Evidence for multiple introductions of Centaurea stoebe micranthos (spotted knapweed, Asteraceae) to North America

Authors: MARSS, R - Colorado State University; SFORZA - European Biological Control Laboratory (EBCL); HUFBAUER, RUTH - Colorado State University

Submitted to: Molecular Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/17/2008
Publication Date: 9/1/2008
Citation: Marss, R.A., Sforza, Hufbauer, R.A. 2008. Evidence for multiple introductions of Centaurea stoebe micranthos (spotted knapweed, Asteraceae) to North America. Molecular Ecology. 17:4197-4208.

Interpretive Summary: Biological invasions are initiated by one or more introduc- tions of a species into an area where it previously was not present. Introduction events can consist of many or only a few individuals, and both the number of introductions and the number of propagules introduced during each event can have a large effect on the genetic outcome of an invasion. Because genetic diversity provides the raw materials necessary for adaptive evolution, it has been hypothesized that multiple introductions may lead to especially problematic invaders capable of swift evolutionary response to selection pressure. Spotted knapweed, Centaurea stoebe subspecies micranthos L. (Asteraceae), is an especially problematic invader in North America for which little information on genetic diversity and population structure exists. It was first recorded in North America in Victoria, British Columbia in 1893 and was probably imported as a contaminant of alfalfa seed or soil carried as ships’ ballast. After over 110 years of invasion, C. stoebe micranthos has spread over much of North America, and is considered noxious in 15 US states (USDA Natural Resources. Conservation Service Plants Database). Here we use genotypes of individuals from the native and introduced ranges of spotted knapweed to investigate three sets of questions regarding the invasion of this species: (i) Is genetic diversity higher in the native range than in its introduced range? (ii) How is genetic variation structured among sampling sites in the European and North American ranges of C. stoebe micranthos? Is isolation by distance significant in either range? (iii) Are multiple introductions likely to have occurred, and can we infer native range origins of the invasive populations we sampled? Our results show that Multiple introductions and the maintenance of high genetic diversity through the introduction process may be partially responsible for the invasive success of C. stoebe micranthos.

Technical Abstract: Invasive species’ success may depend strongly on the genetic resources they maintain through the invasion process. We ask how many introductions have occurred in the North American weed Centaurea stoebe micranthos (Asteraceae), and explore whether genetic diversity and population structure have changed as a result of introduction. We surveyed individuals from 15 European native range sites and 11 North American introduced range sites at six polymorphic microsatellite loci. No significant difference existed in the total number of alleles or in the number of private alleles found in each range. Shannon–Weaver diversity of phenotype frequencies was also not significantly different between the ranges, while expected heterozygosity was significantly higher in the invasive range. Population structure was similar between the native range and the invasive range, and isolation by distance was not significant in either range. Traditional assignment methods did not allocate any North American individuals to the sampled European populations, while Bayesian assignment methods grouped individuals into nine genetic clusters, with three of them shared between North America and Europe. Invasive individuals tended to have genetically admixed profiles, while natives tended to assign more strongly to a single cluster. Many North American indi- viduals share assignment with Romania and Bulgaria, suggesting two separate invasions that have undergone gene flow in North America. Samples from three other invasive range sites were genetically distinct, possibly representing three other unique introductions. Multiple introductions and the maintenance of high genetic diversity through the introduction process may be partially responsible for the invasive success of C. stoebe micranthos.