Nine polymorphic microsatellite markers in Centaurea stoebe L. [subspecies C. s. stoebe and C. s. micranthos (S. G. Gmelin ex Gugler) Hayek] and C. diffusa Lam. (Asteraceae)

Centaurea stoebe (subspecies C. s. stoebe and C. s. micranthos[S. G. Gmelin ex Gugler) Hayek] and Centaurea diffusa are Eurasian plant species that have invaded much of North America. We isolated seven microsatellite loci from C. stoebe and two loci from C. diffusa. All loci described here amplify in both species and have between six and 25 alleles each. These markers will be useful in examining population structure and addressing questions regarding these invasions.     

Revised methods for the mass-rearing of the spotted knapweed biological control agent,Cyphocleonus achates (Coleoptera: Curculionidae), in field corrals

Cyphocleonus achates (Fahraeus), a root-feeding weevil introduced from Eurasia, is an effective biological control agent against spotted knapweed, Centaurea stoebe L. ssp. micranthos. Because C. achates is univoltine and does not fly, distribution of the weevil has been slow. To hasten the weevil's distribution, a rearing effort using field corrals was initiated at a facility in Corvallis, Montana. Procedures for mass-rearing the weevil in field corrals are described, with an emphasis on improvements over earlier methods. The described field-corral approach is effective and appropriate for producing C. achates for distribution in the western United States.     

The lesser of two weevils: physiological responses of spotted knapweed (Centaurea stoebe) to above- and belowground herbivory by Larinus minutus and Cyphocleonus achates

The physiological responses of plants to variable levels of root and shoot herbivory in the field may yield valuable insights regarding potential compensation or tolerance responses for herbivory. In an infestation of Centaurea stoebe (spotted knapweed) located in the Colorado foothills, we measured physiology, biomass, and flower production of individual plants undergoing a natural range of herbivory by the above- and belowground biological control insects, Larinus minutus and Cyphocleonus achates. Over the growing season, net carbon assimilation rate, transpiration, stomatal conductance, and intercellular leaf [CO₂] (C _i) all decreased, while water use efficiency increased. The decrease in these physiological traits was due to an increase in the intensity of L. minutus damage over time; effects of C. achates root damage to plant physiology, including transpiration were only marginally significant. The effects of these two species on plant physiology were not interactive, and Larinus minutus was found to exert larger negative effects on this invasive plant in terms of plant physiology and potential reproductive output than C. achates. While previous studies have shown C. achates to have significant negative effects on population densities of spotted knapweed, the addition of Larinus minutus to the suite of insects used in biological control of spotted knapweed should facilitate continued or enhanced reduction in densities of this noxious weed.     

Molecular evidence for multiple introductions of garlic mustard (Alliaria petiolata, Brassicaceae) to North America

Invasive species offer excellent model systems for studying rapid evolutionary change. In this context, molecular markers play an important role because they provide information about pathways of introduction, the amount of genetic variation introduced, and the extent to which founder effects and inbreeding after population bottlenecks may have contributed to evolutionary change. Here, we studied microsatellite variation in eight polymorphic loci among and within 27 native and 26 introduced populations of garlic mustard (Alliaria petiolata), a European herb which is a current serious invader in North American deciduous forests. Overall, introduced populations were genetically less diverse. However, considerable variability was present and when compared to the probable source regions, no bottleneck was evident. Observed heterozygosity was very low and resulted in high inbreeding coefficients, which did not differ significantly between native and introduced populations. Thus, selfing seems to be equally dominant in both ranges. Consequently, there was strong population differentiation in the native (F(ST) = 0.704) and the introduced (F(ST) = 0.789) ranges. The high allelic diversity in the introduced range strongly suggests multiple introductions of Alliaria petiolata to North America. Out of six European regions, the British Isles, northern Europe, and central Europe had significantly higher proportions of alleles, which are common to the introduced range, and are therefore the most probable source regions. The genetic diversity established by multiple introductions, and the lack of inbreeding depression in this highly selfing species, may have contributed to the invasion success of Alliaria petiolata.     

