Gene flow is an integral element in the evolutionary biology of plant species. Gene flow promotes genetic variability, allows gene migration, and can enable more rapid evolutionary dynamics (Ehrlich and Raven, 1969; Mallory-Smith et al., 2015). Defined as the movement of genes by gametes, diasporas, or individuals from one location to another (Golenburg, 1986), gene flow in plants can occur through the dispersal of pollen, seeds, and/or vegetative propagules. In highly self-pollinated species, gene flow occurs mainly by seed dispersal, whereas pollen movement may potentially contribute more to gene flow in highly outcrossing species (Darmency, 1996).
In the past, gene flow was not generally investigated within agricultural ecosystems. With the introduction of herbicide-resistant (HR) transgenic crops in North America in 1996, there was renewed interest in quantifying intraspecific pollen-mediated gene flow (PMGF) from glyphosate (GR)- or glufosinate-resistant cultivars of crops such as Brassica napus L. (oilseed rape or canola) to conventional (non-HR) cultivars. Understanding and addressing PMGF was central to coexistence measures implemented in various jurisdictions (Beckie and Hall, 2015). Additionally, the potential for PMGF from genetically-engineered HR crops to wild or weedy relatives was important in environmental risk assessments. Before and during that time, less attention
was paid to the movement of HR alleles via pollen or seed within and among weed populations of the same species. From an agronomic perspective, a weed population constitutes individuals of a weedy species present within a field, the largest spatial scale where management practices and selection pressures are relatively homogeneous (Délye et al., 2010).
This paper outlines three case studies of weed species that have demonstrated a great propensity for HR gene flow: B. scoparia in western North America, Amaranthus palmeri S. Watson (Palmer amaranth) in the United States (U.S.), and Lolium rigidum Gaud. (annual or rigid ryegrass) in Australia. These three species share three common features: (1) a top troublesome and economically damaging weed in their respective jurisdictions; (2) high incidence of multiple resistance in populations; and (3) rapid expansion of resistance incidence across jurisdictions in a short period of time.
Authors: Hugh J. Beckie, Roberto Busi, Muthukumar V. Bagavathiannan, Sara L. Martin