Hybridization and invasion: one of North America's most devastating invasive plants shows evidence for a history of interspecific hybridization

Genotyping-by-sequencing provides new genetic and taxonomic insights in the critical group of Centaurea tenorei

Centaurea L. is one of the most widespread, differentiated, and critical genera of Asteraceae in the Euro-Mediterranean area, with more than 100 currently recognized species inhabiting the region. The controversial C. tenorei group, narrowly endemic to the Peninsula of Sorrento (Campania region, southern Italy), includes three weakly differentiated microspecies: C. tenorei Guss. ex Lacaita, C. montaltensis (Fiori) Peruzzi and C. lacaitae Peruzzi. However, their taxonomic distinctiveness and relationships with close or sympatric species are still unclear. In particular, the existence in several localities of individuals with intermediate morphology suggests inadequate taxonomic assessment within the group or hybridization and introgression with other species. In this study we aimed at defining population structure in this complex. With this objective, we sampled the three currently accepted species from their loci classici (i.e., the localities in which the taxa were originally described) and from other localities throughout the range, including populations of difficult identification occurring where the ranges of different taxa overlap. We employed a panel of SNPs obtained via genotyping-by-sequencing for investigations on genetic structure, admixture and ploidy inference, the latter also compared with chromosome counts. Our results showed that Centaurea tenorei s.l. is consistently tetraploid, contradicting the current taxonomy that was also based on ploidy level. Population structure analyses indicated the presence of four to seven clusters, most of which with clear evidence of admixture. Furthermore, contrarily to what previously supposed, we demonstrated a remarkable contribution of C. deusta, more that of C. cineraria in the genetic make-up of C. tenorei. However, we found a population of C. cineraria outside its ecological range, probably driven by climate change, which could be responsible in the future of further hybridization phenomena.

A lack of population structure characterizes the invasive Lonicera japonica in West Virginia and across eastern North America

Invasive plant species cause massive ecosystem damage globally, yet represent powerful case studies in population genetics and rapid adaptation to new habitats. The availability of digitized herbarium collections data, and the ubiquity of invasive species across the landscape make them highly accessible for studies of invasion history and population dynamics associated with their introduction, establishment, spread, and ecological interactions. Here we focus on Lonicera japonica, one of the most damaging invasive vine species in North America. We leveraged digitized collections data and contemporary field collections to reconstruct the invasion history and characterize patterns of genomic variation in the eastern USA, using a straightforward method for generating nucleotide polymorphism data and a recently published, chromosome-level genome for the species. We found an overall lack of population structure among sites in northern West Virginia, USA, as well as across sites in the central and eastern USA. Heterozygosity and population differentiation were both low based on Fst, analysis of molecular variance, principal components analysis, and cluster-based analyses. We also found evidence of high inbreeding coefficients and significant linkage disequilibrium, in line with the ability of this otherwise outcrossing, perennial species to propagate vegetatively. Our findings corroborate earlier studies based on allozyme data, and suggest that intentional, human-assisted spread explains the lack of population structure, as this species was planted for erosion control and as an ornamental, escaping cultivation repeatedly across the USA. Finally, we discuss how plant invasion genomics can be incorporated into experiential undergraduate education as a way to integrate teaching and research.

Expression Level Dominance and Homeolog Expression Bias Upon Cold Stress in the F1 Hybrid Between the Invasive Sphagneticola trilobata and the Native S. calendulacea in South China, and Implications for Its Invasiveness

The role of hybridization is significant in biological invasion, and thermotolerance is a trait critical to range expansions. The South American Sphagneticola trilobata is now widespread in South China, threatening the native S. calendulacea by competition and hybridization. Furthermore, upon formation, their F1 hybrid can quickly replace both parents. In this study, the three taxa were used as a model to investigate the consequences of hybridization on cold tolerance, particularly the effect of subgenome dominance in the hybrid. Upon chilling treatments, physiological responses and transcriptome profiles were compared across different temperature points to understand their differential responses to cold. While both parents showed divergent responses, the hybrid’s responses showed an overall resemblance to S. calendulacea, but the contribution of homeolog expression bias to cold stress was not readily evident in the F1 hybrid possibly due to inherent bias that comes with the sampling location. Our findings provided insights into the role of gene expression in differential cold tolerance, and further contribute to predicting the invasive potential of other hybrids between S. trilobata and its congeners around the world.

