Gerhard Karrer1, Rea Maria Hall1, Valerie Le Corre2 and Matthias Kropf1
1Department of Integrative Biology and Biodiversity Research; University of Natural Resources and Life Science Vienna, Gregor Mendel Street 33, A-1180 Vienna, Austria
2INRA, UMR1347 Agroécologie, Dijon, France
Western ragweed (Ambrosia psilostachya DC.) is native to North America and naturalized in Europe. This perennial species is reproducing clonally by root sprouts which enable the species to invade a wide range of climatic regions in Europe – from southern Italy to Scandinavia and Scotland, and from Southwestern Spain to Russia. However, the process and patterns of spreading, the mutual relationship of the various populations, as well as the genetic and demographic structure of the invasive populations is still unexplored. Therefore, we sampled 61 populations throughout Europe and analysed genetic diversity and population genetic structure based on 15 microsatellite loci.
Our results indicate meaningful genetic differentiation among populations of A. psilostachya. Allelic richness (Na) within populations varied between 1.5 and 4.7. With a populational Fst-value of 0.20, the genetic differentiation among populations was moderate, whereas the genetic diversity within populations, based on Shannon index (I) was very high. This was confirmed by the results of the non-hierarchical AMOVA indicating that approx. 80% of genetic variation occur within populations. We found a significant deviation from the Hardy-Weinberg Equilibrium in all populations which was confirmed by the unbiased estimate of excepted heterozygosity (uHe = 0.46) indicating high homozygosity levels. Thirty-four populations showed at least one monomorphic locus, particularly, the Northern populations which were characterized by approx. 70% homozygote loci. In general, populations originating from the same region showed reduced genetic distance compared to populations from remote regions. In eight populations, we found at least one clone which also occurred in one other population, indicating an exchange of clones between these populations. Two of those populations that exchanged clones were rather nearby (0.3 and 7 km, connected by continuous anthropogenic disturbance and a river bed, respectively). The others were far from each other (162 km and 106 km, disconnected across the Adriatic Sea and along the southern Adriatic coastline, respectively.). While the first case can be interpreted as post-introductory expansion by obvious vectors, the second cases can be explained either by very rare far distance shoot fragment dispersal or by concerted introduction from identical sources. In addition, significant clonal structures were detected from genetic data in 55.7% of all populations studied.