Cultivated Helianthus annuus (Asteraceae) volunteers as a genetic "bridge" to weedy sunflower populations in North America

In many crops, unharvested seeds can create populations of volunteer plants that increase opportunities for crop-to-wild gene flow. Pollen-mediated gene flow between cultivated and wild sunflower (both Helianthus annuus, Asteraceae) is well documented, but the role of seed dispersal and volunteers has not been investigated. We compared flowering times and other phenotypic traits of volunteers from both "normal" and "off-type" (multi-headed) crop plants with those of wild sunflowers. Normal and off-type volunteers typically had a maternally inherited, crop-specific DNA marker. Seedlings of wild plants, normal volunteers, and off-type volunteers from Colorado were cultivated in a greenhouse and at a field site in Ohio. We used a classification tree approach to differentiate the three plant types and identify phenotypic traits that can be used to recognize volunteers in the field in future surveys. In greenhouse and field experiments, we observed sufficient overlap in flowering times to allow gene flow among the three plant types. Volunteers from off-type crop plants were more likely to cross-pollinate with wild plants than volunteers from normal crop plants. Our results suggest that both types of crop volunteers have the potential to act as conduits for gene exchange between cultivated and wild sunflowers.