Selection of preadapted populations allowed Senecio inaequidens to invade Central Europe

Invasive species often evolve rapidly in response to the novel biotic and abiotic conditions in their introduced range. Such adaptive evolutionary changes might play an important role in the success of some invasive species. Here, we investigated whether introduced European populations of the South African ragwort Senecio inaequidens (Asteraceae) have genetically diverged from native populations. We carried out a greenhouse experiment where 12 South African and 11 European populations were for several months grown at two levels of nutrient availability, as well as in the presence or absence of a generalist insect herbivore. We found that, in contrast to a current hypothesis, plants from introduced populations had a significantly lower reproductive output, but higher allocation to root biomass, and they were more tolerant to insect herbivory. Moreover, introduced populations were less genetically variable, but displayed greater plasticity in response to fertilization. Finally, introduced populations were phenotypically most similar to a subset of native populations from mountainous regions in southern Africa. Taking into account the species' likely history of introduction, our data support the idea that the invasion success of Senecio inaequidens in Central Europe is based on selective introduction of specific preadapted and plastic genotypes rather than on adaptive evolution in the introduced range.     

Ploidy level and origin of the European invasive weed Senecio inaequidens (Asteraceae)

Native to South-Africa, species of the Senecio inaequidens complex are presently invasive in Europe, Australia and South-America. Previously, different ploidy levels have been found in these different areas, with only tetraploid individuals reported in Europe, and only diploids in South-Africa and Australia. In the present study chromosome counts and flow cytometry were used to survey DNA ploidy levels in a large sample of 66 native and 21 European invasive populations. One Mexican individual was also added to the study. We found only tetraploid individuals occurring in Europe, whereas both ploidy levels, diploid and tetraploid, were found in South-Africa. Moreover, based on genome size, we suggest that two largely allopatric varieties of diploids exist in South-Africa. The Mexican individual was diploid. We suggest that European tetraploid individuals come from South-Africa and hypothesize that a hybridization event between the two DNA types of diploids occurred in the Lesotho area. The taxonomic difficulties surrounding species of theS. inaequidens complex are briefly discussed.     

The genetic ghost of an invasion past: colonization and extinction revealed by historical hybridization in Senecio

Hybridization is an important evolutionary factor in the diversification of many plant and animal species. Of particular interest is that historical hybridization resulting in the origin of new species or introgressants has occurred between species now geographically separated by great distances. Here, we report that Senecio massaicus, a tetraploid species native to Morocco and the Canary Islands, contains genetic material of two distinct, geographically separated lineages: a Mediterranean lineage and a mainly southern African lineage. A time-calibrated internal transcribed spacer phylogeny indicates that the hybridization event took place up to 6.18 Ma. Because the southern African lineage has never been recorded from Morocco or the Canary Islands, we hypothesize that it reached this area in the distant past, but never became permanently established. Interestingly, the southern African lineage includes S. inaequidens, a highly invasive species that has recently become widespread throughout Europe and was introduced at the end of the 19th century as a 'wool alien'. Our results suggest that this more recent invasion of Europe by S. inaequidens represents the second arrival of this lineage into the region.     

Invasive species of Heracleum in Europe: an insight into genetic relationships and invasion history

Several species of the genus Heracleum (Umbelliferae) were introduced into Europe from south-west Asia in the 19th century and are now widespread in many countries. At least three invasive taxa with unresolved relationships to one another are thought to occur in Europe: Heracleum mantegazzianum Sommier & Levier, H. sosnowskyi Manden, and H. persicum Desf. ex Fischer. They are tall plants forming extensive stands with a high cover. To elucidate genetic relationships between the species, and gain insight into their invasion history, samples were collected from native ranges in Asia and invaded ranges of the three species in Europe and analysed using amplified fragment length polymorphism. Five other Heracleum species were also studied and in total, 189 samples from 72 populations were analysed. The results confirmed that there are three distinct tall Heracleum species invading in Europe. Within each of the three species, plants collected in the invaded range are genetically close to those from their native ranges. A close genetic relationship between the three invasive Heracleum species in Europe was also found. A high overall genetic variability detected in the invaded range suggests that the majority of invading populations were not affected by a genetic bottleneck and that rapid evolution, drift, or hybridization played a role in genetic structuring of invading populations. For H. mantegazzianum , genetic distance of populations in the native range significantly decreased with geographical distance, but not in the invaded range. It is likely that the current pattern of genetic diversity in Europe resulted from multiple introductions of all three species.     

