Feral rice from introgression of weedy rice genes into transgenic herbicide-resistant hybrid-rice progeny

Pollen-mediated transgenic flow of herbicide resistance occurs bidirectionally between transgenic cultivated rice and weedy rice. The potential risk of weedy traits introgressing into hybrid rice has been underestimated and is poorly understood. In this study, two glufosinate-resistant transgenic rice varieties, hybrid rice (F1), and their succeeding generations (F2–F4) were planted for 3 years in field plots free of weedy rice adjacent to experimental weedy-rice fields. Weedy-rice-like (feral) plants that were both glufosinate-resistant and had red-pericarp seed were initially found only among the F3 generations of the two glufosinate-resistant transgenic hybrid cultivars. The composite fitness (an index based on eight productivity and weediness traits) of the feral progeny was significantly higher than that of the glufosinate-resistant transgenic hybrid (the original female parent of the feral progeny) under monoculture common garden conditions. The hybrid rice progeny segregated into individuals of variable height and extended flowering. The hybrid rice F2 generations had higher outcrossing rates by pollen reception (0.96–1.65%) than their progenitors (0.07–0.98%). The results show that herbicide-resistant weedy rice can rapidly arise by pollen-mediated gene flow from weedy to transgenic hybrid rice, and their segregating pollen-receptive progeny pose a greater agro-ecological risk than transgenic varieties. The safety assessment and management regulations for transgenic hybrid rice should take into account the risk of bidirectional gene flow.