Crested floating heart [Nymphoides cristata (Roxb.) Kuntze] is an invasive aquatic plant in the southeastern United States. For clonal plants like N. cristata, clonal diversity may influence response to control tactics and/or evolutionary potential. However, little is known about the diversity of introduced N. cristata. In this study, we used genotyping by sequencing to quantify N. cristata diversity in the southeastern United States and determine how that diversity is distributed across the invaded range. Our results show that at least three distinct genetic lineages of N. cristata are present in the southeastern United States. Geographic distribution of the lineages varied, with one widespread lineage identified across several states and others only found in a single water body. There is also evidence of extensive asexual reproduction, with invaded water bodies often host to a single genetic lineage. The genetic diversity reported in this study likely results from multiple introductions of N. cristata to the southeastern United States and should be considered by managers when assessing control tactics, such as screening for biocontrol agents or herbicide testing. The extent and distribution of genetic diversity should also be considered by researchers studying the potential for invasive spread of N. cristata within the United States or hybridization with native Nymphoides species.

Invasive populations of Dalmation toadflax [Linaria dalmatica (L.) Mill.] and yellow toadflax (Linaria vulgaris Mill.) are widespread throughout the Intermountain West, where gene flow between these nonnative species is producing vigorous and fertile hybrids. These hybrid toadflax populations are less responsive to herbicides than either parent species, and biocontrol agents routinely released on L. dalmatica and L. vulgaris often fail to establish on hybrid hosts. Early detection of hybrid Linaria populations is therefore essential for effective management, but resources are limited for scouting large expanses of range and wildland. We used species distribution modeling to identify environmentally suitable areas for these invasive Linaria taxa in Montana, Wyoming, and Colorado. Areas suitable for hybrid Linaria establishment were estimated using two different modeling approaches: first, based on known hybrid occurrence and associated environmental conditions, and second, based on zones environmentally suitable for co-occurrence of the parent species. This also allowed comparison of different model outputs, especially relevant when modeling emerging invasives, such as novel hybrids, with minimal occurrence data. Combining the two model outputs identified areas at greatest risk of hybrid Linaria invasion, including parts of north-central Montana, where model estimates indicate the hybrid may spread without prior co-invasion of the parents. Potential hybrid hot spots were also identified in western Montana; northwestern, northeastern, and southeastern Wyoming; and the Western Slope and Front Range of Colorado. Despite relatively few confirmed occurrences of hybrid populations to date, our results indicate that extensive spread of hybrid populations is possible within the studied area. Model-based maps of potential Linaria distributions will allow area weed managers to direct limited resources more effectively for locating and controlling these invaders.

Glyphosate-resistant (GR) Palmer amaranth (Amaranthus palmeri S. Watson) is considered one of the most troublesome weeds in the southern and central United States, but results of previous research to determine the mode of inheritance of this trait have been conflicting and inconclusive. In this study, we examined segregation patterns of EPSPS gene-copy numbers in F1 and F2 generations of A. palmeri and found no evidence of a Mendelian single-gene pattern of inheritance. Transgressive segregation for copy number was exhibited by several F1 and all of the F2 families, most likely the product of EPSPS copy-number variation within each plant. This variation was confirmed by assaying gene-copy number across clonal generations and among individual shoots on the same plant, demonstrating that EPSPS amplification levels vary significantly within a single plant. Increases and decreases in copy number occurred in a controlled, stress-free environment in the absence of glyphosate, indicating that EPSPS gene amplification is a random and variable process within the plant. The ability of A. palmeri to gain or lose EPSPS gene copies is a valuable adaptive trait, allowing this species to respond rapidly to selection pressures and changing environments.

In a recent essay, Harker and coauthors stated that considering herbicide resistance as a wicked problem “without clear causes or solutions” ignores what weed scientists know about the biology and management of herbicide-resistant weeds. In this response, we argue that this misrepresents what is meant by “wicked” and that the wicked problem concept is valuable in understanding the multifaceted nature of herbicide resistance as a human-caused phenomenon.