No evolutionary shift in the mating system of north American Ambrosia artemisiifolia (Asteraceae) following its introduction to China

The mating system plays a key role during the process of plant invasion. Contemporary evolution of uniparental reproduction (selfing or asexuality) can relieve the challenges of mate limitation in colonizing populations by providing reproductive assurance. Here we examined aspects of the genetics of colonization in Ambrosia artemisiifolia, a North American native that is invasive in China. This species has been found to possess a strong self-incompatibility system and have high outcrossing rates in North America and we examined whether there has been an evolutionary shift towards the dependence on selfing in the introduced range. Specifically, we estimated outcrossing rates in one native and five invasive populations and compared levels of genetic diversity between North America and China. Based on six microsatellite loci we found that, like the native North American population, all five Chinese populations possessed a completely outcrossing mating system. The estimates of paternity correlations were low, ranging from 0.028-0.122, which suggests that populations possessed ~8-36 pollen donor parents contributing to each maternal plant in the invasive populations. High levels of genetic diversity for both native and invasive populations were found with the unbiased estimate of gene diversity ranging from 0.262-0.289 for both geographic ranges based on AFLP markers. Our results demonstrate that there has been no evolutionary shift from outcrossing to selfing during A. artemisiifolia's invasion of China. Furthermore, high levels of genetic variation in North America and China indicate that there has been no erosion of genetic variance due to a bottleneck during the introduction process. We suggest that the successful invasion of A. artemisiifolia into Asia was facilitated by repeated introductions from multiple source populations in the native range creating a diverse gene pool within Chinese populations.     

Factors regulating the invasive success of an alien frog: a comparison of the ecology of the native and alien populations

We examined niche occupancy of Discoglossus pictus, an anuran recently established in Europe, comparing the niches of native (North Africa) and alien populations (south-western Europe) at two spatial scales to determine whether adaptive divergence had occurred between these two populations. Additionally, we determine whether the alien species showed a wider larvae niche and higher phenotypic variability compared with co-occurring anurans. We characterized the breeding habitats and the climatic space occupied by native and alien groups of populations of D. pictus and examined morphological traits of D. pictus and sympatric anuran larvae. Our results revealed no divergence in breeding habitat use between native and alien populations. A shift was observed between the realized niches occupied by the native and alien populations, but this shift might only reflect cryptic niche conservatism. The range of reproductive habitats selected by D. pictus was not wider than those of most native species. In the invaded range, D. pictus showed morphological overlap with some native species and broader phenotypic variability, but the adaptive advantages of this latter attribute were uncertain. Our results suggest that the invasive capacity of this species depends on favourable abiotic conditions rather than on its adaptive advantages over native anurans.     

Resistance in introduced populations of a freshwater snail to native range parasites

Introduced species provide an opportunity to examine responses to novel ecological conditions, in particular to the absence of co-evolved enemies. Introduced populations could evolve lower investment in resistance or could down-regulate their immune system as a plastic response to enemy absence. The response might have consequences for the success of introduced species. Assuming a trade-off between resistance and traits related to demographic success, an evolved change or reallocation from resistance could increase the chances of invasions. On the other hand, introduced populations could have increased resistance as a correlate of greater vigour and competitive ability among successful invaders [Sampling Bias hypothesis (SBH)]. These hypotheses make different predictions about investment in resistance in introduced populations. Using a New Zealand clonal snail (Potamopyrgus antipodarum), we examined the resistance of three introduced genotypes (one from the US and two from Europe) to several populations of a native range parasite (Microphallus sp.). One genotype (Euro A) was resistant to all native range parasite populations, consistent with the SBH. However, two remaining genotypes (Euro C and US 1) were less susceptible to parasite populations that were allopatric to their source populations. Furthermore, resistance of one genotype (US 1) collected from the introduced range was indistinguishable from its resistance when collected from the range of the parasite. Hence, there was no evidence for decreased resistance in the absence of native enemies, which is inconsistent with hypotheses that envision reduced allocation to resistance or a trade-off between competitive ability and resistance.     