×Sorbaronia mitschurinii: from an artificially created species to an invasion in Europe: repeating the fate of invasive Amelanchier ×spicata, a review

By intervening in natural events, relocating species to other areas, purposefully hybridizing them, as well as reducing the habitats required for them, humans have created conditions for new hybrid species to emerge. As long as hybrids exist in our gardens and fields, we have no reason to worry. However, problems arise with the expansion of these hybrid species into natural habitats, where such hybrid species cannot always be recognized and remain unnoticed, and in most cases they are often mistaken for natural parent species. Two hybrid species, Amelanchier ×spicata and ×Sorbaronia mitschurinii have historically developed in Europe with different scenarios of origin. It has been suggested in the past that both species are of hybrid origin, and recent molecular studies have confirmed the previous assumptions. There is no doubt that A. ×spicata originated in a natural way of hybridization, when the two parental species came into contact in Europe, but ×S. mitschurinii is a purposefully created species, crossing ×Sorbaronia fallax with Aronia melanocarpa. Produced as a result of different scenarios, these two hybrid species have become invasive in Europe, and ×S. mitschurinii has started to follow in the footsteps of the highly invasive A. ×spicata, spreading in the wild, where it predominantly expands in pine forests and in wetland forests along water bodies and bogs. Moreover, ×S. mitschurinii occupies the same habitats in Europe as one of its parent plants, A. melanocarpa in North America, and this species is a threat to wetland forest habitats. Given that ×S. mitschurinii has long been regarded as one of Aronia species, the exact distribution of the species in Europe is unknown, but the following countries are currently reporting the presence of the species in Europe: Belarus, Belgium, Czech Republic, Estonia, Finland, Latvia, Lithuania, Netherlands, Norway, Poland, Russian Federation, Sweden, Ukraine and United Kingdom. In the light of current knowledge, further studies on ×S. mitschurinii invasion in Europe are needed, as well as the need to correct the information in international databases, such as CABI, NOBANIS, to separate ×S. mitschurinii data from what applies to Aronia taxa.

Adaptive Introgression across Semipermeable Species Boundaries between Local Helicoverpa zea and Invasive Helicoverpa armigera Moths

Hybridization between invasive and native species has raised global concern, given the dramatic increase in species range shifts and pest outbreaks due to anthropogenic dispersal. Nevertheless, secondary contact between sister lineages of local and invasive species provides a natural laboratory to understand the factors that determine introgression and the maintenance or loss of species barriers. Here, we characterize the early evolutionary outcomes following secondary contact between invasive Helicoverpa armigera and native H. zea in Brazil. We carried out whole-genome resequencing of Helicoverpa moths from Brazil in two temporal samples: during the outbreak of H. armigera in 2013 and 2017. There is evidence for a burst of hybridization and widespread introgression from local H. zea into invasive H. armigera coinciding with H. armigera expansion in 2013. However, in H. armigera, the admixture proportion and the length of introgressed blocks were significantly reduced between 2013 and 2017, suggesting selection against admixture. In contrast to the genome-wide pattern, there was striking evidence for adaptive introgression of a single region from the invasive H. armigera into local H. zea, including an insecticide resistance allele that increased in frequency over time. In summary, despite extensive gene flow after secondary contact, the species boundaries are largely maintained except for the single introgressed region containing the insecticide-resistant locus. We document the worst-case scenario for an invasive species, in which there are now two pest species instead of one, and the native species has acquired resistance to pyrethroid insecticides through introgression.

Islands as a crossroad of evolutionary lineages: A case study of Centaurea sect. Centaurea (Compositae) from Sardinia (Mediterranean Basin)

