Outcrossing breeding system does not compromise invasiveness in Buddleja davidii

Introductions of alien plant species are often likely to consist of a few individuals. Thus, invasion success may strongly depend on their reproductive biology. A high number of self-compatible plants species are known to be successful colonizers of new habitats, even able to establish populations from single propagules. However, many other invasive species require pollen vectors. Here, we investigated the mating system of Buddleja davidii, a fast growing shrub native to China that colonizes quickly in disturbed habitats such as quarries, river banks, along railways and roads, both in its native and invasive regions. It was intentionally introduced to Europe as an ornamental plant because of its fragrant and showy flowers. We additionally studied its vulnerability to biparental inbreeding depression by performing a controlled crossing experiment using pollen from the same population or from geographically close and distant populations, respectively. As a measure for pollination success, we used capsule weight, seed number per capsule and seed weight for each treatment.The self-incompatibility index for B. davidii was found to be 96% suggesting that successful reproduction strongly depends on cross-pollination and the presence of appropriate pollen vectors. Since cross-pollination did not reveal significant differences in measured traits, it is assumed that invasive B. davidii-populations do not suffer from biparental inbreeding depression. B. davidii has fragrant and rewarding flowers that mainly attract butterflies. We conclude that the long distance pollen transfer performed by these insects may have prevented inbreeding so far and thus contributes to the invasive spread of B. davidii in Europe.