Genetic consequences of an invasion through a patchy environment — the cynipid gallwasp Andricus quercuscalicis (Hymenoptera: Cynipidae)

Over the last 300–400 years, the cynipid gallwasp Andricus quercuscalicis has invaded northern and western Europe following human introduction of an obligate host plant, the Turkey oak ( Quercus cerris ) from south-eastern Europe. In the introduced range, distances between Turkey oak patches are greater and the numbers of oaks in each patch are far lower than in its native range. These changes in spatial distribution of Turkey oak are predicted to result in high genetic subdivision of A. quercuscalicis in invaded areas relative to its native range. Allozyme electrophoresis was used to examine genetic variation in 823 gall wasps from 39 populations of A. quercuscalicis. No new electrophoretic variants were found in the invaded range and both allelic diversity and mean heterozygosity decreased significantly with distance from the native range. Spatial autocorrelation analysis and values of Wright's F_st indicate that differences in allele frequences between populations were substantially greater in the invaded range than in the native range. Spatial autocorrelation analysis also suggests that changes in allele frequencies across Europe are unlikely to be the results of selection, but rather of strong directional migration followed by limited gene flow between populations. Patterns of genetic differentiation across Europe suggest that populations of A. quercuscalicis have been founded sequentially from the east through a process of random genetic subsampling and not by source-sink colonization directly from the native range.     

Evolution under changing climates: climatic niche stasis despite rapid evolution in a non-native plant

A topic of great current interest is the capacity of populations to adapt genetically to rapidly changing climates, for example by evolving the timing of life-history events, but this is challenging to address experimentally. I use a plant invasion as a model system to tackle this question by combining molecular markers, a common garden experiment and climatic niche modelling. This approach reveals that non-native Lactuca serriola originates primarily from Europe, a climatic subset of its native range, with low rates of admixture from Asia. It has rapidly refilled its climatic niche in the new range, associated with the evolution of flowering phenology to produce clines along climate gradients that mirror those across the native range. Consequently, some non-native plants have evolved development times and grow under climates more extreme than those found in Europe, but not among populations from the native range as a whole. This suggests that many plant populations can adapt rapidly to changed climatic conditions that are already within the climatic niche space occupied by the species elsewhere in its range, but that evolution to conditions outside of this range is more difficult. These findings can also help to explain the prevalence of niche conservatism among non-native species.     

The population genomic basis of geographic differentiation in North American common ragweed (Ambrosia artemisiifolia L.)

Common ragweed (Ambrosia artemisiifolia L.) is an invasive, wind‐pollinated plant nearly ubiquitous in disturbed sites in its eastern North American native range and present across growing portions of Europe, Africa, Asia, and Australia. Phenotypic divergence between European and native‐range populations has been described as rapid evolution. However, a recent study demonstrated major human‐mediated shifts in ragweed genetic structure before introduction to Europe and suggested that native‐range genetic structure and local adaptation might fully explain accelerated growth and other invasive characteristics of introduced populations. Genomic differentiation that potentially influenced this structure has not yet been investigated, and it remains unclear whether substantial admixture during historical disturbance of the native range contributed to the development of invasiveness in introduced European ragweed populations. To investigate fine‐scale population genetic structure across the species' native range, we characterized diallelic SNP loci via a reduced‐representation genotyping‐by‐sequencing (GBS) approach. We corroborate phylogeographic domains previously discovered using traditional sequencing methods, while demonstrating increased power to resolve weak genetic structure in this highly admixed plant species. By identifying exome polymorphisms underlying genetic differentiation, we suggest that geographic differentiation of this important invasive species has occurred more often within pathways that regulate growth and response to defense and stress, which may be associated with survival in North America's diverse climatic regions.     