Intraspecific genetic variation may contribute significantly to invasiveness and control problems, but has been characterized to date in relatively few invasive weed species. We examined 56 intersimple sequence repeat (ISSR) loci in 220 individuals from 11 invading populations of yellow toadflax sampled across five western states. All populations showed high levels of genetic diversity. Estimated values for Shannon's diversity measure ranged from 0.217 to 0.388, and for expected heterozygosity from 0.178 to 0.260. Nei's total gene diversity index (HT), on the basis of all individuals across all populations, was 0.267. Partitioning of genetic variance using analysis of molecular variance revealed 1.7% of genetic variation among regional population groups, 29.1% among populations within groups, and 69.2% within populations, consistent with expectations for an outcrossing species but suggesting little geographic differentiation. Pairs of adjacent individuals identical at all ISSR loci that appeared to be ramets of a single clone were detected in only one population. This indicates that patch expansion in yellow toadflax is driven more by sexual reproduction via seed than by rhizomatous clonal spread, at least at the spatial scale of sampling for this study. Eight populations had significant values for Mantel's R at P = 0.05, suggesting some fine-scale positive genetic structuring, possibly from restricted gene flow. Population clustering on the basis of Nei's genetic distance between populations and unweighted pair group method with arithmetic mean did not reflect geographic location. It is likely that multiple introductions of this species have occurred across the Intermountain West, followed by extensive genetic recombination. High levels of genetic diversity within yellow toadflax populations pose management challenges, as already seen in reports of variable response to herbicide application and limited impacts of biocontrol agent releases.

Recent advances in molecular methods and statistical analyses provide weed scientists with powerful tools for examining the genetic structure of weedy plant populations. The value of these studies depends on effective sampling protocols; however, there is little consensus on how to sample plant populations for genetic diversity analyses. In this review, we draw on published literature that incorporates sampling theory and spatial statistics in population genetic analyses to identify key factors to consider when designing a sampling strategy. We discuss how sampling design is affected by research objectives, biology of the study species, population structure, marker choice, and the genetic parameters to be investigated, and we offer suggestions on defining sampling units and developing sampling protocols.

Although there is evidence that interspecific hybridization can initiate invasion by nonnative plants, there are few documented examples of novel hybridization events between introduced plant species already exhibiting invasive behavior. We conducted morphometric and molecular analyses of toadflax plants with intermediate morphology found at two sites in Montana, which were co-invaded by yellow toadflax and Dalmatian toadflax. Field-collected putative hybrid plants had intermediate morphometric scores (mean 0.47, on a scale of 0.0 = indistinguishable from Dalmatian toadflax to 1.0 = indistinguishable from yellow toadflax) for a suite of phenotypic traits that differentiate the parent species (leaf length : width ratio, growth form, seed morphology, inflorescence type, and ventral petal shape). Inter-simple sequence repeat (ISSR) analysis of a subset of these putative hybrids revealed combinations of species-diagnostic bands, confirming the presence of DNA from both parent species. Controlled interspecific hand-pollinations generated viable first generation (F1) hybrid plants that also had intermediate morphometric scores (mean 0.46) and a mix of species-diagnostic ISSR bands from both parents. The hand-generated F1 hybrids crossed readily with both parent species to produce viable first generation backcrossed (BC1) plants. Our results confirm that hybridization is occurring between invasive populations of yellow toadflax and Dalmatian toadflax, and that the hybrid progeny are viable and fertile. This example of hybridization between alien congeners is of concern as the parent taxa are already known to be highly invasive. Further research is needed to assess the invasive potential of hybrid toadflax populations, and the likelihood of introgressive trait transfer between the parent species.

The inheritance of resistance to the auxinic herbicide dicamba was examined in a kochia population from Nebraska. An inbred, resistant line was developed by selection and selfing over seven generations to ensure any resistance alleles would be homozygous in the parents. An inbred, susceptible line was similarly developed, but without selection. Dose–response experiments with dicamba determined a glyphosate-resistant concentration required to inhibit dry weight accumulation by 50% (GR50) of 45 and 1,331 g ae ha−1 for the susceptible and resistant populations, respectively. F1 crosses were made between resistant and susceptible inbred individuals by hand-pollination, and the F1 plants were selfed to produce F2 plants. The F2 population was screened with 280 g ha−1 dicamba, a rate that could discriminate between susceptible and resistant plants. A total of eight F2 families were screened twice. In the first screen, seven F2 families segregated in a 3:1 ratio, consistent with a single dominant allele controlling resistance, and in the second screen six F2 families segregated in a 3:1 ratio. F2 individuals were selfed, the F3 progeny were tested with 280 g ha−1 dicamba, and the genotype of each F2 parent was determined based on F3 progeny segregation. F3 family segregation was consistent with the F2 parents having a 1:2:1 homozygous-susceptible:heterozygote:homozygous-resistant pattern, confirming that resistance to dicamba in kochia is likely conferred by a single allele with a high degree of dominance.