The Mediterranean Basin is a biodiversity hotspot, where islands play a key role because of their high biological diversity, degree of endemicity and human pressure. One of these islands, Sardinia, is a good evolutionary laboratory, especially for the study of complex genera, such as Centaurea. In particular, endemic species of Centaurea sect. Centaurea from Sardinia provides an interesting case study of plant evolution on continental islands. We attempted to clarify the processes leading to the diversification of Centaurea species on Sardinia using bi-parentally inherited nuclear markers and maternally inherited plastid markers. Our plastid results revealed the presence of five lineages of sect. Centaurea on the island. Three of them were defined as three species: C. ferulacea, C. filiformis and C. horrida. The other two lineages highlighted the complex evolutionary history of the two polyploids C. corensis and C. magistrorum. Multiple colonization events from the mainland involving the C. deusta and C. paniculata lineages among others, have led to the diversity of sect. Centaurea on Sardinia. One colonization event likely followed a southern path via the land connection between the mainland, the Calabrian Plate and Sardinia. A second pathway likely followed a northern connection, probably through the Tuscan Archipelago. Implications of these findings on conservation efforts for Centaurea endemics on Sardinia are also discussed.

Associations between genomic ancestry, genome size and capitula morphology in the invasive meadow knapweed hybrid complex (Centaurea × moncktonii) in eastern North America

Plant invasions are prime opportunities for studying hybridization and the nature of species boundaries, but hybrids also complicate the taxonomic treatment and management of introduced taxa. In this study, we use population genomics to estimate the extent of genomic admixture and test for its association with morphology and genome size in a hybrid complex of knapweeds invasive to North America: meadow knapweed (Centaurea × moncktonii) and its parental species (C. jacea and C. nigra). We sampled 20 populations from New York and Vermont, USA, and used genotyping by sequencing to identify single nucleotide polymorphisms in order to estimate genome-wide ancestry and classify individuals into hybrid genotype classes. We then tested for association between degree of genomic introgression and variation in a subset of traits diagnostic for the parental taxa, namely capitula morphology and monoploid genome size. Genomic clustering revealed two clearly defined lineages, as well as many admixed individuals forming a continuous gradation of introgression. Individual assignments to hybrid genotype classes revealed many advanced generation intercrosses and backcrosses, suggesting introgression has been extensive and unimpeded by strong reproductive barriers between taxa. Variation in capitula traits between the two unadmixed, presumed parental, lineages exhibited continuous, and in some cases transgressive, segregation among introgressed hybrids. Genome size was also divergent between lineages, although advanced generation hybrids had smaller genomes relative to additive expectations. Our study demonstrates deep introgression between the porous genomes of a hybrid invasive species complex. In addition to strong associations among genomic ancestry, genome size and morphology, hybrids expressed more extreme phenotypic values for capitula traits and genome size, indicating transgressive segregation, as well as a bias towards smaller genomes, possibly due to genomic downsizing. Future studies will apply these results to experimentally test how introgression, transgressive segregation and genome size reduction interact to confer invasiveness.

The Occurrence of Seedlessness in Higher Plants; Insights on Roles and Mechanisms of Parthenocarpy

Parthenocarpy in a broad sense includes those processes that allow the production of seedless fruits. Such fruits are favorable to growers, because they are set independently of successful pollination, and to processors and consumers, because they are easier to deal with and to eat. Seedless fruits however represent a biological paradox because they do not contribute to offspring production. In this work, the occurrence of parthenocarpy in Angiosperms was investigated by conducting a bibliographic survey. We distinguished monospermic (single seeded) from plurispermic (multiseeded) species and wild from cultivated taxa. Out of 96 seedless taxa, 66% belonged to plurispermic species. Of these, cultivated species were represented six times higher than wild species, suggesting a selective pressure for parthenocarpy during domestication and breeding. In monospermic taxa, wild and cultivated species were similarly represented. The occurrence of parthenocarpy in wild species suggests that seedlessness may have an adaptive role. In monospermic species, seedless fruits are proposed to reduce seed predation through deceptive mechanisms. In plurispermic fruit species, parthenocarpy may exert an adaptive advantage under suboptimal pollination regimes, when too few embryos are formed to support fruit growth. In this situation, parthenocarpy offers the opportunity to accomplish the production and dispersal of few seeds, thus representing a selective advantage. Approximately 20 sources of seedlessness have been described in tomato. Excluding the EMS induced mutation parthenocarpic fruit (pat), the parthenocarpic phenotype always emerged in biparental populations derived from wide crosses between cultivated tomato and wild relatives. Following a theory postulated for apomictic species, we argument that wide hybridization could also be the force driving parthenocarpy, following the disruption of synchrony in time and space of reproductive developmental events, from sporogenesis to fruit development. The high occurrence of polyploidy among parthenocarpic species supported this suggestion. Other commonalities between apomixis and parthenocarpy emerged from genetic and molecular studies of the two phenomena. Such insights may improve the understanding of the mechanisms underlying these two reproductive variants of great importance to modern breeding.

