Limiting Unwanted Gene Flow in Cotton Seed Production Fields.

Maintaining varietal purity is a priority for seed producers. As most cotton grown in Arizona is transgenic for the Bacillus thuringiensis (Bt) trait, there is strong potential for gene flow into non-Bt cotton varieties grown for seed. To identify strategies for reducing unwanted gene flow, we monitored gene flow in 15 commercial non-Bt cotton seed production fields. We quantified the density of pollinating insects and abundance of adventitious transgenic Bt plants in the fields, and obtained GIS maps of Bt and non-Bt cotton fields surrounding seed production fields. We measured both pollen- and seed-mediated gene flow, as evidenced by seeds and plants that contained the Bt toxin Cry1Ac, and used statistical methods to identify probable causes of each. Pollen-mediated gene flow increased with increasing abundance of Bt cotton fields within a distance of 750m from seed production fields, density of flower foraging honeybees, and density of adventitious Bt plants in the non-Bt cotton fields. Adventitious Bt cotton plants in fields resulted primarily from contamination in the planted seed bags and human error. Thus, unwanted gene flow can be minimized by 1) spacing cotton varieties 750m apart, particularly when honey bees are abundant, 2) screening seed for previous contamination before planting, and 3) minimizing human error at planting.