Preadaptation and post‐introduction evolution facilitate the invasion of Phragmites australis in North America

Compared with non‐invasive species, invasive plant species may benefit from certain advantageous traits, for example, higher photosynthesis capacity and resource/energy‐use efficiency. These traits can be preadapted prior to introduction, but can also be acquired through evolution following introduction to the new range. Disentangling the origins of these advantageous traits is a fundamental and emerging question in invasion ecology. We conducted a multiple comparative experiment under identical environmental condition with the invasive haplotype M lineage of the wetland grass Phragmites australis and compared the ecophysiological traits of this invasive haplotype M in North America with those of the European ancestor and the conspecific North American native haplotype E lineage, P. australis ssp. americanus. The invasive haplotype M differed significantly from the native North American conspecific haplotype E in several ecophysiological and morphological traits, and the European haplotype M had a more efficient photosynthetic apparatus than the native North American P. australis ssp. americanus. Within the haplotype M lineage, the introduced North American P. australis exhibited different biomass allocation patterns and resource/energy‐use strategies compared to its European ancestor group. A discriminant analysis of principal components separated the haplotype M and the haplotype E lineages completely along the first canonical axis, highly related to photosynthetic gas‐exchange parameters, photosynthetic energy‐use efficiency and payback time. The second canonical axis, highly related to photosynthetic nitrogen use efficiency and construction costs, significantly separated the introduced P. australis in North America from its European ancestor. Synthesis. We conclude that the European P. australis lineage was preadapted to be invasive prior to its introduction, and that the invasion in North America is further stimulated by rapid post‐introduction evolution in several advantageous traits. The multicomparison approach used in this study could be an effective approach for distinguishing preadaptation and post‐introduction evolution of invasive species. Further research is needed to link the observed changes in invasive traits to the genetic variation and the interaction with the environment.     

Phylogenetic analyses of <Emphasis Type="Italic">Phragmites</Emphasis> spp. in southwest China identified two lineages and their hybrids

The species/lineage delimitation and possible hybridization/introgression are prerequisites in the management of invasive organism. Phragmites australis invaded diverse habitats and displaced the native lineages in North America as a consequence of the introduction from the Eurasia. Such species threatened the biodiversity safety of the invaded regions, in particular the biodiversity hot spots. Southwest (SW) China is a biodiversity hot spot with the occurrence of Phragmites species, both native and introduced. However, the genetic identity of Phragmites species in this biodiversity hot spot remains unclear, hampering effective ecological managements. In this study, we explored the phylogenetic lineages of Phragmites species in SW China. A total of 44 accessions sampled across SW China were analyzed using two chloroplast DNA (cpDNA) markers and amplified fragment length polymorphisms. Two genetic lineages were recovered, i.e., (1) the tropical lineage which primarily consisted of native Phragmites species represented by cpDNA haplotypes I, Q, and U in relatively low altitude and (2) the common lineage including native species at higher elevations in the Hengduan Mountains as well as artificially planted species represented by cpDNA haplotype P. The between-lineage hybridization was suggested for five analyzed accessions collected from either natural or artificial habitats. The putative hybrids might have originated from the maternal native tropical lineages and paternal introduced common lineage. Our results suggest the likelihood of introgressive hybridizations in SW China and thus provided implications for future research and ecological management.     

Molecular data provide strong evidence of natural hybridization between native and introduced lineages of <Emphasis Type="Italic">Phragmites australis</Emphasis> in North America

Hybridization, both within and between taxa, can be an important evolutionary stimulus for bioinvasions. Novel intra-taxon hybridizations may arise either between formerly allopatric introduced lineages, or between native and introduced lineages. The latter can occur following a cryptic invasion of a non-native lineage, such as the nineteenth century introduction to North America of a European lineage of the common reed Phragmites australis. Previous studies found no evidence of natural hybridization between native and introduced lineages of P. australis, but produced some F₁ hybrids under experimental conditions when the seed parent was native and the pollen parent was introduced. In this study we used microsatellite data to compare genotypes of P. australis along a transect of approximately 2,000 km in eastern North America. Although hybridization appears uncommon, simulations and principle component analysis of genetic data provided strong evidence for natural hybridization at two sites adjacent to Lake Erie in which native and introduced lineages were sympatric. The seed parent was the native lineage in some cases, and the introduced lineage in other cases. There is now the potential for P. australis hybrids to become increasingly invasive, and managers should consider as a priority the removal of introduced stands from sites where they co-exist with native stands.     

