Reproductive strategies of Xanthium italicum differ from those of native Xanthium sibiricum, and they are key to its invasiveness.
The reproductive traits of alien plants are an important factor contributing to their successful invasion. Xanthium italicum (Compositae) is a widespread invasive weed in China. In this study, we compared the reproductive traits of this species with those of its native relative X. sibiricum, with the aim to provide a scientific basis for characterizing the invasion mechanisms of X. italicum. The traits compared were flowering phenology, stigma receptivity, pollen viability, pollination agent, male and female inflorescence functions, and breeding system, based on field investigations, experimental observations, and controlled pollination treatments. Xanthium italicum presented a longer flowering period and stigma receptivity stage, higher pollen viability, higher average number of male and female inflorescences per plant, and larger amount of pollen, which increase the probability of stigma pollination and seed setting, than X. sibiricum. Although both species are wind pollinated, the pollen grains of X. italicum are more widely dispersed than those of X. sibiricum. Moreover, whilst X. sibiricum is strictly wind pollinated, X. italicum is characterized by insect pollination. The dual pollination mechanism and longer pollination distance of X. italicum constitute advantageous pollination strategies of this invasive plant. Controlled pollination studies revealed that the breeding systems of the two plants are mixed, including both outcrossing and selfing, although the amount of seeds produced by X. italicum was higher than that by X. sibiricum. Overall, compared with the native species, the successful invasion of X. italicum can be attributed to the differences in reproductive strategies.