Release from Parasites as Natural Enemies: Increased Performance of a Globally Introduced Marine Crab

Introduced species often seem to perform better than conspecifics in their native range. This is apparent in the high densities they may achieve or the larger individual sizes they attain. A prominent hypothesis explaining the success of introduced terrestrial species is that they are typically free of or are less affected by the natural enemies (competitors, predators, and parasites) they encounter in their introduced range compared to their native range. To test this hypothesis in a marine system, we conducted a global assessment of the effect of parasitism and predation on the ecological performance of European green crab populations. In Europe, where the green crab is native, crab body size and biomass were negatively associated with the prevalence of parasitic castrators. When we compared native crab populations with those from introduced regions, limb loss (an estimator of predation) was not significantly lower in introduced regions, parasites infected introduced populations substantially less and crabs in introduced regions were larger and exhibited a greater biomass. Our results are consistent with the general prediction that introduced species suffer less from parasites compared to populations where they are native. This may partly explain why the green crab is such a successful invader and, subsequently, why it is a pest in so many places.     

Variations in parasitism in sympatric populations of three invasive leaf miners

Abstract:  Parasitism was investigated in sympatric populations of three invasive gracillariid leaf miners in Europe, Cameraria ohridella , Phyllonorycter robiniella and Phyllonorycter platani to test the hypotheses that C. ohridella is less heavily parasitized by native parasitoids and attacked by fewer species than the two other invasive species. In all regions investigated, C. ohridella showed a lower parasitism rate, and its parasitoid complex was poorer in species than those of either Phyllonorycter spp. Comparisons were made between sympatric populations of C. ohridella on its main host tree, Aesculus hippocastanum , and an occasional host, Acer pseudoplatanus . Parasitism rates were similarly low and composed of the same parasitoid species on both trees. In contrast, a sympatric population of Phyllonorycter geniculella , a native species mining A. pseudoplatanus , was heavily parasitized by a totally different parasitoid complex. These results suggest that the low parasitism in C. ohridella by native polyphagous leaf miner parasitoids is due neither to its host tree, nor to a problem of synchronization between the phenology of the pest and that of its parasitoids. Instead, it probably results from the inability of the native parasitoids to locate, attack, or develop on a new host that does not have any native congener in Europe.     

Global realized niche divergence in the African clawed frog Xenopus laevis

Although of crucial importance for invasion biology and impact assessments of climate change, it remains widely unknown how species cope with and adapt to environmental conditions beyond their currently realized climatic niches (i.e., those climatic conditions existing populations are exposed to). The African clawed frog Xenopus laevis, native to southern Africa, has established numerous invasive populations on multiple continents making it a pertinent model organism to study environmental niche dynamics. In this study, we assess whether the realized niches of the invasive populations in Europe, South, and North America represent subsets of the species’ realized niche in its native distributional range or if niche shifts are traceable. If shifts are traceable, we ask whether the realized niches of invasive populations still contain signatures of the niche of source populations what could indicate local adaptations. Univariate comparisons among bioclimatic conditions at native and invaded ranges revealed the invasive populations to be nested within the variable range of the native population. However, at the same time, invasive populations are well differentiated in multidimensional niche space as quantified via n‐dimensional hypervolumes. The most deviant invasive population are those from Europe. Our results suggest varying degrees of realized niche shifts, which are mainly driven by temperature related variables. The crosswise projection of the hypervolumes that were trained in invaded ranges revealed the south‐western Cape region as likely area of origin for all invasive populations, which is largely congruent with DNA sequence data and suggests a gradual exploration of novel climate space in invasive populations.     

Functional gametophytic self-incompatibility in a peripheral population of Solanum peruvianum (Solanaceae)

