multiple source populations accelerates invasive range expansion

A wealth of population genetic studies have documented that many successful biological invasions stem from multiple introductions from genetically distinct source populations. Yet, mechanistic understanding of whether and how genetic mixture promotes invasiveness has lagged behind documentation that such mixture commonly occurs. We conducted a laboratory experiment to test the influence of genetic mixture on the velocity of invasive range expansion. The mechanistic basis for effects of genetic mixture could include evolutionary responses (mixed invasions may harbour greater genetic diversity and thus elevated evolutionary potential) and/or fitness advantages of between-population mating (heterosis). If driven by evolution, positive effects of source population mixture should increase through time, as selection sculpts genetic variation. If driven by heterosis, effects of mixture should peak following first reproductive contact and then dissipate. Using a laboratory model system (beetles spreading through artificial landscapes), we quantified the velocity of range expansion for invasions initiated with one, two, four or six genetic sources over six generations. Our experiment was designed to test predictions corresponding to the evolutionary and heterosis mechanisms, asking whether any effects of genetic mixture occurred in early or later generations of range expansion. We also quantified demography and dispersal for each experimental treatment, since any effects of mixture should be manifest in one or both of these traits. Over six generations, invasions with any amount of genetic mixture (two, four and six sources) spread farther than single-source invasions. Our data suggest that heterosis provided a 'catapult effect', leaving a lasting signature on range expansion even though the benefits of outcrossing were transient. Individual-level trait data indicated that genetic mixture had positive effects on local demography (reduced extinction risk and enhanced population growth) during the initial stages of invasion but no consistent effects on dispersal ability. Our work is the first to demonstrate that genetic mixture can alter the course of spatial expansion, the stage of invasion typically associated with the greatest ecological and economic impacts. We suggest that similar effects of genetic mixture may be a common feature of biological invasions in nature, but that these effects can easily go undetected.

Recent admixture generates heterozygosity-fitness correlations during the range expansion of an invading species

Admixture, the mixing of historically isolated gene pools, can have immediate consequences for the genetic architecture of fitness traits. Admixture may be especially important for newly colonized populations, such as during range expansion and species invasions, by generating heterozygosity that can boost fitness through heterosis. Despite widespread evidence for admixture during species invasions, few studies have examined the demographic history leading to admixture, how admixture affects the heterozygosity and fitness of invasive genotypes, and whether such fitness effects are maintained through time. We address these questions using the invasive plant Silene vulgaris, which shows evidence of admixture in both its native Europe and in North America where it has invaded. Using multilocus genotype data in conjunction with approximate Bayesian computation analysis of demographic history, we showed that admixture during the invasion of North America was independent from and much younger than admixture in the native range of Europe. We tested for fitness consequences of admixture in each range and detected a significant positive heterozygosity-fitness correlation (HFC) in North America; in contrast, no HFC was present in Europe. The lack of HFC in Europe may reflect the longer time since admixture in the native range, dissipating associations between heterozygosity at markers and fitness loci. Our results support a key short-term role for admixture during the early stages of invasion by generating HFCs that carry populations past the threat of extinction from inbreeding and demographic stochasticity.

Higher genetic diversity in recolonized areas than in refugia of Alnus glutinosa triggered by continent-wide lineage admixture

Genetic admixture is supposed to be an important trigger of species expansions because it can create the potential for selection of genotypes suitable for new climatic conditions. Up until now, however, no continent-wide population genetic study has performed a detailed reconstruction of admixture events during natural species expansions. To fill this gap, we analysed the postglacial history of Alnus glutinosa, a keystone species of European swamp habitats, across its entire distribution range using two molecular markers, cpDNA and nuclear microsatellites. CpDNA revealed multiple southern refugia located in the Iberian, Apennine, Balkan and Anatolian Peninsulas, Corsica and North Africa. Analysis of microsatellites variation revealed three main directions of postglacial expansion: (i) from the northern part of the Iberian Peninsula to Western and Central Europe and subsequently to the British Isles, (ii) from the Apennine Peninsula to the Alps and (iii) from the eastern part of the Balkan Peninsula to the Carpathians followed by expansion towards the Northern European plains. This challenges the classical paradigm that most European populations originated from refugial areas in the Carpathians. It has been shown that colonizing lineages have met several times and formed secondary contact zones with unexpectedly high population genetic diversity in Central Europe and Scandinavia. On the contrary, limited genetic admixture in southern refugial areas of A. glutinosa renders rear-edge populations in the Mediterranean region more vulnerable to extinction due to climate change.

The roles of demography and genetics in the early stages of colonization

Colonization success increases with the size of the founding group. Both demographic and genetic factors underlie this relationship, yet because genetic diversity normally increases with numbers of individuals, their relative importance remains unclear. Furthermore, their influence may depend on the environment and may change as colonization progresses from establishment through population growth and then dispersal. We tested the roles of genetics, demography and environment in the founding of Tribolium castaneum populations. Using three genetic backgrounds (inbred to outbred), we released individuals of four founding sizes (2-32) into two environments (natal and novel), and measured establishment success, initial population growth and dispersal. Establishment increased with founding size, whereas population growth was shaped by founding size, genetic background and environment. Population growth was depressed by inbreeding at small founding sizes, but growth rates were similar across genetic backgrounds at large founding size, an interaction indicating that the magnitude of the genetic effects depends upon founding population size. Dispersal rates increased with genetic diversity. These results suggest that numbers of individuals may drive initial establishment, but that subsequent population growth and spread, even in the first generation of colonization, can be driven by genetic processes, including both reduced growth owing to inbreeding depression, and increased dispersal with increased genetic diversity.