Genetic structure reveals a history of multiple independent origins followed by admixture in the allopolyploid weed Salsola ryanii

It has recently become clear that many invasive species have evolved in situ via hybridization or polyploidy from progenitors which themselves are introduced species. For species formed by hybridization or polyploidy, genetic diversity within the newly formed species is influenced by the number of independent evolutionary origins of the species. For recently formed species, an analysis of genetic structure can provide insight into the number of independent origin events involved in the formation of the species. For a putative invasive allopolyploid species, the number of origins involved in the species formation, the genetic diversity present within these origins, and the level of gene flow between independent origins determines the genetic composition of the neospecies. Here we analyze the genetic structure of the newly formed allopolyploid species, Salsola ryanii, a tumbleweed which evolved within the last 20–100 years in California. We utilize the genetic structure analysis to determine that this new species is the result of at least three independent allopolyplodization events followed by gene flow between the descendants of independent origins.

Shift in precipitation regime promotes interspecific hybridization of introduced Coffea species

The frequency of plant species introductions has increased in a highly connected world, modifying species distribution patterns to include areas outside their natural ranges. These introductions provide the opportunity to gain new insight into the importance of flowering phenology as a component of adaptation to a new environment. Three Coffea species, C. arabica, C. canephora (Robusta), and C. liberica, native to intertropical Africa have been introduced to New Caledonia. On this archipelago, a secondary contact zone has been characterized where these species coexist, persist, and hybridize spontaneously. We investigated the impact of environmental changes undergone by each species following its introduction in New Caledonia on flowering phenology and overcoming reproductive barriers between sister species. We developed species distribution models and compared both environmental envelopes and climatic niches between native and introduced hybrid zones. Flowering phenology was monitored in a population in the hybrid zone along with temperature and precipitation sequences recorded at a nearby weather station. The extent and nature of hybridization events were characterized using chloroplast and nuclear microsatellite markers. The three Coffea species encountered weak environmental suitability compared to their native ranges when introduced to New Caledonia, especially C. arabica and C. canephora. The niche of the New Caledonia hybrid zone was significantly different from all three species' native niches based on identity tests (I Similarity and D Schoener's Similarity Indexes). This area appeared to exhibit intermediate conditions between the native conditions of the three species for temperature-related variables and divergent conditions for precipitation-related ones. Flowering pattern in these Coffea species was shown to have a strong genetic component that determined the time between the triggering rain and anthesis (flower opening), specific to each species. However, a precipitation regime different from those in Africa was directly involved in generating partial flowering overlap between species and thus in allowing hybridization and interspecific gene flow. Interspecific hybrids accounted for 4% of the mature individuals in the sympatric population and occurred between each pair of species with various level of introgression. Adaptation to new environmental conditions following introduction of Coffea species to New Caledonia has resulted in a secondary contact between three related species, which would not have happened in their native ranges, leading to hybridization and gene flow.

Adaptive plasticity and niche expansion in an invasive thistle

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

A rare case of a natural contact zone in Morocco between an autopolyploid and an allopolyploid of Centaurea aspera with sterile tetraploid hybrids

A new contact zone between Centaurea aspera and Centaurea seridis was found in Morocco. Chromosome counts and flow cytometry showed that both taxa were tetraploid (4x = 44). A literature review and morphometric analysis established that C. aspera corresponds to the autopolyploid C. aspera subsp. gentilii and C. seridis corresponds to the allopolyploid C. seridis var. auriculata. This contact area was compared with the homologous contact zones in Spain formed by the diploid C. aspera subsp. stenophylla and the tetraploid C. seridis subsp. maritima. Natural hybrids between parental species were frequent in both areas. In Spain, hybrids were triploid (from reduced gametes A and gamete AB), highly sterile and exerted a 'triploid block'. In Morocco, cytometry showed that hybrids were tetraploid and, therefore, probably fertile, but all the capitula lacked achenes. It is likely that the resulting genome of the new tetraploid hybrid (AAAB), through the fusion of reduced gametes AA (from subsp. gentilii) and AB (from var. auriculata), could explain irregularities in meiosis through formation of aneuploid gametes and, therefore, infertility of the hybrid. Moroccan sterile tetraploid hybrids develop, but have the identical irregularities to Spanish triploids, probably due to the odd number of homologous chromosomes. The new hybrid is first described as C. x subdecurrens nothosubsp. paucispinus. In addition, distribution and ecological traits are analysed.

