Herbicide resistance gene flow in weeds: Under-estimated and under-appreciated

Interest in the magnitude and consequences of intra- and inter-specific herbicide resistance gene flow, particularly that mediated by pollen, increased in the mid-1990s with the introduction of herbicide-resistant (HR) transgenic crops. During that time, less attention was paid to the movement of HR alleles via pollen or seed among weed populations. Incidence of HR weeds in a region is often attributed to independent evolution through herbicide selection; the role or contribution of HR allele movement via pollen, seeds and/or vegetative propagules is often under-estimated and under-appreciated. Once a new HR weed biotype has been confirmed in a jurisdiction, how often have we been surprised at its rapid areawide expansion? In genotypic studies of HR weed populations, the contribution of gene flow to incidence of resistance is frequently similar or greater than that of independent evolution. Simulation models have consistently predicted that frequent widespread applications of highly effective herbicides (e.g., acetolactate synthase inhibitors, glyphosate) provide connective high-fitness habitats across the landscape, which facilitate a rapid increase in the frequency and movement of an HR trait within and among populations. Such habitats, characterized by minimal heterogeneity but high selection intensity due to frequent herbicide 'on' vs. 'off' exposure periods, strongly favour the fitness of HR propagules at great disadvantage of herbicide-susceptible individuals. The unanticipated speed of areawide expansion of some HR weed biotypes has spurred numerous calls over the past decade for a collective community or regional response to mitigate this unhindered spread of HR alleles. The best mitigation strategy is minimizing weed population abundance and seed bank replenishment in fields and adjacent ruderal areas. This goal is difficult, but necessary for preserving the remaining public good and common pool resource of herbicide susceptibility.