Interspecific interactions between the gall-fly Urophora affinis Frfld. (Diptera: Tephritidae) and the weevil Larinus minutus Gyll. (Coleoptera: Curculionidae), two biological control agents released against spotted knapweed, Centaurea stobe L. ssp. micranthos

Interspecific competition has been suggested as an explanation for the failure of some insects as biological control agents for weeds. Enclosure and exclusion cages were used, in southern British Columbia, Canada to evaluate the importance of interspecific competition between a seedhead weevil, Larinus minutus, and a gall-inducing fly, Urophora affinis, two biocontrol agents released against spotted knapweed in North America. At the seedhead scale, U. affinis, which is an inferior biological control agent based on knapweed seed mortality, was the superior competitor. Larinus minutus attack rates were significantly lower in the presence of U. affinis compared to release treatments where L. minutus was attacking alone. Reduced L. minutus attack rates were apparent in seed heads expected to contain both species, assuming insect distributions were random, but instead only contained U. affinis. L. minutus did not significantly affect U. affinis density. Although overall attack rates on knapweed seedheads were higher when both species were together at a site, the consequence of the antagonistic interaction is that overall seed destruction was not as high as it could have been if the weevil were attacking on its own. These results support minimizing the number of biocontrol agents released that use similar resources on the target weed, to avoid negative interactions between control agents and potential reductions in biocontrol efficacy resulting from competitive exclusion.     

Increased population growth rate in invasive polyploid Centaurea stoebe in a common garden

Biological invasions are inherently demographic processes, but trait differences between native and introduced genotypes are rarely linked to population growth rates. Native European Centaurea stoebe occurs as two cytotypes with different life histories (monocarpic diploids, polycarpic tetraploids); however, only tetraploids have been found in its introduced range in North America. In a common garden experiment using artificial populations, we compared the demographic performance of the three geo-cytotypes in the presence and absence of a specialist herbivore using periodic matrix models. We found no difference in population growth rate between the two European cytotypes and no significant effects of herbivory in all geo-cytotypes. However, there was a pronounced increase in population growth rate for North American compared with European tetraploids due to increased seed production and juvenile establishment. These results suggest that genetic drift or rapid evolution, rather than pre-adaptation through polyploidy may explain the invasion success of tetraploids.     

Novelty in Centaurea sect. Acrocentron (Asteraceae,Cardueae) from Iran

A new species, Centaurera bojnordensis Ranjbar, Negaresh & Joharchi (Asteraceae), endemic to Khorasan Province in northeastern Iran and belonging to Centaurea sect. Acrocentron, is described and illustrated. This species is mainly distinguished from C. sintenisiana of C. sect. Acrocentron by its indumentum of stem, shape and size of involucres, cilia and spine length. In addition, geographical distribution of the two species is presented and shown. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Genetic patterns across multiple introductions of the globally invasive crab genus Carcinus

The European green crab Carcinus maenas is one of the world's most successful aquatic invaders, having established populations on every continent with temperate shores. Here we describe patterns of genetic diversity across both the native and introduced ranges of C. maenas and its sister species, C. aestuarii, including all known non‐native populations. The global data set includes sequences from the mitochondrial cytochrome c oxidase subunit I gene, as well as multilocus genotype data from nine polymorphic nuclear microsatellite loci. Combined phylogeographic and population genetic analyses clarify the global colonization history of C. maenas, providing evidence of multiple invasions to Atlantic North America and South Africa, secondary invasions to the northeastern Pacific, Tasmania, and Argentina, and a strong likelihood of C. maenas × C. aestuarii hybrids in South Africa and Japan. Successful C. maenas invasions vary broadly in the degree to which they retain genetic diversity, although populations with the least variation typically derive from secondary invasions or from introductions that occurred more than 100 years ago.     

A new Centaurea species (Asteraceae) from Mt Sakar,South-eastern Bulgaria

A new species of Centaurea, C. sakarensis, known from a single locality north-east of the village of Lessovo on Mt Sakar, is described and illustrated. It belongs to C. sect. Acrolophus, and its morphologically closest relative is C. cariensis Boiss., a Turkish endemic from western and south-western Anatolia. The new species is currently known from a single population of about 1000 individuals, growing on open stony calcareous places, at 475 m a.s.l., and should be classified as Critically Endangered, (B1ab[i, ii, iii]+2ab [i, ii, iii]).     

