Effects of the cultivation of genetically modified Bt crops on nontarget soil organisms - Microbial Ecology in Sustainable Agroecosystems - page 210

enforcement or regulations to monitor Bt (and other transgenic) crops is of concern as
almost half of the global hectarage of GM crops is grown in developing nations where
this level of oversight may not be feasible. It is therefore imperative that GM crops be
carefully evaluated for nontarget effects and the potential for gene flow under a variety
of environmental and experimental scenarios, even after they have been approved for
commercial use in the United States.
Examples of gene flow from transgenic crops to SCWR are becoming more prevalent,
and as more and different types of transgenic crops are introduced each year, gene flow
between them will become inevitable (Snow, 2002). In one of the first studies to examine
“transgene escape,” Quist and Chapela (2001) reported that genes from Bt maize had intro-
gressed into ancient landraces of traditional maize in Oaxaca, Mexico. This was despite
a 6-year moratorium, implemented in 1998, on the cultivation of GM maize in Mexico. A
follow-up study, conducted in 2005, found no traces of the genetically engineered traits
(specifically the cauliflower mosaic virus promoter) in any of the Mexican maize samples
tested, but rather than attempting to disprove the findings of Quist and Chapela, Ortiz-
Garcia et al. (2005) concluded that the moratorium, as well as increased education among
the farmers, likely led to the lack of genetic contamination detected in their study. Pineyro-
Nelson et al. (2009) employed new molecular techniques to confirm the presence of trans-
genes in 3 of 23 localities sampled in Oaxaca in 2001 and demonstrated that the persistence
or reintroduction of transgenes from Bt maize continued until at least 2004. It is not clear
how changes in plant physiology due to flow of transgenes might influence rhizosphere
ecology, if at all, but as new and different types of GM crops are developed, it is important
to consider the potential impacts of gene flow on nontarget soil organisms in different
agricultural and nonagricultural environments.
Escape of transgenes has also been reported in other GM crops. In Oregon, for exam-
ple, EPA researchers discovered gene escape (CP4 EPSPS protein and the corresponding
transgene) from glyphosate-resistant creeping bentgrass ( Agrostis stolonifera ) grown in
USDA-approved Monsanto-Scotts test plots (Reichman et al., 2006). The researchers found
the transgenes incorporated into resident populations of compatible Agrostis species up
to 3.8 km away from the USDA-APHIS permitted field test site, supporting a previous
study in which gene flow between glyphosate-resistant creeping bentgrass and sentinel
and resident plants occurred up to 21 and 14 km away, respectively, from the perimeter of
the GM bentgrass test plot (Watrud et al., 2004). These studies provide evidence that novel
traits can spread to wild-type plants and related species over much greater distances than
previously realized.
Gene flow and introgression of some GM traits may also affect plant population or
plant-soil dynamics and may indirectly aid in the spread of invasive species via the altera-
tion of the soil community. When the effect of glyphosate drift (10% application rate of
Roundup Ready ® ) was evaluated on mixed-species mesocosm communities consisting of
GM glyphosate resistant Brassica , two sexually compatible Brassica relatives, and a variety
of annual weeds ( Digitaria sanguinalis, Panicum capillare, and Lapsana communis ), Watrud et
al. (2011) found that crabgrass was the dominant weed in the control treatments, and that
Brassica dominated in the glyphosate treatments, increasing the incidence of the Roundup
resistance gene in the plant community. When Trifolium incarnatum (crimson clover) was
planted in soil mesocosms that had received glyphosate drift treatments, shoot biomass
and AMF colonization were reduced compared with plants grown in mesocosms that
received no herbicide treatment (Watrud et al., 2011). These results suggest that glypho-
sate drift associated with HT GM crops could contribute to the persistence and spread of
certain invasive species, such as Brassica sp., which are nonmycorrhizal, and thus could