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Title: When invasion increases population genetic structure: A study with Centaurea diffusa

Author
item MARSS, R - Colorado State University
item SFORZA, RENE - European Biological Control Laboratory (EBCL)
item HUFBAUER, RUTH - Colorado State University

Submitted to: Biological Invasions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/26/2007
Publication Date: 8/15/2007
Citation: Marss, R.A., Sforza, R., Hufbauer, R.A. 2007. When invasion increases population genetic structure: A study with Centaurea diffusa. Biological Invasions. 10:561-572.

Interpretive Summary: Human-mediated transport of species into novel ecosystems can substantially alter the genetic variation of introduced populations relative to native populations, both in total amount and in how variation is partitioned within and between populations. . It is becoming widely recognized that adaptive evolution of introduced species to novel ecosystems plays an important role in biological invasions. Because genetic variation provides the raw materials for adaptive evolution, it is critical to understand how introductions affect the amount and structure of genetic variation. Here, we examine genetic diversity and population structure in the native and introduced ranges of a prominent rangeland weed, Centaurea diffusa Lam. (Asteraceae) using microsatellite loci. Our questions are: 1) Is genetic diversity in the introduced range of C. diffusa reduced or increased within populations relative to the native range? 2) Is genetic variation structured similarly between the two ranges? If so, is that structure organized in accordance with the expectations of isolation by distance? 3) Are multiple introductions likely to have occurred in this system? Do the Eurasian samples represent possible areas of origin of our invasive populations? Our main results show that 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.

Technical Abstract: 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.