The transition from self-incompatibility to self-compatibility is a common transition in angiosperms often reported in populations at the edge of species range limits. Geographically distinct populations of wild tomato species (Solanum section Lycopersicon (Solanaceae)) have been described as polymorphic for mating system with both self-incompatible and self-compatible populations. Using controlled pollinations and sequencing of the S-RNase mating system gene, we test the compatibility status of a population of S. peruvianum located near its southern range limit. Pollinations among plants of known genotypes revealed strong self-incompatibility; fruit set following compatible pollinations was significantly higher than following incompatible pollinations for all tested individuals. Sequencing of the S-RNase gene in parents and progeny arrays was also as predicted under self-incompatibility. Molecular variation at the S-RNase locus revealed a diverse set of alleles, and heterozygosity in over 500 genotyped individuals. We used controlled crosses to test the specificity of sequences recovered in this study; in all cases, results were consistent with a unique allelic specificity for each tested sequence, including two alleles sharing 92% amino-acid similarity. Site-specific patterns of selection at the S-RNase gene indicate positive selection in regions of the gene associated with allelic specificity determination and purifying selection in previously characterized conserved regions. Further, there is broad convergence between the present and previous studies in specific amino-acid positions inferred to be evolving under positive selection.     

A review of growth and life-history traits of native and non-native European populations of black bullhead <Emphasis Type="Italic">Ameiurus melas</Emphasis>

North American black bullhead, Ameiurus melas, which were introduced to Europe in the nineteenth and twentieth centuries, have received relatively little study. With focus on growth and reproduction, this extensive review, which includes new European data, aims to inform the risk analysis process concerning this non-native species in Europe. Surprisingly, the new data for Europe were more comprehensive than for native populations, with data available mainly from Oklahoma, and North and South Dakota (USA). In terms of relative growth, juvenile A. melas were found to have a relatively uniform body shape regardless of the population’s origin, whereas adults developed different phenotypes depending upon location. Overall growth trajectory was significantly faster for native than for non-native populations. Growth index values decreased significantly with increasing latitude in non-native but not native populations—the latter decreasing weakly with increasing altitude in the populations located at latitudes <40°. Mean general condition (slope ‘b’), mean sex ratio and mean egg diameter did not differ significantly between native and non-native populations. Absolute fecundity was slightly (but not significantly) higher in non-native than native populations. GSI data, which were very scarce for native populations, suggest gonad production may be slightly higher in native than in non-native populations. Precise data on age at maturity (AaM) are lacking for the native range, where 2–5 years is reported. Whereas, in the introduced range the greatest AaM was 3.5 years, and AaM decreases with increasing juvenile growth (TL at age 3). The populations with fastest juvenile growth tended to be from warmer water bodies where they are considered to be invasive. The great growth and life-history plasticity of black bullhead affords the species great potential to invade and establish viable populations in new areas.     

The invasive shrub Buddleja davidii performs better in its introduced range

It is commonly assumed that invasive plants grow more vigorously in their introduced than in their native range, which is then attributed to release from natural enemies or to microevolutionary changes, or both. However, few studies have tested this assumption by comparing the performance of invasive species in their native vs. introduced ranges. Here, we studied abundance, growth, reproduction, and herbivory in 10 native Chinese and 10 invasive German populations of the invasive shrub Buddleja davidii (Scrophulariaceae; butterfly bush). We found strong evidence for increased plant vigour in the introduced range: plants in invasive populations were significantly taller and had thicker stems, larger inflorescences, and heavier seeds than plants in native populations. These differences in plant performance could not be explained by a more benign climate in the introduced range. Since leaf herbivory was substantially reduced in invasive populations, our data rather suggest that escape from natural enemies, associated with increased plant growth and reproduction, contributes to the invasion success of B. davidii in Central Europe.     

Differentiation in the status of self-incompatibility among Calibrachoa species (Solanaceae)

The overall status of self-incompatibility, as assessed by the rate of capsule-set after self-pollination, was investigated in the genus Calibrachoa (Solanaceae). Thirty-two species were surveyed using a total of 655 individuals collected in 102 different native populations in Argentina, Brazil, Mexico, and Uruguay. The rate of capsule-set in 278 voucher specimens collected from the same native habitats was also measured to obtain additional information to assess the degree of self-(in)compatibility. Only one species, Calibrachoa parviflora, was self-compatible (SC, autogamous) and the other 31 species were found to be self-incompatible (SI). A mixed population (SI and SC individuals in the same population) was not found. The differentiation of C. parviflora as an autogamous species is associated with a successful occupation of different (riparian) habitats within a larger range of geographic distribution compared to the rest of the species in the principally SI genus of Calibrachoa.     