Comparative genomics in the Asteraceae reveals little evidence for parallel evolutionary change in invasive taxa

Asteraceae, the largest family of flowering plants, has given rise to many notorious invasive species. Using publicly available transcriptome assemblies from 35 Asteraceae, including six major invasive species, we examined evidence for micro- and macro-evolutionary genomic changes associated with invasion. To detect episodes of positive selection repeated across multiple introductions, we conducted comparisons between native and introduced genotypes from six focal species and identified genes with elevated rates of amino acid change (dN/dS). We then looked for evidence of positive selection at a broader phylogenetic scale across all taxa. As invasive species may experience founder events during colonization and spread, we also looked for evidence of increased genetic load in introduced genotypes. We rarely found evidence for parallel changes in orthologous genes in the intraspecific comparisons, but in some cases we identified changes in members of the same gene family. Using among-species comparisons, we detected positive selection in 0.003-0.69% and 2.4-7.8% of the genes using site and stochastic branch-site models, respectively. These genes had diverse putative functions, including defence response, stress response and herbicide resistance, although there was no clear pattern in the GO terms. There was no indication that introduced genotypes have a higher proportion of deleterious alleles than native genotypes in the six focal species, suggesting multiple introductions and admixture mitigated the impact of drift. Our findings provide little evidence for common genomic responses in invasive taxa of the Asteraceae and hence suggest that multiple evolutionary pathways may lead to adaptation during introduction and spread in these species.

Rapid evolution of an invasive weed

Trade-offs between performance and the ability to tolerate abiotic and biotic stress have been suggested to explain both the success of invasive species and phenotypic differentiation between native and invasive populations. It is critical to sample broadly across both ranges and to account for latitudinal clines and maternal effects when testing this premise. Wild-collected Centaurea diffusa seeds were grown in benign and stressful conditions (drought, flooding, nutrient stress and simulated herbivory), to evaluate whether native and introduced individuals differ in performance or life history phenotypes. A second experiment used glasshouse-grown seeds to evaluate whether patterns remain comparable when maternal environment is consistent. Many traits differed between ranges, and in all cases but one, invasive individuals grew larger, performed better, or matured later. No trade-off in performance with herbivore defense was evident. Invasive populations may have been released from a trade-off between growth and drought tolerance apparent in the native range. Larger individuals with delayed maturity and greater reproductive potential have evolved in invasive populations, a pattern evident across broad population sampling, and after latitude and maternal environment were considered. Release from abiotic stress tolerance trade-offs may be important for the invasion success of Centaurea diffusa.

Hybridization of an invasive shrub affects tolerance and resistance to defoliation by a biological control agent

Evolution has contributed to the successful invasion of exotic plant species in their introduced ranges, but how evolution affects particular control strategies is still under evaluation. For instance, classical biological control, a common strategy involving the utilization of highly specific natural enemies to control exotic pests, may be negatively affected by host hybridization because of shifts in plant traits, such as root allocation or chemical constituents. We investigated introgression between two parent species of the invasive shrub tamarisk (Tamarix spp.) in the western United States, and how differences in plant traits affect interactions with a biological control agent. Introgression varied strongly with latitude of origin and was highly correlated with plant performance. Increased levels of T. ramosissima introgression resulted in both higher investment in roots and tolerance to defoliation and less resistance to insect attack. Because tamarisk hybridization occurs predictably on the western U.S. landscape, managers may be able to exploit this information to maximize control efforts. Genetic differentiation in plant traits in this system underpins the importance of plant hybridization and may explain why some biological control releases are more successful than others.