Influence of Centaurea biebersteinii patch size on Urophora quadrifasciata (Dipt. Tephritidae) in Michigan, USA

Abstract:  Host plant patch size plays an important role in the distribution and densities of many insect species. Understanding how the host plant patch size influences the population of a biological control agent is necessary to monitor the success of a biological control programme. Urophora quadrifasciata (Dipt. Tephritidae) was released in the early 1970s to control the spotted knapweed, Centaurea biebersteinii (Asteraceae) in North America. The studies reported here investigated the influences of spotted knapweed patch size and stem density on U. quadrifasciata larval populations overwintering within capitula in the Upper Peninsula of Michigan, USA. An inverse relationship was found between the percentage of capitula with U. quadrifasciata larvae and spotted knapweed patch size in 2002 from Houghton county, MI. In 2003, an inverse relationship was also found in the south region of the study area in Houghton and Keweenaw counties, Michigan. The percentage of capitula with U. quadrifasciata larvae and spotted knapweed patch size were positively related for the central region. No relationship was found for the North region in 2003. Dispersal of U. quadrifasciata is directed north-east along the Keweenaw Peninsula, Michigan. With this directed spread, areas with well-established populations of U. quadrifasciata would provide source individuals for new colonization. Spotted knapweed patch size may play an important role in the overwintering populations of U. quadrifasciata as a result of dispersal.     

Phenology and dispersal of Larinus minutus Gyllenhal and Larinus obtusus Gyllenhal (Coleoptera: Curculionidae), two biological control agents of Centaurea stoebe ssp. micranthos (spotted knapweed) in Michigan

Centaurea stoebe L. ssp. micranthos (Gugler) (spotted knapweed) is an invasive plant that has been the target of classical biological control in North America for more than four decades. The seedhead-feeding weevils Larinus minutus Gyllenhal and Larinus obtusus Gyllenhal (Coleoptera: Curculionidae) are two of the most-widely released C. stoebe control agents, and have more recently been introduced into the eastern US. While there have been many studies focusing on their ability to impact C. stoebe in the western US and Canada, there have been few studies from eastern North America, and basic knowledge of important aspects of their biology is lacking. Here we report on the phenology and dispersal of L. minutus and L. obtusus in Michigan. We regularly sampled two established Larinus spp. populations in southern Michigan in 2012 and 2013, and found that while adult abundance fluctuates during the growing season, they remained at easily detectable levels from mid-June through the end of August. We also used previously established populations of L. minutus and L. obtusus released in 1996 (n = 1), 2007 (n = 2), and 2010 (n = 5) to determine how dispersal of Larinus spp. into the surrounding landscape changes with time since release. Populations of Larinus spp. weevils showed little dispersal for 2 years post-release. However, after initial establishment dispersal rates increased rapidly, resulting in average dispersal rates that increased exponentially with time since release. These findings can inform future biological control release and sampling programs for Larinus spp. in eastern North America.     

Genetic diversity and population structure of Lantana camara in India indicates multiple introductions and gene flow

Lantana camara is a highly invasive plant, which has spread over 60 countries and island groups of Asia, Africa and Australia. In India, it was introduced in the early nineteenth century, since when it has expanded and gradually established itself in almost every available ecosystem. We investigated the genetic diversity and population structure of this plant in India in order to understand its introduction, subsequent range expansion and gene flow. A total of 179 individuals were sequenced at three chloroplast loci and 218 individuals were genotyped for six nuclear microsatellites. Both chloroplasts (nine haplotypes) and microsatellites (83 alleles) showed high genetic diversity. Besides, each type of marker confirmed the presence of private polymorphism. We uncovered low to medium population structure in both markers, and found a faint signal of isolation by distance with microsatellites. Bayesian clustering analyses revealed multiple divergent genetic clusters. Taken together, these findings (i.e. high genetic diversity with private alleles and multiple genetic clusters) suggest that Lantana was introduced multiple times and gradually underwent spatial expansion with recurrent gene flow.     