Contemporary evolution during invasion: evidence for differentiation, natural selection, and local adaptation

Biological invasions are 'natural' experiments that can improve our understanding of contemporary evolution. We evaluate evidence for population differentiation, natural selection and adaptive evolution of invading plants and animals at two nested spatial scales: (i) among introduced populations (ii) between native and introduced genotypes. Evolution during invasion is frequently inferred, but rarely confirmed as adaptive. In common garden studies, quantitative trait differentiation is only marginally lower (~3.5%) among introduced relative to native populations, despite genetic bottlenecks and shorter timescales (i.e. millennia vs. decades). However, differentiation between genotypes from the native vs. introduced range is less clear and confounded by nonrandom geographic sampling; simulations suggest this causes a high false-positive discovery rate (>50%) in geographically structured populations. Selection differentials (¦s¦) are stronger in introduced than in native species, although selection gradients (¦β¦) are not, consistent with introduced species experiencing weaker genetic constraints. This could facilitate rapid adaptation, but evidence is limited. For example, rapid phenotypic evolution often manifests as geographical clines, but simulations demonstrate that nonadaptive trait clines can evolve frequently during colonization (~two-thirds of simulations). Additionally, QST-FST studies may often misrepresent the strength and form of natural selection acting during invasion. Instead, classic approaches in evolutionary ecology (e.g. selection analysis, reciprocal transplant, artificial selection) are necessary to determine the frequency of adaptive evolution during invasion and its influence on establishment, spread and impact of invasive species. These studies are rare but crucial for managing biological invasions in the context of global change.

Biological invasions, climate change and genomics

The rate of biological invasions is expected to increase as the effects of climate change on biological communities become widespread. Climate change enhances habitat disturbance which facilitates the establishment of invasive species, which in turn provides opportunities for hybridization and introgression. These effects influence local biodiversity that can be tracked through genetic and genomic approaches. Metabarcoding and metagenomic approaches provide a way of monitoring some types of communities under climate change for the appearance of invasives. Introgression and hybridization can be followed by the analysis of entire genomes so that rapidly changing areas of the genome are identified and instances of genetic pollution monitored. Genomic markers enable accurate tracking of invasive species' geographic origin well beyond what was previously possible. New genomic tools are promoting fresh insights into classic questions about invading organisms under climate change, such as the role of genetic variation, local adaptation and climate pre-adaptation in successful invasions. These tools are providing managers with often more effective means to identify potential threats, improve surveillance and assess impacts on communities. We provide a framework for the application of genomic techniques within a management context and also indicate some important limitations in what can be achieved.

No clear effect of admixture between two European invading outbreaks of Diabrotica virgifera virgifera in natura

In this study, we challenged the hypothesis that admixture may have had a positive impact in the context of the European invasion of the western corn rootworm (WCR), Diabrotica virgifera virgifera, LeConte. This beetle was introduced in Europe from the USA several times since the 1980's. The multiple introductions of this major pest of cultivated corn led to the formation of two major outbreaks in North Western (NW) Italy and in Central and South Eastern (CSE) Europe that eventually merged into a secondary contact zone where insects from both outbreaks interbreed. We collected about 600 insects from this contact zone and genotyped them using 13 microsatellite markers. Three types of information were obtained from the collected individuals: (i) their survival under starvation; (ii) their admixed status, determined through a Bayesian method of genetic clustering and (iii) their mating probability, studied via the detection, isolation and genotyping of sperm in female spermathecae. Twenty six % and 12% of the individuals were assigned to the NW Italy or the CSE Europe parental types, respectively, and 23% and 39% to the F1 and backcross hybrid types, respectively. Globally, our results do not reveal any significant impact of the admixed status on the mating probability and on the choice of mating partners. However the admixed status had a sex- and sampling site-dependent effect on survival in adults under starvation. In addition sex had an effect on survival, with mortality hazard about 3 times larger in males than in females. The consequences of these findings for the evolution of the admixture zone of northern Italy are discussed.

Population admixture, biological invasions and the balance between local adaptation and inbreeding depression

When previously isolated populations meet and mix, the resulting admixed population can benefit from several genetic advantages, including increased genetic variation, the creation of novel genotypes and the masking of deleterious mutations. These admixture benefits are thought to play an important role in biological invasions. In contrast, populations in their native range often remain differentiated and frequently suffer from inbreeding depression owing to isolation. While the advantages of admixture are evident for introduced populations that experienced recent bottlenecks or that face novel selection pressures, it is less obvious why native range populations do not similarly benefit from admixture. Here we argue that a temporary loss of local adaptation in recent invaders fundamentally alters the fitness consequences of admixture. In native populations, selection against dilution of the locally adapted gene pool inhibits unconstrained admixture and reinforces population isolation, with some level of inbreeding depression as an expected consequence. We show that admixture is selected against despite significant inbreeding depression because the benefits of local adaptation are greater than the cost of inbreeding. In contrast, introduced populations that have not yet established a pattern of local adaptation can freely reap the benefits of admixture. There can be strong selection for admixture because it instantly lifts the inbreeding depression that had built up in isolated parental populations. Recent work in Silene suggests that reduced inbreeding depression associated with post-introduction admixture may contribute to enhanced fitness of invasive populations. We hypothesize that in locally adapted populations, the benefits of local adaptation are balanced against an inbreeding cost that could develop in part owing to the isolating effect of local adaptation itself. The inbreeding cost can be revealed in admixing populations during recent invasions.