Genomics of Compositae weeds: EST libraries, microarrays, and evidence of introgression

PREMISE OF STUDY

Weeds cause considerable environmental and economic damage. However, genomic characterization of weeds has lagged behind that of model plants and crop species. Here we describe the development of genomic tools and resources for 11 weeds from the Compositae family that will serve as a basis for subsequent population and comparative genomic analyses. Because hybridization has been suggested as a stimulus for the evolution of invasiveness, we also analyze these genomic data for evidence of hybridization.

METHODS

We generated 22 expressed sequence tag (EST) libraries for the 11 targeted weeds using Sanger, 454, and Illumina sequencing, compared the coverage and quality of sequence assemblies, and developed NimbleGen microarrays for expression analyses in five taxa. When possible, we also compared the distributions of Ks values between orthologs of congeneric taxa to detect and quantify hybridization and introgression.

RESULTS

Gene discovery was enhanced by sequencing from multiple tissues, normalization of cDNA libraries, and especially greater sequencing depth. However, assemblies from short sequence reads sometimes failed to resolve close paralogs. Substantial introgression was detected in Centaurea and Helianthus, but not in Ambrosia and Lactuca.

CONCLUSIONS

Transcriptome sequencing using next-generation platforms has greatly reduced the cost of genomic studies of nonmodel organisms, and the ESTs and microarrays reported here will accelerate evolutionary and molecular investigations of Compositae weeds. Our study also shows how ortholog comparisons can be used to approximately estimate the genome-wide extent of introgression and to identify genes that have been exchanged between hybridizing taxa.

Allopolyploid origin of highly invasive Centaurea stoebe s.l. (Asteraceae)

Spotted knapweed (Centaurea stoebe) occurs from Western Asia to Western Europe both as diploid and tetraploid cytotypes, predominantly in single-cytotype populations with higher frequency of diploid populations. Interestingly, only tetraploids have been recorded so far from its introduced range in North America where they became highly invasive. We performed phylogenetic and network analyses of more than 40 accessions of the C. stoebe and C. paniculata groups and other related taxa using cloned internal transcribed spacer (ITS) and sequences of the chloroplast trnT-trnL and atpBrbcL regions to (i) assess the evolutionary origin of tetraploid C. stoebe s.l., and (ii) uncover the phylogeny of the C. stoebe group. Both issues have not been studied so far and thus remained controversial. Cloned ITS sequences showed the presence of two slightly divergent ribotypes occurring in tetraploid cytotype, while only one major ribotype was present in diploid C. stoebe s.str. This pattern suggests an allopolyploid origin of tetraploids with contribution of the diploid C. stoebe s.str. genome. Although we were not able to detect the second parental taxon, we hypothesize that hybridization might have triggered important changes in morphology and life history traits, which in turn may explain the colonization success of the tetraploid taxon. Bayesian relaxed clock estimations indicate a relatively recent--Pleistocene origin of the tetraploid C. stoebe s.l. Furthermore, our analyses showed a deep split between the C. paniculata and C. stoebe groups, and a young diversification of the taxa within the C. stoebe group. In contrast to nrDNA analyses, the observed pattern based on two cpDNA regions was inconclusive with respect to the origin and phylogeny of the studied taxa, most likely due to shared ancient polymorphism and frequent homoplasies.

Hybridization and invasion: an experimental test with diffuse knapweed (Centaurea diffusa Lam.)

A number of studies have suggested a link between hybridization and invasion. In this study, we experimentally test the potential for hybridization to influence invasion through a greenhouse common garden study. Diffuse knapweed (DK) (Centaurea diffusa Lam.) was introduced to North America with admixture from spotted knapweed (SK) (Centaurea stoebe subsp. stoebe L.). Comparisons between North American DK (including hybrid phenotypes) and native (European) DK in a common garden did not reveal enhanced performance or increased phenotypic variance, suggesting that pre-introduction hybridization or, more generally, post-introduction evolutionary change has not significantly contributed to the invasion of DK. In contrast, early generation hybrids [artificially created Backcross 1 (BC1) plants] exhibited increased variance for eight of the examined traits, and greater leaf and reproductive shoot production when compared to North American DK. Individual BC1 lines differed for several traits, suggesting the importance of the cross for drawing conclusions from such comparisons. When compared to the parental species (DK and SK), the BC1 plants were not transgressive for any of the measured traits. Overall, these findings suggest that if diploid SK is introduced to North America, interspecific hybridization has the potential to result in even more aggressive invaders.

