Genetically engineered (GE) stacks, combinations of two or more single transgenic events (i.e. single-locus insertions) that have been produced by crossing sexually compatible parents, are an important and growing sector of the crop seed market. Stacked traits covered 26% of the global transgenic crop area in 2011 and were the fastest growing trait group, with a 31% increase in the area planted compared with 2010 (James, 2011). Stacked traits already dominate the market in some regions. For example, 95% of the cotton (Gossypium spp.) grown in Australia during 2011 had both herbicide tolerance and insect resistance traits (James, 2011). Worldwide, at least 12 countries are now growing stacked varieties, of which nine are developing countries (James, 2011). The rapid adoption of GE stacks has focused attention on whether the safety of such products differs from that of the individual events.
At issue is whether combining two or more events via conventional breeding creates changes that require additional safety assessment, even though the safety of each event in the stack has been assessed previously. The two main concerns are (1) whether combining two or more events into a plant by conventional breeding increases genomic instability and (2) whether potential interactions between the products of the transgenes in GE stacks impact safety. This paper draws on insights from plant breeding, describes the plasticity of conventional plant genomes over generations of crossing and selection, and considers the implications of event stacking on food and feed safety in the context of the normal plant genome.
The term GE is used here to distinguish the process of specific, intentional, and directed physical modification of the genome of a plant from random genetic modifications that occur in conventional breeding or by mutagenesis. The term GE is preferred over the term “genetically modified” (commonly referred to as GM) for these reasons. The term “event” refers to a single-locus insertion of recombinant DNA into the host genome regardless of the number of genes contained on the inserted piece of DNA. The term “conventional breeding” refers to methods of crossing plants with desired characteristics to generate offspring combining those desirable characteristics. These characteristics may include both non-GE and GE traits.