Transgenic Mitigation of Transgene Dispersal by Pollen and Seed

The containment strategies to keep transgenes within a crop, and to keep transgenic crop seed (or hybrids between transgenic crops and their closely related nontransgenic neighbors) from spreading are not absolute. Containers eventually leak, and some containment systems are inherently leaky. Once there is a leak, the containment strategies cannot stop the spread. Some containment methods described in other chapters are unilateral, they prevent gene flow in one direction only (and are not even near absolute in that direction). For example, if a transgene is chloroplast encoded, and the pollen of the species does not transmit chloroplast DNA, the relative can cross with the crop, and its hybrid and backcrosses can be the recurrent pollen parent, moving the transgene to the relative. Additionally, even in species where there is maternal inheritance, plastid encoded traits do manage to be pollen transmitted by about 0.5% of pollen grains (Wang et al., 2004; Svab andMaliga 2007). Crops can be engineered to lack pollen production and be male sterile (rarely 100%) (Moon et al., 2010), but the wild or weedy relative can pollinate the crop. If various containment systems such as the genetic use restriction technologies (GURTs) that are discussed in other chapters are absolute, there still can be pollen transmission in the seed production fields before the GURTs are activated. Some seeds from transgenic crops are typically not harvested. If they persist to the next growing seasons, they will become “volunteer weeds” in the rotational crops that follow. There is selection pressure on such volunteer weeds to lose some of the recessive domestication traits such as uniform germination (lack of secondary dormancy that hedges chances against being controlled during the following and subsequent seasons), lack of seed shattering, and in green revolution crops, dwarfism. The feral traits are mostly dominant back mutations from crop to weedy traits such as shattering, dormancy, and height. They render the volunteer cum feral weeds more competitive with their domesticated cohorts, as well as with other weeds. The stress of being a volunteer weed in an agroecosystem may actually facilitate a more rapid evolution of back mutations to ferality, as stresses can increase the rate of mutations (Gressel and Levy, 2009). Thus, there is a need to have a second line of defense after containment to mitigate against genetic leaks from establishing in the field, whether leaks are of pollen or seed.