Morphology delimits more species than molecular genetic clusters of invasive Pilosella.

UNLABELLED: • PREMISE OF THE STUDY: Accurate assessments of biodiversity are paramount for understanding ecosystem processes and adaptation to change. Invasive species often contribute substantially to local biodiversity; correctly identifying and distinguishing invaders is thus necessary to assess their potential impacts. We compared the reliability of morphology and molecular sequences to discriminate six putative species of invasive Pilosella hawkweeds (syn. Hieracium, Asteraceae), known for unreliable identifications and historical introgression. We asked (1) which morphological traits dependably discriminate putative species, (2) if genetic clusters supported morphological species, and (3) if novel hybridizations occur in the invaded range.• METHODS: We assessed 33 morphometric characters for their discriminatory power using the randomForest classifier and, using AFLPs, evaluated genetic clustering with the program structure and subsequently with an AMOVA. The strength of the association between morphological and genotypic dissimilarity was assessed with a Mantel test.• KEY RESULTS: Morphometric analyses delimited six species while genetic analyses defined only four clusters. Specifically, we found (1) eight morphological traits could reliably distinguish species, (2) structure suggested strong genetic differentiation but for only four putative species clusters, and (3) genetic data suggest both novel hybridizations and multiple introductions have occurred.• CONCLUSIONS: (1) Traditional floristic techniques may resolve more species than molecular analyses in taxonomic groups subject to introgression. (2) Even within complexes of closely related species, relatively few but highly discerning morphological characters can reliably discriminate species. (3) By clarifying patterns of morphological and genotypic variation of invasive Pilosella, we lay foundations for further ecological study and mitigation.

Evaluating the use of common grasses by the wheat stem sawfly (Hymenoptera: Cephidae) and its native parasitoids in rangeland and Conservation Reserve Program grasslands.

The wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae), is a major pest of wheat (Triticum aestivum L., Poales: Poaceae) across the northern Great Plains of North America. Cephus cinctus has a wide host range, attacking numerous wild grasses and cultivated cereals in crop and grassland habitats, where it is, in turn, attacked by 2 native braconid parasitoids. Quantitative assessments of C. cinctus infestation and parasitism levels in different grass species across the full spectrum of available hosts are important in assessing the extent to which grasslands, or specific constituent grass species, may be reservoirs of pests or parasitoids moving into wheat. We quantified infestation and parasitism levels in over 25,000 stems collected from 17 grass species and wheat spanning 35 sites in central Montana, United States, over 2 yr. Infestation levels in 5 grass species, primarily wheatgrasses, were high (38%-65%) and similar to the levels observed in wheat (55%). In contrast, the majority of grass species (12 of 17) had significantly lower levels of infestation (<10%), suggesting that most grasses are not important reservoirs of C. cinctus. Parasitism levels in highly infested wheatgrasses were, on average, 3 times higher than those in cultivated wheat, suggesting that these grasses could provide important conservation habitat for parasitoids. Future work examining the relative performance of pests and parasitoids in these grasses will be important in gauging their relative value as plant materials to bolster parasitoid conservation in reseeded grassland habitats.

Propagule pressure, genetic structure, and geographic origins of Chondrilla juncea (Asteraceae): an apomictic invader on three continents.

PREMISE OF THE STUDY: Assessing propagule pressure and geographic origins of invasive species provides insight into the invasion process. Rush skeletonweed (Chondrilla juncea; Asteraceae) is an apomictic, perennial plant that is invasive in Australia, South America (Argentina), and North America (Canada and the United States). This study comprehensively compares propagule pressure and geographic structure of genotypes to improve our understanding of a clonal invasion and enhance management strategies. • METHODS: We analyzed 1056 native range plants from Eurasia and 1156 plants from three invaded continents using amplified fragment length polymorphism (AFLP) techniques. We used measures of diversity (Simpson's D) and evenness (E), analysis of molecular variance, and Mantel tests to compare invasions, and genotype similarity to determine origins of invasive genotypes. • KEY RESULTS: We found 682 unique genotypes in the native range, but only 13 in the invaded regions. Each invaded region contained distinct AFLP genotypes, suggesting independent introduction events, probably with different geographic origins. Relatively low propagule pressure was associated with each introduction around the globe, but levels of among-population variation differed. We found exact AFLP genotype matches between the native and invaded ranges for five of the 13 invasive genotypes. • CONCLUSIONS: Invasion dynamics can vary across invaded ranges within a species. Intensive sampling for molecular analyses can provide insight for understanding intraspecific invasion dynamics, which can hold significance for the management of plant species, especially by finding origins and distributions of invasive genotypes for classical biological control efforts.