Species introduction promotes hybridization and introgression in Coregonus: is there sign of selection against hybrids?

Species introductions are considered one of the major drivers of biodiversity loss via ecological interactions and genetic admixture with local fauna. We examined two well-recognized fish species, native whitefish (Coregonus lavaretus) and introduced vendace (Coregonus albula), as well as their morphological hybrids in a single lake to test for selection against hybrids and backcrosses in the wild. A representative random subsample of 693 individuals (27.8%) was taken from the total catch of coregonids. This subsample was examined with the aim to select c. 50 individuals of pure whitefish (n = 52), pure vendace (n = 55) and putative hybrid (n = 19) for genetic analyses. The subsequent microsatellites and mitochondrial (mt) DNA analyses provided compelling evidence of hybridization and introgression. Of the 126 fish examined, four were found to be F(1) , 14 backcrosses to whitefish and seven backcrosses to vendace. The estimates of historical gene flow suggested higher rates from introduced vendace into native whitefish than vice versa, whereas estimates of contemporary gene flow were equal. Mitochondrial introgression was skewed, with 18 backcrosses having vendace mtDNA and only three with whitefish mtDNA. Hybrids and backcrosses had intermediate morphology and niche utilization compared with parental species. No evidence of selection against hybrids or backcrosses was apparent, as both hybrid and backcross growth rates and fecundities were high. Hybrids (F(1) ) were only detected in 2 year-classes, suggesting temporal variability in mating between vendace and whitefish. However, our data show that hybrids reached sexual maturity and reproduced actively, with backcrosses recorded from six consecutive year-classes, whereas no F(2) individuals were found. The results indicate widespread introgression, as 10.8% of coregonids were estimated to be backcrosses.

Amplified fragment length polymorphism and sequence analyses reveal massive gene introgression from the European fungal pathogen Heterobasidion annosum into its introduced congener H. irregulare

The paucity of fungal species known to be currently hybridizing has significantly hindered our understanding of the mechanisms driving gene introgression in these eukaryotic microbes. Here, we describe an area of hybridization and gene introgression between the invasive plant pathogen Heterobasidion irregulare (introduced from North America) and the native H. annosum in Italy. A STRUCTURE analysis of amplified fragment length polymorphism data for 267 individuals identified gene introgression in 8-42% of genotypes in the invasion area, depending on site. Data indicate that introgression is mostly occurring unilaterally from the native to the invasive species and is responsible for 5-45% of genomes in admixed individuals. Sequence analysis of 11 randomly selected and unlinked loci for 30 individuals identified introgression at every locus, thus confirming interspecific gene flow involves a large number of loci. In 37 cases, we documented movement of entire alleles between the two species, but in 7 cases, we also documented the creation of new alleles through intralocus recombination. Sequence analysis did not identify enrichment of either transcriptionally different nonsynonymous alleles or of transcriptionally identical synonymous alleles. These findings may suggest introgression is occurring randomly for extant alleles without an obvious enrichment process driven by selection. However, further studies are needed to ensure selection is not at work elsewhere in the genome.

Can things get worse when an invasive species hybridizes? The harlequin ladybird Harmonia axyridis in France as a case study

So far, only a few studies have explicitly investigated the consequences of admixture for the adaptative potential of invasive populations. We addressed this question in the invasive ladybird Harmonia axyridis. After decades of use as a biological control agent against aphids in Europe and North America, H. axyridis recently became invasive in four continents and has now spread widely in Europe. Despite this invasion, a flightless strain is still sold as a biological control agent in Europe. However, crosses between flightless and invasive individuals yield individuals able to fly, as the flightless phenotype is caused by a single recessive mutation. We investigated the potential consequences of admixture between invasive and flightless biological control individuals on the invasion in France. We used three complementary approaches: (i) population genetics, (ii) a mate-choice experiment, and (iii) a quantitative genetics experiment. The invasive French population and the biological control strain showed substantial genetic differentiation, but there are no reproductive barriers between the two. Hybrids displayed a shorter development time, a larger size and a higher genetic variance for survival in starvation conditions than invasive individuals. We discuss the potential consequences of our results with respect to the invasion of H. axyridis in Europe.