Ancestral origins and invasion pathways in a globally invasive bird correlate with climate and influences from bird trade

Invasive species present a major threat to global biodiversity. Understanding genetic patterns and evolutionary processes that reinforce successful establishment is paramount for elucidating mechanisms underlying biological invasions. Among birds, the ring‐necked parakeet (Psittacula krameri) is one of the most successful invasive species, established in over 35 countries. However, little is known about the evolutionary genetic origins of this species and what population genetic signatures tell us about patterns of invasion. We reveal the ancestral origins of populations across the invasive range and explore the potential influence of climate and propagule pressure from the pet trade on observed genetic patterns. Ring‐necked parakeet samples representing the ancestral native range (n = 96) were collected from museum specimens, and modern samples from the invasive range (n = 855) were gathered from across Europe, Mauritius and Seychelles, and sequenced for two mitochondrial DNA markers comprising 868 bp of cytochrome b and control region, and genotyped at 10 microsatellite loci. Invasive populations comprise birds that originate predominantly from Pakistan and northern areas of India. Haplotypes associated with more northerly distribution limits in the ancestral native range were more prevalent in invasive populations in Europe, and the predominance of Asian haplotypes in Europe is consistent with the higher number of Asian birds transported by the pet trade outside the native range. Successful establishment of invasive species is likely to be underpinned by a combination of environmental and anthropogenic influences.     

Niche conservatism among non‐native vertebrates in Europe and North America

Niche conservatism, the hypothesis that niches remain constant through time and space, is crucial for the study of biological invasions as it underlies native‐range based predictions of invasion risk. Niche changes between native and non‐native populations are increasingly reported. However, it has been argued that these changes arise mainly because in their novel range, species occupy only a subset of the environments they inhabit in their native range, and not because they expand into environments entirely novel to them. Here, using occurrences of 29 vertebrate species native to either Europe or North America and introduced into the other continent, we assess the prevalence of niche changes between native and non‐native populations and assess whether the changes detected are caused primarily by native niche unfilling in the non‐native range rather than by expansion into novel environments. We show that niche overlap between native and non‐native populations is generally low because of a large degree of niche unfilling in the non‐native range. This most probably reflects an ongoing colonization of the novel range, as niche changes were smaller for species that were introduced longer ago and into a larger number of locations. Niche expansion was rare, and for the few species exhibiting larger amounts of niche overlap, an unfilling of the niche in the native range (e.g. through competition or dispersal limitations) is the most probable explanation. The fact that for most species, the realized non‐native niche is a subset of the realized native niche allows native‐range based niche models to generate accurate predictions of invasion risk. These results suggest that niche changes arising during biological invasions are strongly influenced by propagule pressure and colonization processes, and we argue that introduction history should be taken into account when evaluating niche conservatism in the context of biological invasions.     

A few north Appalachian populations are the source of European black locust

The role of evolution in biological invasion studies is often overlooked. In order to evaluate the evolutionary mechanisms behind invasiveness, it is crucial to identify the source populations of the introduction. Studies in population genetics were carried out on Robinia pseudoacacia L., a North American tree which is now one of the worst invasive tree species in Europe. We realized large‐scale sampling in both the invasive and native ranges: 63 populations were sampled and 818 individuals were genotyped using 113 SNPs. We identified clonal genotypes in each population and analyzed between and within range population structure, and then, we compared genetic diversity between ranges, enlarging the number of SNPs to mitigate the ascertainment bias. First, we demonstrated that European black locust was introduced from just a limited number of populations located in the Appalachian Mountains, which is in agreement with the historical documents briefly reviewed in this study. Within America, population structure reflected the effects of long‐term processes, whereas in Europe it was largely impacted by human activities. Second, we showed that there is a genetic bottleneck between the ranges with a decrease in allelic richness and total number of alleles in Europe. Lastly, we found more clonality within European populations. Black locust became invasive in Europe despite being introduced from a reduced part of its native distribution. Our results suggest that human activity, such as breeding programs in Europe and the seed trade throughout the introduced range, had a major role in promoting invasion; therefore, the introduction of the missing American genetic cluster to Europe should be avoided.