Risk of alfalfa transgene dissemination and scale-dependent effects

Pollen can function as a vehicle to disseminate introduced, genetically eng ineered 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 considere d include the rate of pollen spread, the maximal dispersion distance of pol len, and the spatial dynamics of pollen movement within seed production fie lds; none of which are known for alfalfa (Medicago sativa L.), an insect-po llinated crop species. Using a rare, naturally occurring molecular marker, alfalfa pollen movement was tracked from seed and hay production fields. Re sults indicated that leafcutter bees ((Megachile spp.) used in commercial s eed 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 see d production fields occurs at distances up to 1000 m. By examining widely d ispersed, individual escaped alfalfa plants and their progeny using RAPD ma rkers, gene movement among escaped alfalfa plants has been confirmed for di stances up to 230 m. The outcrossing frequency for large fields was nearly 10-times greater than that of research-sized plots. A minimum isolation dis tance of 1557 m may be required to prevent gene flow in alfalfa. Data sugge st that complete containment of transgenes within alfalfa seed or hay produ ction fields would be highly unlikely using current production practices.