Submitted to: Proceedings of Southern Weed Science Society
Publication Type: Abstract only
Publication Acceptance Date: 1/23/2004
Publication Date: 1/23/2004
Citation: Tebeest, D.O., Gealy, D.R., Wheeler, C.C. 2004. Outcrossing rates between gufosinate-resistant transgenic rice cultivars and several red rice ecotypes. Proceedings of Southern Weed Science Society. 57:209. Interpretive Summary:
Technical Abstract: Recently developed herbicide resistant rice cultivars have provided rice growers in the southern U.S. a viable, and effective new method to control red rice and other weeds. Outcrossing between rice and red rice may compromise the long term effectiveness of this technology. Outcrossing rates between three transgenic glufosinate-resistant rice cultivars and eight genetically and phenotypically distinct red rice ecotypes were estimated in outdoor pot experiments at Fayetteville, AR in 1998, 2000, and 2001. The transgenic rice cultivars were Gulfmont, Cypress, and Bengal (each of these specific transgenic lines has since been discontinued). Red rice ecotypes included strawhull awnless types (Stuttgart strawhull, 15A), blackhull awned types (Stuttgart blackhull, 14F, TX4, 10A, and #8), and a strawhull awned type (11D). Ten plants each of the eight red rice types and three transgenic rice cultivars were pre-germinated and transplanted to numerous pots for placement into the field. Pots containing the eight red rice types were placed in a circular fashion around a single pot of transgenic rice in round plastic pools. Distance between the transgenic rice pot and each red rice pot was about 0.25 m. There were two replicate pools in 1998 and three replicates in 2000 and 2001. Seed from all red rice ecotypes and rice cultivars were collected from mature plants, subsampled, grown up as seedlings in a greenhouse to the one- to three-leaf stage, and sprayed with glufosinate at 0.42 kg/ha (1998) or 2.24 kg/ha (2000 and 2001; to minimize numbers of 'false positive' survivors). Surviving plants were counted and their DNA later evaluated for presence of the glufosinate resistance transgene (BAR) using PCR. More than 35000 seedlings have been screened in the greenhouse. Although not yet complete for all three years, PCR-confirmed outcrossing to date has occurred only between Bengal and 10A red rice, and was estimated at 0.12%, apparently because flowering periods were well synchronized. Outcrossing estimates based on herbicide survival alone were substantially higher than those based on PCR. The glufosinate-tolerant TX4 red rice type had glufosinate survival rates averaging 20% in 1998 and 8% in 2000, while survival rates of 15A were always less than 1%. These results suggest that the particular combination of transgenic rice cultivar and red rice ecotype present in rice fields can affect actual outcrossing rates. Also, the screening method and herbicide screening dosage can markedly affect estimated outcrossing rates.