The potential for gene flow between cultivated and wild sunflower (Helianthus annuus) in the United States.

The transfer of genes from crop plants to their wild relatives via hybridization has emerged as one of the primary risks associated with the commercialization of genetically engineered crops. Although previous studies have revealed relatively high levels of hybridization when crop plants come into contact with their wild relatives, the frequency of such contact across the range of cultivation of any crop taxon is unknown. Here we report the results of a multi-year, range-wide survey of the potential for reproductive contact between cultivated and common sunflower (Helianthus annuus). The results of this work indicate that the opportunity for crop-wild hybridization exists throughout the range of sunflower cultivation. Approximately two-thirds of all cultivated fields occurred in close proximity to, and flowered coincidentally with, common sunflower populations. In these populations, the phenological overlap was extensive, with 52-96% of all wilds flowering coincidentally with the adjacent cultivar field. Moreover, there was morphological evidence of hybridization in 10-33% of the populations surveyed within a given year. These findings indicate that crop-wild hybridization is likely across the range of sunflower cultivation in the USA.