Using hand-crosses and field observation to investigate pollen flow between American bittersweet (Celastrus scandens) and Oriental bittersweet (C. orbiculatus)

Background/Question/Methods

The threat that invasive plant species pose to native biodiversity through competition and habitat alteration is widely recognized, but the threat of hybridization with native congeners has only recently gained attention. Hybridization is especially detrimental when asymmetric pollen flow occurs, meaning that more pollen flows from the invasive species to the native than vice versa. Genetic data from our previous research has demonstrated that the native American bittersweet (Celastrus scandens) and the invasive Oriental bittersweet (C. orbiculatus) are hybridizing in the wild. In order to determine the rate of hybridization and directionality of pollen flow, we conducted manipulative hand-crosses in the field and observations of flower fate on both vines. We used nuclear microsatellite DNA markers to determine the species identity of the pollen parent for seedlings from both species. 

Results/Conclusions

Results from manipulative hand-crosses demonstrated that American bittersweet is significantly more likely to accept pollen from Oriental bittersweet than vice versa. The observational study and germination trials demonstrated that American bittersweet pistillate plants have lower flower fertilization rate and a greater hybridization rate, likely due in part to the interaction with staminate congeners. Asymmetry in pollen flow is likely due to differences in recognition and rejection of interspecific pollen, and a large advantage of Oriental bittersweet in terms of male fitness. This work demonstrates the importance of eradicating Oriental bittersweet at the early stages of invasion, before hybridization can threaten the genetic identity of American bittersweet populations.