Effects of Agaricus lilaceps fairy rings on soil aggregation and microbial community structure in relation to growth stimulation of western wheatgrass (Pascopyrum smithii) in Eastern Montana rangeland.

Stimulation of plant productivity caused by Agaricus fairy rings has been reported, but little is known about the effects of these fungi on soil aggregation and the microbial community structure, particularly the communities that can bind soil particles. We studied three concentric zones of Agaricus lilaceps fairy rings in Eastern Montana that stimulate western wheatgrass (Pascopyrum smithii): outside the ring (OUT), inside the ring (IN), and stimulated zone adjacent to the fungal fruiting bodies (SZ) to determine (1) soil aggregate proportion and stability, (2) the microbial community composition and the N-acetyl-ß-D-glucosaminidase activity associated with bulk soil at 0-15 cm depth, (3) the predominant culturable bacterial communities that can bind to soil adhering to wheatgrass roots, and (4) the stimulation of wheatgrass production. In bulk soil, macroaggregates (4.75-2.00 and 2.00-0.25 mm) and aggregate stability increased in SZ compared to IN and OUT. The high ratio of fungal to bacteria (fatty acid methyl ester) and N-acetyl-ß-D-glucosaminidase activity in SZ compared to IN and OUT suggest high fungal biomass. A soil sedimentation assay performed on the predominant isolates from root-adhering soil indicated more soil-binding bacteria in SZ than IN and OUT; Pseudomonas fluorescens and Stenotrophomonas maltophilia isolates predominated in SZ, whereas Bacillus spp. isolates predominated in IN and OUT. This study suggests that growth stimulation of wheatgrass in A. lilaceps fairy rings may be attributed to the activity of the fungus by enhancing soil aggregation of bulk soil at 0-15 cm depth and influencing the amount and functionality of specific predominant microbial communities in the wheatgrass root-adhering soil.

Variation in reproductive mode across the latitudinal range of invasive Russian knapweed.

For invading species, reproduction is a critical determinant of population establishment as well as spread into new areas. When species have multiple modes of reproduction, the prevalence of different modes can influence management decisions. We used genetic markers to determine the prevalent method of recruitment for invasive Russian knapweed (Acroptilon repens). This species forms patches and can spread by both rhizomic growth and seed from outcrossing. We found no shared genotypes between 41 western North American populations, indicating at the macroscale, Russian knapweed is spreading via seed to distant locations. We also examined drivers of reproductive mode by comparing clonality with large-scale environmental factors across the invasion. We found a correlation between latitude and clonal versus seed reproduction, with clonality higher in northern latitude populations. This trend was associated most parsimoniously with decreasing maximum annual temperature and 30-year average of available growing degree days, and increasing soil organic carbon content. These results have management implications: if not properly temporally implemented, grazing or herbicide applications that create open spaces for recruitment may increase the likelihood of Russian knapweed patch persistence through seed, and recently released galling biological control agents in North America may be less effective in northern latitudes where Russian knapweed spread by seed is less prevalent.

Increased ploidy of Butomus umbellatus in introduced populations is not associated with higher phenotypic plasticity to N and P.

Separate introductions or post-introduction evolution may lead to multiple invader genotypes or cytotypes that differ in growth rates, biomass or chemical profile responses (phenotype) to a range of environments. If the invader has high trait plasticity to a range of resource levels, then sediment N or P enrichment may enhance invasiveness. However, the ways in which ploidy, plasticity, and available N or P interact are unknown for most species despite the potential to explain spread and impacts by invaders with multiple introduced lineages. We conducted a common garden experiment with four triploid and six diploid populations of Butomus umbellatus, collected from across its invasive range in the USA. Plants were grown under different N or P nutrient levels (4, 40, 200, 400 mg L-1 N; 0.4, 4, 40 mg L-1 P) and we measured reaction norms for biomass, clonal reproduction and tissue chemistry. Contrary to our expectation, triploid B. umbellatus plants were less plastic to variation in N or P than diploid B. umbellatus in most measured traits. Diploid plants produced 172 % more reproductive biomass and 57 % more total biomass across levels of N, and 158 % more reproductive biomass and 33 % more total biomass across P than triploid plants. Triploid plants had lower shoot:root ratios and produced 30 % and 150 % more root biomass than diploid plants in response to increases in N and P, respectively. Tissue chemistry differed between cytotypes but plasticity was similar; N was 8 % higher and C:N ratio was 30 % lower in triploid than diploid plants across levels of N and plant parts, and N was 22 % higher and C:N ratio 27 % lower across levels of P and plant parts. Our results highlight differences in nutrient response between cytotypes of a widespread invader, and we call for additional field studies to better understand the interaction of nutrients and ploidy during invasion.