Remnant native <Emphasis Type="Italic">Phragmites australis</Emphasis> maintains genetic diversity despite multiple threats

Over the past century, an increasing number of species have been negatively impacted by anthropogenic factors such as habitat disturbance and introduced species. One such plant, Phragmites australis subsp. americanus is a perennial emergent grass found in tidal and inland marshes of the Atlantic coast of the United States. While rarely dominant, it grows in mixed communities and across much of this area its distribution has been reduced dramatically, likely due to eutrophication and the invasion of conspecific P. australis introduced from Europe. In this study, two noncoding cpDNA markers and six microsatellite loci were used to characterize genetic diversity among 58 remnant native P. australis stands from North Carolina to Maine. Five chloroplast DNA haplotypes were identified along with 42 multilocus genotypes. Bayesian exploration detected no population structure (e.g., optimal K = 1), indicating that these individuals form a single population. The analysis also detected no presence of hybrids of native and introduced P. australis in the samples, despite the close proximity of individuals to each other in many cases. These results suggest that the genetic composition of native P. australis across the region remains homogeneous and pure, providing managers with justification for its conservation and a potentially large source of germplasm for use in restoration activities.     

Vigna mungo, V. radiata and V. unguiculata plants sampled in different agronomical–ecological–climatic regions of India are nodulated by Bradyrhizobium yuanmingense

Vigna mungo, Vigna radiata and Vigna unguiculata are important legume crops cultivated in India, but little is known about the genetic resources in native rhizobia that nodulate these species. To identify these bacteria, a core collection of 76 slow-growing isolates was built from root nodules of V. mungo, V. radiata and V. unguiculata plants grown at different sites within three agro-ecological-climatic regions of India. The genetic diversity of the bacterial collection was assessed by restriction fragment length polymorphism (RFLP) analysis of PCR-amplified DNA fragments of the 16S–23S rDNA intergenic spacer (IGS) region, and the symbiotic genes nifH and nodC. One rDNA IGS type grouped 91% of isolates, but more diversity was found at the symbiotic loci (17 symbiotic genotypes). Overall, no host plant specificity was shown, the three host plant species sharing common bradyrhizobial genotypes that represented 62% of the collection. Similarly, the predominant genotypes were found at most sampling sites and in all agro-ecological-climatic regions. Phylogenies inferred from IGS sequencing and multi-locus sequence analysis of the dnaK, glnII and recA genes indicated that all isolates but one were clustered with the Bradyrhizobium yuanmingense species. The nifH phylogeny also grouped the different nif haplotypes within a cluster including B. yuanmingense, except for one infrequent nif haplotype which formed a new lineage within the Bradyrhizobium genus. These results may reflect a long history of co-evolution between B. yuanmingense and Vigna spp. in India, while intra-species polymorphism detected in the symbiotic loci may be linked with the long history of diversification of B. yuanmingense coinciding with that of its host legumes.     

