Invasion of Ambrosia artemisiifolia in Italy: Assessment via analysis of genetic variability and herbarium data

Investigations of the genetic pattern and colonisation sources and the routes of invasion by alien species populations are crucial for identifying invasion mechanisms and the reasons for the bio-ecological success of invasive species. The aim of our work was to study the genetic pattern of Italian populations of Ambrosia artemisiifolia in comparison with that of some French and Canadian populations and to use herbarium records to characterise the colonisation areas of A. artemisiifolia across Italy. Molecular investigations were based on a set of nuclear SSR marker loci, which we used to analyse a number of Italian, Canadian and France populations. The time-spatial spread of A. artemisiifolia was reconstructed through the distributional pathway of 194 herbarium specimens.Ambrosia artemisiifolia Italian populations (HE = 0.687) had higher values of genetic diversity when compared to Canadian (HE = 0.639) and French (HE = 0.643) populations. Nevertheless, the time of residence of the Italian populations was positively correlated with the observed and expected heterozygosity (HO, HE). Genetic clustering inferred in STRUCTURE suggested admixture of populations with different ancestry. Historical distributional data highlighted that A. artemisiifolia first colonised the Po plain and different phased localities of the Mediterranean region. This study synthesises genetic and historical distributional data, highlighting that several invasion events have occurred across the Italian peninsula in different spatio-temporal steps, establishing high levels of historic gene flow between populations with mixed ancestry.