Biology of an Adventive Population of the Armored Scale Rhizaspidiotus donacis, a Biological Control Agent of Arundo donax in California.

Arundo donax (giant reed) is invasive in Mediterranean, sub-, and tropical riparian systems worldwide. The armored scale Rhizaspidiotus donacis is approved for biocontrol in North America, but an adventive population was recently discovered in southern California. We documented this population's distribution, phylogeny, phenology, potential host spillover to Phragmites spp., and potential for parasitism by a common biocontrol parasitoid of citrus scale. The adventive scale was found within a single watershed and is genetically closest to Iberian scale genotypes. Rhizaspidiotus donacis developed on Phragmites haplotypes but at much lower densities than Arundo. The adventive population is univoltine, producing crawlers from March-June. Aphytis melinus parasitoids exhibited sustained interest in R. donacis during choice and no-choice trials and oviposition resulted in a small second generation. Rhizaspidiotus donacis appears limited in distribution by its univoltinism and sessile adult females. This presents challenges for broad biocontrol implementation but allows for targeted application. The genetic differentiation between imported biocontrol samples and adventive populations presents an opportunity for exploring benefits of hybrids and/or alternative genotypes where establishment has been difficult. While unlikely to occur in situ, spillover to vulnerable endemic Phragmites or deleterious parasitoid effects on scale biocontrol agents warrants consideration when planning use of R. donacis.

Molecular evidence of hybridization in Florida's sheoak (Casuarina spp.) invasion.

The presence of hybrids in plant invasions can indicate a potential for rapid adaptation and an added level of complexity in management of the invasion. Three Casuarina tree species, Casuarina glauca, Casuarina cunninghamiana and Casuarina equisetifolia, native to Australia, are naturalized in Florida, USA. Many Florida Casuarina trees are considered unidentifiable, presumably due to interspecific hybridization. We collected tissue from over 500 trees from Australia and Florida and genotyped these using amplified fragment length polymorphisms. Our goal was to determine the exact identity of the Florida species, including any putative hybrid combinations. In Australia, we found high assignment values to the three parental species, and no evidence of hybridization. In Florida, we found many trees with strong assignment to any one of the three species, as well as 49 trees with assignment values intermediate to C. glauca and C. equisetifolia, suggesting hybridization between these species. One population of 10 trees had assignment values intermediate to C. cunninghamiana and C. glauca, suggesting additional hybridization. For 69 of these putative hybrid and parental types, we sequenced a low-copy intron of nuclear G3pdh, and these sequences indicated that some Florida trees contain heterozygotic combinations of C. glauca and C. equisetifolia haplotypes. The presence of novel hybrids in the Florida invasion may enhance evolution of invasive traits in these species. Novel Casuarina hybrids in Florida have no coevolutionary history with any insects or diseases, which may be problematic for biological control efforts.

An unexpected genetic diversity pattern and a complex demographic history of a rare medicinal herb, Chinese asparagus (Asparagus cochinchinensis) in Korea.

Range-wide population studies of wide spread species are often associated with complex diversity patterns resulting from genetically divergent evolutionary significant units (ESUs). The compound evolutionary history creating such a pattern of diversity can be inferred through molecular analyses. Asparagus cochinchinensis, a medicinally important perennial herb, is in decline due to overharvesting in Korea. Eight A. cochinchinensis populations in Korea and three populations from neighboring countries (China, Japan and Taiwan) were examined using nine nuclear microsatellite loci and three chloroplast microsatellite loci to characterize molecular diversity patterns. The average within-population diversity was limited likely due to long-term bottlenecks observed in all eight populations. High pairwise FST values indicated that the populations have largely diverged, but the divergences were not correlated with geographic distances. Clustering analyses revealed a highly complex spatial structure pattern associated with two ESUs. Approximate Bayesian Computation (ABC) suggests that the two ESUs split about 21,000 BP were independently introduced to Korea approximately 1,800 years ago, and admixed in secondary contact zones. The two ESUs found in our study may have different habitat preferences and growth conditions, implying that the two genetically divergent groups should be considered not only for conservation and management but also for breeding programs in agricultural areas.