Jack-and-Master Trait Responses to Elevated CO2 and N: A Comparison of Native and Introduced Phragmites australis

Global change is predicted to promote plant invasions world-wide, reducing biodiversity and ecosystem function. Phenotypic plasticity may influence the ability of introduced plant species to invade and dominate extant communities. However, interpreting differences in plasticity can be confounded by phylogenetic differences in morphology and physiology. Here we present a novel case investigating the role of fitness trait values and phenotypic plasticity to global change factors between conspecific lineages of Phragmites australis. We hypothesized that due to observed differences in the competitive success of North American-native and Eurasian-introduced P. australis genotypes, Eurasian-introduced P. australis would exhibit greater fitness in response to global change factors. Plasticity and plant performance to ambient and predicted levels of carbon dioxide and nitrogen pollution were investigated to understand how invasion pressure may change in North America under a realistic global change scenario. We found that the introduced Eurasian genotype expressed greater mean trait values in nearly every ecophysiological trait measured – aboveground and belowground – to elevated CO₂ and nitrogen, outperforming the native North American conspecific by a factor of two to three under every global change scenario. This response is consistent with “jack and master” phenotypic plasticity. We suggest that differences in plant nitrogen productivity, specific leaf area, belowground biomass allocation, and inherently higher relative growth rate are the plant traits that may enhance invasion of Eurasian Phragmites in North America. Given the high degree of genotypic variability within this species, and our limited number of genotypes, our results must be interpreted cautiously. Our study is the first to demonstrate the potential importance of jack-and-master phenotypic plasticity in plant invasions when facing imminent global change conditions. We suggest that jack-and-master invasive genotypes and/or species similar to introduced P. australis will have an increased ecological fitness, facilitating their invasion in both stressful and resource rich environments.     

Population genetic structure of three tree species in the mangrove genus Ceriops (Rhizophoraceae) from the Indo West Pacific

Ceriops is a viviparous mangrove with widespread species Ceriops decandra and C. tagal, and an endemic species C. australis. Genetic diversity of the three species was screened in 30 populations collected from 23 locations in the Indo West Pacific (IWP) using Inter-simple sequence repeats (ISSR) and sequences of partial nuclear gene (G3pdh) and chloroplast DNA (trnV-trnM). At the species level, the total gene diversity (Ht) revealed by ISSRs was 0.270, 0.118, and 0.089 in C. decandra, C. tagal, and C. australis, respectively. A total of six haplotypes of G3pdh and five haplotypes of trnV-trnM were recognized among the three species. Only C. decandra was detected containing more than one haplotype from each sequence data set (four G3pdh haplotypes and three trnV-trnM haplotypes). At the population level, genetic diversity of Ceriops was relatively low inferred from ISSRs (He = 0.028, 0.023, and 0.053 in C. decandra, C. tagal, and C. australis, respectively). No haplotype diversity within population was detected from any of the three species. Cluster analysis based on ISSRs identified three major geographical groups in correspond to the East Indian Ocean (EIO), South China Sea (SCS), and North Australia (NA) in both C. decandra and C. tagal. The cladogram from DNA sequences also detected the same three geographical groups in C. decandra. Analysis of molecular variance (AMOVA) revealed that most of the total variation was accounted for by differentiation between the three major geographical regions of both C. decandra and C. tagal. The significant genetic structure may result from the geological events in these regions during the recent Pleistocene glaciations. This study also provided insights into the phylogenetics of Ceriops. Yelin Huang and Fengxiao Tan contributed equally to this work.     

Genetic variation in photosynthetic characteristics among invasive and native populations of reed canarygrass (Phalaris arundinacea)

With the extensive spread of invasive species throughout North America and Europe there is an urgent need to better understand the morphological and physiological characteristics of successful invasive plants and the evolutionary mechanisms that allow introduced species to become invasive. Most ecological studies have focused on morphological differences and changes in community dynamics, and physiological studies have typically explored the differences between native and invasive species. In this study, 15 different genotypes of Phalaris arundinacea from both its native (European) and invasive (North American) range were grown in a common garden experiment to monitor the physiological differences between native and invasive genotypes. Here we present data that suggests high variability exists in the physiological traits among genotypes of P. arundinacea, yet genotypes from the native range are not necessarily physiologically inferior to the hybridized invasive genotypes. Previous work has shown that multiple introductions of P. arundinacea from various European locations to the United States resulted in numerous hybridization events, yielding more genetic variability and phenotypic plasticity in the invasive range. Of the genotypes studied, both morphological and physiological traits of genotypes with French origin were significantly different from the plants from the Czech Republic, North Carolina, and Vermont. The lack of clear differences between native and invasive genotypes indicates that physiological traits may be highly conserved in P. arundinacea and enhanced photosynthetic rates are not indicative of successful invasive genotypes. Instead, morphological traits and defensive secondary compound metabolism may play a more important role in the success of P. arundinacea within its invasive range, and patterns of genetic variation in physiological traits between invasive and native range may be more important than the mean traits of each region when explaining reed canarygrass’ invasive potential in North America.     