The rise of on-line open access (OA) has profound implications for academic publishing, not least the shift from subscribers to authors as the primary transactional partners for peer-reviewed journals. Although OA offers many benefits, it also paves the way for predatory publishers, who exploit the author-as-customer model to obtain revenue from author fees while providing few of the editorial services associated with academic publishing. Predatory journals publish papers with little or no peer review, and often disguise their real geographical location while exaggerating their scope and editorial expertise. Such journals also attempt to attract authors by promising unrealistically rapid editorial decisions while falsely claiming peer review, and fabricating impact factors and inclusion in academic indexes. The explosive increase in predatory OA journals is not only a risk to inexperienced authors, but also threatens to undermine the OA model and the legitimate communication of research.

The rate at which plant invasions occur is accelerating globally, and a growing amount of recent research uses genetic analysis of invasive plant populations to better understand the histories, processes, and effects of plant invasions. The goal of this review is to provide natural resource managers with an introduction to this research. We discuss examples selected from published studies that examine intraspecific genetic diversity and the role of hybridization in plant invasion. We also consider the conflicting evidence that has emerged from recent research for the evolution of increased competitiveness as an explanation for invasion, and the significance of multiple genetic characteristics and patterns of genetic diversity reported in the literature across different species invasions. High and low levels of genetic diversity have been found in different invading plant populations, suggesting that either selection leading to local adaptation, or pre-adapted characteristics such as phenotypic plasticity, can lead to aggressive range expansion by colonizing nonnative species. As molecular techniques for detecting hybrids advance, it is also becoming clear that hybridization is a significant component of some plant invasions, with consequences that include increased genetic diversity within an invasive species, generation of successful novel genotypes, and genetic swamping of native plant gene pools. Genetic analysis of invasive plant populations has many applications, including predicting population response to biological or chemical control measures based on diversity levels, identifying source populations, tracking introduction routes, and elucidating mechanisms of local spread and adaptation. This information can be invaluable in developing more effectively targeted strategies for managing existing plant invasions and preventing new ones. Future genetic research, including the use of high throughput molecular marker systems and genomic approaches such as microarray analysis, has the potential to contribute to better understanding and more effective management of plant invasions.

Since its discovery in 2005, glyphosate-resistant Palmer amaranth has become a major problem for many farmers in the southern United States. One mechanism of resistance found in a Georgia population of glyphosate-resistant Palmer amaranth is amplification of the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene throughout the genome, with some resistant plants containing and expressing more than 100 EPSPS genes. Such high numbers of EPSPS genes and protein production could result in a fitness cost to resistant plants due to (1) metabolic cost of overproduction of this enzyme and (2) disruption of other genes after insertion of the EPSPS gene. A greenhouse experiment was set up to investigate differences in growth and reproduction between glyphosate-susceptible and -resistant Palmer amaranth plants. Measurements included growth rate, plant height/volume ratio, final biomass, photosynthetic rate, inflorescence length, pollen viability, and seed set. This study found no significant fitness costs for plants with the resistance trait. This study also provided a clear example of how controlling for genetic background is important in fitness cost studies and how potentially misleading results can be obtained if only a few fitness traits are measured. These results indicate that glyphosate-resistant Palmer amaranth plants with high EPSPS gene copy numbers are likely to persist in field populations, even in the absence of glyphosate, potentially leading to long-term loss of glyphosate as a control option for Palmer amaranth.

Two broad aims drive weed science research: improved management and improved understanding of weed biology and ecology. In recent years, agricultural weed research addressing these two aims has effectively split into separate subdisciplines despite repeated calls for greater integration. Although some excellent work is being done, agricultural weed research has developed a very high level of repetitiveness, a preponderance of purely descriptive studies, and has failed to clearly articulate novel hypotheses linked to established bodies of ecological and evolutionary theory. In contrast, invasive plant research attracts a diverse cadre of nonweed scientists using invasions to explore broader and more integrated biological questions grounded in theory. We propose that although studies focused on weed management remain vitally important, agricultural weed research would benefit from deeper theoretical justification, a broader vision, and increased collaboration across diverse disciplines. To initiate change in this direction, we call for more emphasis on interdisciplinary training for weed scientists, and for focused workshops and working groups to develop specific areas of research and promote interactions among weed scientists and with the wider scientific community.

Herbicides are the foundation of weed control in commercial crop-production systems. However, herbicide-resistant (HR) weed populations are evolving rapidly as a natural response to selection pressure imposed by modern agricultural management activities. Mitigating the evolution of herbicide resistance depends on reducing selection through diversification of weed control techniques, minimizing the spread of resistance genes and genotypes via pollen or propagule dispersal, and eliminating additions of weed seed to the soil seedbank. Effective deployment of such a multifaceted approach will require shifting from the current concept of basing weed management on single-year economic thresholds.