Corrigendum: Geographic population structure in an outcrossing plant invasion after centuries of cultivation and recent founding events.

[This corrects the article DOI: 10.1093/aobpla/ply020.][This corrects the article DOI: 10.1093/aobpla/ply020.].

Geographic population structure in an outcrossing plant invasion after centuries of cultivation and recent founding events.

Population structure and genetic diversity of invasions are the result of evolutionary processes such as natural selection, drift and founding events. Some invasions are also molded by specific human activities such as selection for cultivars and intentional introduction of desired phenotypes, which can lead to low genetic diversity in the resulting invasion. We investigated the population structure, diversity and origins of a species with both accidental and intentional introduction histories, as well as long-term selection as a cultivar. Dyer's woad (Isatis tinctoria; Brassicaceae) has been used as a dye source for at least eight centuries in Eurasia, was introduced to eastern USA in the 1600s, and is now considered invasive in the western USA. Our analyses of amplified fragment length polymorphisms (AFLPs) from 645 plants from the USA and Eurasia did not find significantly lower gene diversity (Hj) in the invaded compared to the native range. This suggests that even though the species was under cultivation for many centuries, human selection of plants may not have had a strong influence on diversity in the invasion. We did find significantly lower genetic differentiation (Fst) in the invasive range but our results still suggested that there are two distinct invasions in the western USA. Our data suggest that these invasions most likely originated from Switzerland, Ukraine and Germany, which correlates with initial biological control agent survey findings. Genetic information on population structure, diversity and origins assists in efforts to control invasive species, and continued combination of ecological and molecular analyses will help bring us closer to sustainable management of plant invasions.

Can local adaptation explain varying patterns of herbivory tolerance in a recently introduced woody plant in North America?

Patterns of woody-plant mortality have been linked to global-scale environmental changes, such as extreme drought, heat stress, more frequent and intense fires, and episodic outbreaks of insects and pathogens. Although many studies have focussed on survival and mortality in response to specific physiological stresses, little attention has been paid to the role of genetic heritability of traits and local adaptation in influencing patterns of plant mortality, especially in non-native species. Tamarix spp. is a dominant, non-native riparian tree in western North America that is experiencing dieback in some areas of its range due to episodic herbivory by the recently introduced northern tamarisk leaf beetle (Diorhabda carinulata). We propose that genotype × environment interactions largely underpin current and future patterns of Tamarix mortality. We anticipate that (i) despite its recent introduction, and the potential for significant gene flow, Tamarix in western North America is generally adapted to local environmental conditions across its current range in part due to hybridization of two species; (ii) local adaptation to specific climate, soil and resource availability will yield predictable responses to episodic herbivory; and (iii) the ability to cope with a combination of episodic herbivory and increased aridity associated with climate change will be largely based on functional tradeoffs in resource allocation. This review focusses on the potential heritability of plant carbon allocation patterns in Tamarix, focussing on the relative contribution of acquired carbon to non-structural carbohydrate (NSC) pools versus other sinks as the basis for surviving episodic disturbance. Where high aridity and/or poor edaphic position lead to chronic stress, NSC pools may fall below a minimum threshold because of an imbalance between the supply of carbon and its demand by various sinks. Identifying patterns of local adaptation of traits related to resource allocation will improve forecasting of Tamarix population susceptibility to episodic herbivory.

The role of hybridization in facilitating tree invasion.

Hybridization events can generate additional genetic diversity upon which natural selection can act and at times enhance invasiveness of the species. Invasive tree species are a growing ecological concern worldwide, and some of these invasions involve hybridization events pre- or post-introduction. There are 20 hybrid invasive tree taxa in 15 genera (11 plant families) discussed here. When reported, abundance of hybrids comprised 10-100% of an invasion, the remainder being parental taxa. In seven hybrid taxa researchers identified phenotypes that may make hybrids better invaders. Twelve hybrid tree taxa involved introgression and more hybrids involved all non-native taxa than native x non-native taxa. Three hybrid tree taxa were the result of intentional crosses and all hybrid taxa involved intentional introduction of either one or more parental taxon or the hybrid itself. The knowledge gaps present in some hybrid tree taxa can weaken our effectiveness in predicting and controlling invasions, as hybrids can add a level of complexity to an invasion by being morphologically cryptic, causing genetic pollution of a native parental taxon, presenting novel genotypes for which there may not be coevolved biological control agents, or evolving adaptive traits through increased genetic variation.