When invasion increases population genetic structure: a study with <Emphasis Type="Italic">Centaurea diffusa</Emphasis>

Biological invasions offer excellent systems to study the evolutionary processes involved in introductions of species to new ranges. Molecular markers can reveal invasion histories and the effects of introductions on amounts and structuring of genetic variation. We used five polymorphic microsatellite loci to elucidate genetic diversity and population structure between native range and introduced range populations of a prominent North American rangeland weed, Centaurea diffusa (Asteraceae). We found that the total number of alleles and the number of private alleles was slightly higher in the native Eurasian range, and that allelic richness did not differ between the ranges, indicating overall levels of diversity were similar in Eurasia and North America. It therefore seems unlikely that this invasion has been affected by genetic bottlenecks or founder effects. Indeed, results of assignment tests suggest that multiple introductions have contributed to North America’s C. diffusa invasion. Additionally, assignment tests show that both Eurasian and North American sites had a strong pattern of mixed genetic ancestry. This mixed assignment corresponded to a lack of geographic population structure among Eurasian samples. The lack of population structure in the native range conflicts with general expectations and findings to date for invasion genetics, and cautions that even species’ native ranges may show signs of recent ecological upheaval. Despite the mixed assignments, North American samples showed strong population structure, suggesting that the invasion has been characterized by long-range dispersal of genetically distinct propagules across the introduced range.     

Low Phylogeographic Structure in a Wide Spread Endangered Australian Frog <Emphasis Type="Italic">Litoria aurea</Emphasis> (Anura: Hylidae)

The green and golden bell frog (Litoria aurea) has a widespread distribution along the south-east coast of Australia. The species range, however, is highly fragmented and remaining populations are predominately isolated and restricted to the coastline. Previously, the range extended further inland and the species was considered common. Here we report a study designed to identify the phylogeographic and conservation genetic parameters of L. aurea. Mitochondrial DNA sequences were examined from 263 individuals sampled from 26 locations using both phylogenetic and population analyses. Despite a general consensus that amphibians are highly structured we found no phylogeographic divisions within the species, however, there was significant structure amongst extant populations (F _ST=0.385). Patterns of haplotype relatedness, high haplotypic diversity (mean h=0.547) relative to low nucleotide diversity (mean π=0.003) and mismatch distribution analysis supported a Pleistocene expansion hypothesis with continued restricted dispersal and gene flow. We conclude that the genetic structure of the species may permit ‘well managed’ intervention to mediate gene flow amongst isolated populations and provide some guidelines for the implementation of such conservation strategies.     

Blocking Phragmites australis reinvasion of restored marshes using plants selected from wild populations and tissue culture

