Asynchronous Flowering Periods and Pollen-mediated Gene Flow

Gene flow exposed by genetically engineered crops Gene flow is the transfer of genes between populations of the same or related species. The exchange between crop cultivars of genes selected by traditional breeding that confer characteristics such as shorter stems, higher yield, or increased seed quality will attract little attention. In the same way, crop-weed hybrids having acquired traits such as lower seed shattering or increased crop mimicry will go unnoticed initially. Genetically engineered (GE) crops have unveiled some of these untraced exchanges among and between crops and weeds. GE herbicide resistant plants, either crop-weed hybrids or volunteer plants (subsequent unmanaged generations of the planted seed) selected by herbicide application, will be readily noticed and might require additional control measures. That is the beauty and downside of the herbicide resistance trait: easy detection with the appropriate herbicide but potential crop yield loss due to competition. Transgenes for second generation traits, e.g., drought or salt tolerance and nitrogen use efficiency, will be more difficult to trace, as plants with these novel traits may not be as easy to detect or have an immediate impact on crop yields, and could have increased fitness in unmanaged areas. Fortunately, lessons learned by tracing transgenes conferring herbicide resistance can serve in the development of models to understand the dynamics and outcome of large scale releases of other transgenes. The release in 1995 and subsequent rapid adoption by producers of canola cultivars (mainly Brassica napus L.: argentine canola, oilseed rape) with resistance to three herbicides with different active ingredients has provided a unique setting to examine gene flow and its agro-ecological impacts. Tracing herbicide resistance in canola showed to what extent genetic material is exchanged between and among cultivated plants in commercial fields, volunteer plants in fields, and along roadsides, and between crop plants and a closely related weed (Brassica rapa L.: birdsrape mustard).1