Risk of alfalfa transgene dissemination and scale-dependent effects

Pollen can function as a vehicle to disseminate introduced, genetically engineered genes throughout a plant population or into a related species. The measurement of the risk of inadvertent dispersal of transgenes must include the assessment of accidental dispersion of pollen. Factors to be considered include the rate of pollen spread, the maximal dispersion distance of pollen, and the spatial dynamics of pollen movement within seed production fields; none of which are known for alfalfa (Medicago sativa L.), an insect-pollinated crop species. Using a rare, naturally occurring molecular marker, alfalfa pollen movement was tracked from seed and hay production fields. Results indicated that leafcutter bees ((Megachile spp.) used in commercial seed production show a directional, non-random bias when pollinating within fields, primarily resulting in the movement of pollen directly towards and away from the bee domicile. Within-field pollen movement was detected only over distances of 4 m or less. Dispersal of pollen from alfalfa hay and seed production fields occurs at distances up to 1000 m. By examining widely dispersed, individual escaped alfalfa plants and their progeny using RAPD markers, gene movement among escaped alfalfa plants has been confirmed for distances up to 230 m. The outcrossing frequency for large fields was nearly 10-times greater than that of research-sized plots. A minimum isolation distance of 1557 m may be required to prevent gene flow in alfalfa. Data suggest that complete containment of transgenes within alfalfa seed or hay production fields would be highly unlikely using current production practices.