This study tested a vegetation strategy for controlling Phragmites australis invasion into brackish marshes as an alternative to the current technique of repeated herbicide sprays followed by burning. This strategy involves blocking P. australis by planting desired plants selected from wild populations and/or tissue culture regenerants at key points on the major routes of P. australis invasion. The planting of native species was conducted at three sites in a herbicide-treated P. australis marsh near Salem, NJ. Wild population selections of three upland marsh shrubs, Myrica cerifera, Baccharis halimifolia, and Iva frutescens, as well as two grass species, Spartina alterniflora and Spartina patens, and two rushes, Juncus gerardi and Juncus roemerianus, were planted according to their normal zonation positions. Tissue culture regenerated plants of the two grasses and two rushes, and the sedge species Scirpus robustus, were also planted. Plant growth at each site was monitored each year after planting for up to 3 years. Most plants of B. halimifolia, I. frutescens, J. roemerianus, and S. patens demonstrated a consistent vigorous growth at all three sites, whether or not the plants were collected from wild populations or were tissue culture regenerants. These multi-layered walls of plants demonstrated effectiveness in controlling the P. australis by restricting or inhibiting its spread. Upon screening 48 regenerated plants of S. patens at one of the three sites, we found that some regenerants showed enhanced characteristics for blocking P. australis, such as greater expansion and a high stem density. The availability of the tissue culture-regenerated plants of the native marsh species makes it possible to select lines from local genotypes that have desirable characteristics for wetland restoration projects, such as blocking P. australis reinvasion.     

Nutrient enrichment enhances hidden differences in phenotype to drive a cryptic plant invasion

Many mechanisms of invasive species success have been elucidated, but those driving cryptic invasions of non‐native genotypes remain least understood. In one of the most successful cryptic plant invasions in North America, we investigate the mechanisms underlying the displacement of native Phragmites australis by its Eurasian counterpart. Since invasive Phragmites’ populations have been especially prolific along eutrophic shorelines, we conducted a two‐year field experiment involving native and invasive genotypes that manipulated nutrient level and competitor identity (inter‐ and intra‐genotypic competition) to assess their relative importance in driving the loss of native Phragmites. Inter‐genotypic competition suppressed aboveground biomass of both native and invasive plants regardless of nutrient treatment (～ 27%), while nutrient addition disproportionately enhanced the aboveground biomass (by 67%) and lateral expansion (by > 3 × farther) of invasive Phragmites. Excavation of experimental plots indicated that nutrient addition generates these differences in aboveground growth by differentially affecting rhizome production in invasive vs native plants; invasive rhizome biomass and rhizome length increased by 595% and 32% with nutrient addition, respectively, while natives increased by only 278% and 15%. Regardless of nutrient level, native rhizomes produced twice as many roots compared to invasives, which field surveys revealed are heavily infected with mycorrhizal symbionts. These results suggest that native Phragmites competes well under nutrient‐limited conditions because its rhizomes are laden with nutrient‐harvesting roots and mycorrhizae. Invasive Phragmites’ vigorous aboveground response to nutrients and scarcity of lateral roots, in contrast, may reflect its historic distribution in eutrophic Eurasian wetlands and correspond to its prevalence in New England marshes characterized by elevated nutrient availability and relaxed nutrient competition. These findings reveal that discrete differences in phenotype can interact with anthropogenic modification of environmental conditions to help explain the success of cryptic invaders.     

Palatability to a generalist herbivore,defence and growth of invasive and native <Emphasis Type="Italic">Senecio</Emphasis> species: testing the evolution of increased competitive ability hypothesis

This paper tests the prediction that introduced plants may become successful invaders because they experience evolutionary changes in growth and defence in their new range [evolution of increased competitive ability hypothesis (EICA)]. Interspecific and intraspecific binary feeding choices were offered to the snail Helix aspersa. The choices were between: (1) plants of the invasive Senecio inaequidens and Senecio pterophorus derived from populations in the introduced range (Europe) and plants of three indigenous species (Senecio jacobea, Senecio vulgaris and Senecio malacitanus) from populations in Europe; (2) plants of the invasive S. inaequidens and S. pterophorus from populations in the introduced range (Europe) and from populations in the native range (South Africa). We did not find a clear pattern of preference for indigenous or alien species of Senecio. However, we found that European invasive populations of S. inaequidens and S. pterophorus were less palatable than South African native populations. Moreover, in contrast to the predictions of the EICA hypothesis, the invasive genotypes of both species also showed a higher total concentration of pyrrolizidine alkaloids, and in the case of S. inaequidens we also found higher growth than in native genotypes. Our results are discussed with respect to the refinement of the EICA hypothesis that takes into account the difference between specialist and generalist herbivores and between qualitative and quantitative defences. We conclude that invasive populations of S. inaequidens and S. pterophorus are less palatable than native populations, suggesting that genetic differentiation associated with founding may occur and contribute to the plants’ invasion success by selecting the best-defended genotypes in the introduced range.     