Evidence for multiple sources of invasion and intraspecific hybridization in Brachypodium sylvaticum (Hudson) Beauv. in North America

We compared the levels and distribution of genetic diversity in Eurasian and North American populations of Brachypodium sylvaticum (Huds.) Beauv. (false brome), a newly invasive perennial bunchgrass in western North America. Our goals were to identify source regions for invasive populations, determine the number of independent invasion events, and assess the possibility that postinvasion bottlenecks and hybridization have affected patterns of genetic diversity in the invaded range. We tested the hypothesis that this Eurasian grass was accidentally introduced into two areas in Oregon and one site in California by examining nuclear microsatellites and chloroplast haplotype variation in 23 introduced and 25 native populations. In the invaded range, there was significantly lower allelic richness (R(S)), observed heterozygosity (H(O)) and within-population gene diversity (H(S)), although a formal test failed to detect a significant genetic bottleneck. Most of the genetic variation existed among populations in the native range but within populations in the invaded range. All of the allelic variation in the invaded range could be explained based on alleles found in western European populations. The distribution of identified genetic clusters in the North American populations and the unique alleles associated with them is consistent with two historical introductions in Oregon and a separate introduction to California. Further analyses of population structure indicate that intraspecific hybridization among genotypes from geographically distinct regions of western Europe occurred following colonization in Oregon. The California populations, however, are more likely to be derived from one or perhaps several genetically similar regions in the native range. The emergence and spread of novel recombinant genotypes may be facilitating the rapid spread of this invasive species in Oregon.     

Effects of fragmentation phenomena on the genetic structure and gene flow in Centaurea cineraria group (Asteraceae) in the Mediterranean Basin

The complex history of the Mediterranean region illustrates how ancient and recent phenomena are closely associated with species distribution and the creation of phylogeographic divisions within Mediterranean flora. A good model to explore the genetic consequences of fragmentation can be found in Centaurea cineraria and its close relatives. We applied simple sequence repeat molecular markers to a dense population sampling throughout the distribution area of all C. cineraria taxa to study how fragmentation has altered the genetic structure and distribution of C. cineraria. The average gene diversity (H_e) was 0.286, and the average allelic richness (A_r) was 3.65 and ranged from 2.15 (C. gymnocarpa) to 5.25 (C. busambarensis). The F_IS averaged a relatively high 0.223, ranging from ? 0.724 in C. aeolica subsp. aeolica to 0.589 in C. leucadea. Our results indicate that habitat fragmentation over several generations reduced heterozygosity due to random genetic drift in populations of C. cineraria. This heterozygosity erosion becomes more severe when the inbreeding coefficient is positive and the outcrossing rates show a significant increase. The results observed for outcrossing rates and inbreeding coefficient could also indirectly support the possibility of disrupted gene flow or mating pattern changes in fragmented C. cineraria populations.     

The role of parental and hybrid species in multiple introgression events: evidence of homoploid hybrid speciation in Centaurea (Cardueae,Asteraceae)

Molecular approaches have greatly increased the number of confirmed homoploid hybrids, which suggests that the frequency of this phenomenon was underestimated in the past because it was much more difficult to detect than allopolyploidy. Centaurea is a suitable model group for studying homoploid speciation, as hybridization events have been commonly reported for this genus. Based on this, here we study Centaurea × forsythiana, a naturally occurring homoploid hybrid between two Sardinian endemics, C. horrida and C. filiformis, using a molecular approach involving nuclear and plastid markers, to understand the underlying population dynamics between homoploid hybrids and their parents. Our results confirm that C. × forsythiana is a hybrid between the above‐mentioned species and define the roles of the parents. Plastid markers point towards C. horrida as the maternal progenitor, and nuclear markers reveal that the other parental species, C. filiformis, is itself an old, stabilized homoploid hybrid related to the C. paniculata complex from the Italian mainland. Homoploid hybrid speciation is discussed and C. × forsythiana and C. filiformis are compared with other similar examples. The study confirms the importance of introgression between parental species mediated by hybrids and its potential implications in conservation. Furthermore, it shows how hybridization studies become even more complex when the parents are themselves of probable hybrid origin. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 175 , 453–467.