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.

Microsatellite markers for Russian olive (Elaeagnus angustifolia; Elaeagnaceae).

PREMISE OF THE STUDY: Microsatellite markers were developed for the plant species Elaeagnus angustifolia to assist in future investigations of genetic variability in its native and invasive ranges and the precise origins of the United States/Canada invasion. • METHODS AND RESULTS: Eleven polymorphic microsatellite markers were developed. The number of alleles observed for each locus ranged from three to 11. • CONCLUSIONS: These microsatellites have sufficient potential variability to define population structure and origins of the Russian olive invasion.

Latitudinal variation in cold hardiness in introduced Tamarix and native Populus.

To investigate the evolution of clinal variation in an invasive plant, we compared cold hardiness in the introduced saltcedar (Tamarix ramosissima, Tamarix chinensis, and hybrids) and the native plains cottonwood (Populus deltoides subsp. monilifera). In a shadehouse in Colorado (41°N), we grew plants collected along a latitudinal gradient in the central United States (29-48°N). On 17 occasions between September 2005 and June 2006, we determined killing temperatures using freeze-induced electrolyte leakage and direct observation. In midwinter, cottonwood survived cooling to -70°C, while saltcedar was killed at -33 to -47°C. Frost sensitivity, therefore, may limit northward expansion of saltcedar in North America. Both species demonstrated inherited latitudinal variation in cold hardiness. For example, from September through January killing temperatures for saltcedar from 29.18°N were 5-21°C higher than those for saltcedar from 47.60°N, and on September 26 and October 11, killing temperatures for cottonwood from 33.06°N were >43°C higher than those for cottonwood from 47.60°N. Analysis of nine microsatellite loci showed that southern saltcedars are more closely related to T. chinensis while northern plants are more closely related to T. ramosissima. Hybridization may have introduced the genetic variability necessary for rapid evolution of the cline in saltcedar cold hardiness.

Invasion of Lepidium draba (Brassicaceae) in the western United States: distributions and origins of chloroplast DNA haplotypes.

Advances in phylogeography are of great value for understanding the population structure and origins of invasive genotypes. Such insights provide constructive information for current or future biological control research efforts. In this study, we investigated a highly variable chloroplast DNA (cpDNA) marker for populations of the weed Lepidium draba (Brassicaceae) in its native Eurasian and invasive US ranges. We sequenced DNA from 684 individuals from Eurasia and the US and found 41 different haplotypes. Our comparative study between the native and invasive ranges showed a 33% reduction in allelic richness (A) and a 7% reduction in haplotype diversity (h) since introduction into the US. Most genetic variation in the native range was observed within geographical regions and populations, not between regions, and this result was similar for the invasive range. Assignment tests indicated the most likely origins of many invasive haplotypes. Some of these occurred in western Europe, supporting an expanded native range that had been proposed for the species. Exact locations were identified for a diverse set of invasive haplotypes which can be used in ongoing host-specificity tests of potential biological control agents.

Hybrid Tamarix widespread in U.S. invasion and undetected in native Asian range.

Biological invasions are drastically altering natural habitats and threatening biodiversity on both local and global levels. In one of the United States' worst invasions, Eurasian Tamarix plant species have spread rapidly to dominate over 600,000 riparian and wetland hectares. The largest Tamarix invasion consists of Tamarix chinensis and Tamarix ramosissima, two morphologically similar species. To clarify the identity, origins, and population structuring of this invasion, we analyzed DNA sequence data from an intron of a nuclear gene, phosphoenolpyruvate carboxylase (PepC). This intron proved to be highly variable at the population level, and the 269 native and invasive specimens yielded 58 haplotypes, from which we constructed a gene genealogy. Only four of these haplotypes were common to both the U.S. and Eurasia. Surprisingly, we found that the most common plant in this U.S. invasion is a hybrid combination of two species-specific genotypes that were geographically isolated in their native Eurasian range. Less extensive hybrids exist in the invasion, involving combinations of T. ramosissima and T. chinensis with Tamarix parviflora and Tamarix gallica. The presence of potentially novel hybrids in the U.S. illustrates how importation of exotics can alter population structures of species and contribute to invasions.