Phenotypic plasticity of <Emphasis Type="Italic">Spartina alterniflora</Emphasis> and <Emphasis Type="Italic">Phragmites australis</Emphasis> in response to nitrogen addition and intraspecific competition

Phenotypic plasticity of the two salt marsh grasses Spartina alterniflora and Phragmites australis in salt marshes is crucial to their invasive ability, but the importance of phenotypic plasticity, nitrogen levels, and intraspecific competition to the success of the two species is unclear at present. Spartina alterniflora Loisel. is an extensively invasive species that has increased dramatically in distribution and abundance on the Chinese and European coasts, and has had considerable ecological impacts in the regions where it has established. Meanwhile, Phragmites australis Cav., a native salt marsh species on the east coast of China, has replaced the native S. alterniflora in many marshes along the Atlantic Coast of the US. This study determined the effects of nitrogen availability and culm density on the morphology, growth, and biomass allocation traits of Spartina alterniflora and Phragmites australis. A large number of morphological, growth, and biomass parameters were measured, and various derived values (culm: root ratio, specific leaf area, etc.) were calculated, along with an index of phenotypic plasticity. Nitrogen addition significantly affected growth performance and biomass allocation traits of Spartina alterniflora, and culm density significantly affected morphological characteristics in a negative way, especially for Spartina alterniflora. However, there were no significant interactions between nitrogen levels and culm density on the morphological parameters, growth performances parameters, and biomass allocation parameters of the two species. Spartina alterniflora appears to respond more strongly to nitrogen than to culm density and this pattern of phenotypic plasticity appears to offer an expedition for successful invasion and displacement of Phramites australias in China. The implication of this study is that, in response to the environmental changes that are increasing nitrogen levels, the range of Spartina alterniflora is expected to continue to expand on the east coast of China.     

Phenotypic traits of the Mediterranean <Emphasis Type="Italic">Phragmites australis</Emphasis> M1 lineage: differences between the native and introduced ranges

The environmental conditions in the new ranges of introduced plant species are often different from the conditions in their native ranges, and invasive plant species have been assumed to adapt to different environmental conditions by rapid ecological evolution in the invasive range after the introduction. Another interpretation of the change in plant traits after their introduction, however, is ecological fitting, which is based on the inherently high phenotypic plasticity of the species rather than on evolution. The Mediterranean haplotype M1 lineage of the wetland grass Phragmites australis was introduced to the coastal wetlands along the Gulf Coast of North America, where it is exposed to a different climate compared to its original range. The climate in the native range is arid or temperate with dry and hot summers, whereas the climate in the introduced range is warmer and has a higher and more uniform precipitation than that in the native range. This warmer and more humid environment is likely to pose different selection pressures to the plants in the introduced range and thus cause rapid evolutionary change and phenotypic differentiation in the introduced range. Here, we compared phenotypic traits of the M1 lineage from the native and introduced ranges in a common garden experiment to study the processes assisting the successful spread in the introduced range. Overall, the native and introduced groups were similar, but we detected a few phenotypic traits that diverged. Ecological fitting could be the fundamental mechanism by which the P. australis M1 lineage survives and spreads in the introduced Gulf Coast region. However, further research is needed to assess how the diverging traits observed in our study in Denmark (lower photosynthetic rates, lower chlorophylls concentration and higher leaf K concentration for the introduced than for the native genotypes) are expressed in the two ranges.     

