ja_mageia

Wheat (Triticum aestivum L.) cultivars with novel traits have generated increased interest in pollen-mediated gene flow (PMGF) among commercial grain producers, biotechnology regulators, and seed growers. Three studies were conducted with the objectives of estimating PMGF in wheat at the landscape level, hybridization between wheat and jointed goatgrass (Aegilops cylindrica Host), and the potential for seed of one cultivar to be unintentionally present in a seed lot of another cultivar.

  The first study estimated PMGF in wheat between commercial fields across years and locations, and compared the results to small experimental plots. The study was conducted in 2003, 2004 and 2005 in eastern Colorado. Fifty-six commercial field locations planted to one or more of 18 different imidazolinone-susceptible (IS) winter wheat cultivars bordering an imidazolinone-resistant (IR) cultivar were sampled at distances of 0.23 to 61 m. Experimental plots were planted in 2003 and 2004, and harvested in 2004 and 2005. Commercial field samples were evaluated with field herbicide applications to identify IR plants that resulted from PMGF. A sub-sample of surviving resistant plants was verified as heterozygous. The highest observed PMGF was 5.3% at 0.23 m. The farthest distance at which PMGF was detected was 61 m and the highest PMGF at 61 m was 0.25%. No PMGF was detected in 27% of samples. PMGF was detected in samples from all 56 locations and in all evaluated cultivars. Based on estimates from a generalized linear mixed model with a random effect of location, the distance required to ensure 95% confidence that 95% of locations would have maximum PMGF less than 0.9% is 41.1 m for a relative heading class of one and 0.7 m for a relative heading class of five. Higher levels of PMGF were detected at farther distances in commercial field samples than experimental plot samples. These confidence limits should represent the highest levels of PMGF that could be expected to occur in winter wheat and will be useful for wheat biotechnology regulation.

The second study utilized both designed experiments and commercial field sites in 2003, 2004, and 2005 to detect pollen-mediated gene flow from IR wheat to jointed goatgrass planted side by side and at distances up to 53 m from IR wheat. A greenhouse planting and spraying method was used to screen for imazamox-resistant hybrids in collected jointed goatgrass seed. Jointed goatgrass samples taken within IR wheat showed that hybridization did occur with substantial variation in cross-pollination between sites and years. The average percent cross-pollination across sites and years from IR wheat to jointed goatgrass was 0.29% and the farthest distance over which cross-pollination was observed was 16 m. Imidazolinone herbicides are non-competitive inhibitors of acetolactate synthase (ALS) and IR wheat carries a mutant ALS allele that confers resistance. Percent ALS activity measured with an in-vitro assay was used for a modified generation means analysis of IR and IS wheat, jointed goatgrass, jointed goatgrass by IR wheat, and IS wheat by IR wheat crosses. The mutant ALS allele was found to be partially dominant in a jointed goatgrass by IR wheat cross and additive in an IS wheat by IR wheat cross. The potential exists for movement of imazamox resistance and other wheat genes into jointed goatgrass in eastern Colorado.

The third study investigated seed lot purity by sampling non-IR varieties from certified and farm-saved seed growers, who either produced or had never produced IR wheat. Adventitious IR seeds were detected using a seed soaking technique in samples from each producer type and each IR production history.

Levels of IR seed ranged from 0% to 11.28%. One certified sample and three farm-saved samples exceeded the 0.1% threshold for off-types in certified wheat seed. Using a two-factor analysis, farm-saved production class and positive IR history increased the estimated proportion of adventitious seed.

Based on grower interviews, higher levels of adventitious seed presence were associated with volunteer plants from previous crops of the resistant variety and mechanical mixture during harvesting. Production practices for certified seed address these factors and may need to be strengthened if more stringent purity criteria are adopted. This information is important for risk assessment and policy development for potential commercial release of transgenic wheat varieties.