The evolution of herbicide-resistant weeds is a major concern in the corn- and soybean-producing Pampas region of Argentina, where growers predominantly plant glyphosate-resistant crop varieties and depend heavily on glyphosate for weed control. Currently, 16 weed species in Argentina are resistant to one or more of three different herbicide mechanisms of action, and resistant weed populations continue to increase, posing a serious threat to agricultural production. Implementation of integrated weed management to address herbicide resistance faces significant barriers in Argentina, especially current land ownership and rental patterns in the Pampas. More than 60% of Pampas cropland is rented to tenants for periods that rarely exceed 1 yr, resulting in crop rotation being largely abandoned, and crop export taxes and quotas have further discouraged wheat and corn production in favor of continuous soybean production. In this paper we discuss ways to facilitate new approaches to weed management in Argentina, including legal and economic reforms and the formation of a national committee of stakeholders from public and private agricultural sectors.

Two DNA molecular marker techniques were used to evaluate genetic diversity in 58 accessions of jointed goatgrass and 6 accessions of the related wild species barb goatgrass. Random amplified polymorphic DNA (RAPD) assays were performed on 8 U.S. and 50 Eurasian jointed goatgrass accessions using 30 random decamer primers. The frequency of scorable polymorphic bands within jointed goatgrass was 6 out of 90 (6.7%). Cluster analysis of RAPD data showed small genetic distances (values of 0.005 or less) among jointed goatgrass accessions. To validate the effectiveness of RAPD techniques to detect genetic diversity in tetraploid Aegilops species, six accessions of barb goatgrass were assayed using a subset of 20 decamer primers (from the original 30). RAPD data for barb goatgrass were pooled with jointed goatgrass data from the same primers. A total of 63 scorable bands were generated, of which 27 (43%) were polymorphic between two or more accessions. RAPD analysis readily distinguished between the two species and detected much greater levels of genetic diversity within barb goatgrass than between the jointed goatgrass accessions. Amplified fragment length polymorphism (AFLP) assays were performed on a subset of the 58 jointed goatgrass accessions, 3 U.S. and 13 Eurasian. These accessions were selected to represent a range in geographic diversity within our collection. Ten primer combinations generated 560 scorable bands of which 28 (5%) were polymorphic. Cluster analysis of AFLP data showed a slightly smaller range in genetic distance (0.0002 to 0.0022) among accessions compared with RAPD results; however, AFLPs distinguished among all but 2 of the 16 accessions surveyed. Although AFLP produced more scorable bands than RAPD did, both methods revealed limited genetic diversity in jointed goatgrass.

Gene flow between Dalmatian toadflax (DT) and yellow toadflax (YT), both aggressive invaders throughout the Intermountain West, is creating hybrid populations potentially more invasive than either parent species. To determine the direction of gene flow in these hybrid populations, species-diagnostic cytoplasmic markers were developed. Markers were based on polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) polymorphisms in the trnT-D chloroplast DNA (cpDNA) region digested with Alu1, and single-nucleotide polymorphisms (SNPs) in the matK and trnL-F chloroplast-barcoding regions. Four hybrid toadflax populations sampled from Colorado, Montana, and Washington contained both DT and YT cytoplasm, with YT predominating; 25 individuals from a fifth hybrid population from Idaho all had identical YT cpDNA haplotypes. Thirteen plants from two Colorado populations, assumed to be DT based on morphology and geographic isolation from any known YT population, were found to have YT cpDNA haplotypes. These results indicate that gene flow between invasive YT and DT populations is more widespread that previously realized and confirms that cryptic introgression of YT alleles has occurred in multiple western U.S. DT populations. The presence of YT genetic material in presumed DT populations may negatively affect host recognition and establishment by biocontrol agents used for toadflax management.

In little over 20 yr, Palmer amaranth has risen from relative obscurity to its current status as one of the most widespread, troublesome, and economically damaging agronomic weeds in the southeastern U.S. Numerous factors have enabled Palmer amaranth to become such a dominant and difficult-to-control weed, including its rapid growth rate, high fecundity, genetic diversity, ability to tolerate adverse conditions, and its facility for evolving herbicide resistance. It is both a serious threat to several U.S. cropping systems and a fascinating model weed. In this paper, we review the growing body of literature on Palmer amaranth to summarize the current state of knowledge on the biology, agricultural impacts, and management of this weed, and we suggest future directions for research.