Epigenetic variation within <Emphasis Type="Italic">Phragmites australis</Emphasis> among lineages,genotypes, and ramets

Epigenetics is likely an important factor in morphological and physiological acclimation, phenotypic plasticity, and potentially ecological dynamics such as invasiveness. We propose that Phragmites australis is an ideal model species for studies of epigenetics as a factor in plant invasions and ecology due to natural clonal replication (controlling for genetic variation) and the co-occurrence of subspecies with distinct life history strategies such as differences in invasiveness. In earlier work, genotypes and constituent clonal ramets were identified using microsatellite markers. In this pilot study, we screened the same ramets for epigenetic variation with Methylation-Sensitive AFLPs (MS-AFLPs), a modified type of AFLP dependent on differentially methylation-sensitive restriction enzymes. We found a significant difference in epigenetic signatures between introduced and native subspecies, and found that introduced P. australis demonstrated more epigenetic variation than their native counterparts. In both subspecies we observed moderate variation between genotypes relative to the higher degree of epigenetic variation found within genotypes (among ramets), suggesting that epigenotype may be more closely aligned with microhabitat than within-subspecies genotype. Finally, we observed potential epigenetic variation by site. This is the first study to investigate natural variation in DNA methylation patterns of P. australis and establishes the baseline in our understanding of the ecological relevance of epigenetics in this species.     

Genetic relationships of the marine invasive crab parasite <Emphasis Type="Italic">Loxothylacus panopaei</Emphasis>: an analysis of DNA sequence variation,host specificity,and distributional range

Host specificity is a key variable of the niche breath of parasites that can be an important determinant of a parasite’s ability to invade new areas. There is increasing evidence that many parasite species may comprise a variety of genetically variable lineages, which differ in host specificity and geographic range. In this study, we (1) explored the extent of diversity in the invasive parasitic barnacle Loxothylacus panopaei (Rhizocephala) infecting mud crabs (2) examined the geographic origin for the invasive lineage and (3) assessed if further southward spread of the parasite may be impeded. Along the US Atlantic coast, L. panopaei infects different hosts in its invaded range (Chesapeake Bay to north of Cape Canaveral) compared to one portion of the native range in Southeast Florida. This difference was reflected in genetic lineages on two independent loci, mitochondrial cytochrome oxidase I and nuclear cytochrome c. Both loci were concordant in that they showed one lineage infecting crabs of the genus Panopeus in the native range and one lineage infecting Eurypanopeus depressus and Rhithropanopeus harrisii hosts in the invaded range and in the Gulf of Mexico, thus indicating Gulf of Mexico populations as the most likely source of introduction into Chesapeake Bay. Interestingly, the nuclear marker resolved an additional lineage of parasites infecting panopeid hosts in the native range. All three parasite lineages were well supported, but a decision about species status must await further analyses. Since its introduction in the 1960s, the invasive L. panopaei lineage has expanded its range southward along the US Atlantic coast, now almost reaching the northern limit of native Panopeus-infecting lineages at Cape Canaveral, Florida. We hypothesized that parasite-free E. depressus in Southeast Florida, living in sympatry with infected panopeid populations, might be resistant to infection by the invasive lineage. Our infection experiments rejected this hypothesis, suggesting that any impediment to further southward range expansion might be expected from temperature regimes of the subtropical zoogeographic region south of Cape Canaveral.     

Pollen tube growth in the self‐compatible sweet cherry genotype, ‘Cristobalina’, is slowed down after self‐pollination

Sweet cherry is a self‐incompatible fruit tree species in the Rosaceae. As other species in the family, sweet cherry exhibits S‐RNase‐based gametophytic self‐incompatibility. This mechanism is genetically determined by the S‐locus that encodes the pollen and pistil determinants, SFB and S‐RNase, respectively. Several self‐compatible sweet cherry genotypes have been described and most of them have mutations at the S‐locus leading to self‐compatibility. However, ‘Cristobalina’ sweet cherry is self‐compatible due to a mutation in a pollen function modifier that is not linked to the S‐locus. To investigate the physiology of self‐compatibility in this cultivar, S‐locus segregation in crosses involving ‘Cristobalina’ pollen, and pollen tube growth in self‐ and cross‐pollinations, were studied. In the crosses with genotypes sharing only one S‐haplotype, the non‐self S‐haplotype was inherited more frequently than the self S‐haplotype. Pollen tube growth studies revealed that the time to travel the whole length of the style was longer for self‐pollen tubes than for cross‐pollen tubes. Together, these results suggest that ‘Cristobalina’ pollen tube growth is slower after self‐pollination than after cross‐pollination. This reproductive strategy would allow self‐fertilisation in the absence of compatible pollen but would promote cross‐fertilisation if cross‐compatible pollen is available, a possible case of cryptic self‐incompatibility. This bet‐hedging strategy might be advantageous for an ecotype that is native to the mountains of the Spanish Mediterranean coast, in the geographical limits of the distribution of this species. ‘Cristobalina’ blooming takes place very early in the season when mating possibilities are scarce and, consequently, self‐compatibility may be the only possibility for this genotype to produce offspring.     

Molecular identification of the exotic slug <Emphasis Type="Italic">Arion subfuscus</Emphasis> sensu stricto (Gastropoda: Pulmonata) in California,with comments on the source location of introduced populations

Arion subfuscus sensu lato (s. l.) refers to a cryptic slug species complex that is native to Europe and which, from there, has been introduced into the northeastern U.S. The species complex was detected in California for the first time in 2005 and recently 12 specimens were collected during statewide surveys. The genital morphology of these specimens suggested that only A. subfuscus sensu stricto (s. s.) was present and partial sequences of mitochondrial 16S rDNA (443 bp) showed that they all belonged to a single haplotype of the mitochondrial lineage, S1 sensu Pinceel et al. (Genetica 125: 155–171, 2005). This result was corroborated by a parallel analysis of a 655 bp COI DNA barcode. The 16S rDNA S1 haplotype (S1-03) of the Californian specimens is hitherto known only from the northeastern U.S. Hence, it is likely that populations may have been introduced to California from the northeastern states, rather than directly from the native area of the S1 haplogroup in Europe.     

Growth of Lythrum salicaria and Phragmites australis plants originating from a wide geographical area: response to nutrient and water supply

Aim The response of L. salicaria and P. australis plants originating from a broad latitudinal gradient to mineral nutrient and water supply was studied. We suggest implications for mechanisms possibly underlying the invasiveness of these two plant species. Location A common garden tub experiment was carried out at T?eboň, Czech Republic (49° N 14°47′ E, altitude 433 m). Methods Plants of 30 L. salicaria and 6 P. australis populations originating from a wide geographical area in Europe and in Israel (60°?32° N latitude, 6° W?20° E longitude) were cultivated for one growing season at two water levels and three (L. salicaria) or two (P. australis) nutrient doses, respectively. At the end of cultivation, basic morphological and growth characteristics were recorded. Results The latitude of the original geographical location was the most significant factor, affecting both the phenology and most of the plant morphological and growth characteristics measured in both plant species, with the characteristics related to plant size being negatively correlated with latitude. The effect of nutrient dose was very strong in both species, while the effect of water level was more pronounced in P. australis than in L. salicaria. Main conclusions The results confirmed the existence of a large phenotypic variability in both plant species within their native ranges of occurrence. In both plant species, the latitudinal variation in plant morphological and growth characteristics most probably indicates the differentiation of genotypes adapted to local geographical conditions. The plants of both species originating from all geographical locations tested responded to the respective nutrient treatments in a similar manner. Exceptions were found in growth characteristics related to reproduction in L. salicaria, indicating that better nutrient supply could enhance the reproductive ability of certain genotypes. Based on the results obtained, we suggest implications for mechanisms possibly underlying the invasiveness of the plant species studied.