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Glyphosate-resistant (GR) horseweed has become an especially problematic
weed in different crop production systems across the United States and the
world. In this field study, we used a nondestructive measurement system to
analyze the pollen production, deposition, and dispersion of a Tennessee
glyphosate resistant (TNR) horseweed biotype in Knoxville, TN during the
2013 pollination season. We observed that the pollination season of TNR
horseweed lasted about 2 mo (54 d). About 78.93% of horseweed pollen was
released between 9:00 A.M. and 7:00 P.M. during each sampling day and the
release peak was at about 1:30 P.M. The seasonal release of pollen grains
was estimated to be 5.11 million grains plant−1. The release rate
data indicated that the integrated horizontal flux density and deposition
flux density contributed to 78.17% and 21.83% of the release rate,
respectively. We also found that pollen concentration decreased with
distance from the source field; the average pollen concentration decreased
to 50.69% at a distance of 16 m from the source plot. This is the first
result of a systematic, direct examination of the release rate (emission and
deposition), release pattern (daily and seasonal), and dispersion pattern of
GR horseweed pollen.
The evolution of glyphosate resistance in weedy species places an environmentally benign herbicide in peril. The first report of a dicot plant with evolved glyphosate resistance was horseweed, which occurred in 2001. Since then, several species have evolved glyphosate resistance and genomic information about nontarget resistance mechanisms in any of them ranges from none to little. Here, we report a study combining iGentifier transcriptome analysis, cDNA sequencing, and a heterologous microarray analysis to explore potential molecular and transcriptomic mechanisms of nontarget glyphosate resistance of horseweed. The results indicate that similar molecular mechanisms might exist for nontarget herbicide resistance across multiple resistant plants from different locations, even though resistance among these resistant plants likely evolved independently and available evidence suggests resistance has evolved at least four separate times. In addition, both the microarray and sequence analyses identified non–target-site resistance candidate genes for follow-on functional genomics analysis.
The genetic basis of weedy and invasive traits and their evolution remain poorly understood, but genomic approaches offer tremendous promise for elucidating these important features of weed biology. However, the genomic tools and resources available for weed research are currently meager compared with those available for many crops. Because genomic methodologies are becoming increasingly accessible and less expensive, the time is ripe for weed scientists to incorporate these methods into their research programs. One example is next-generation sequencing technology, which has the advantage of enhancing the sequencing output from the transcriptome of a weedy plant at a reduced cost. Successful implementation of these approaches will require collaborative efforts that focus resources on common goals and bring together expertise in weed science, molecular biology, plant physiology, and bioinformatics. We outline how these large-scale genomic programs can aid both our understanding of the biology of weedy and invasive plants and our success at managing these species in agriculture. The judicious selection of species for developing weed genomics programs is needed, and we offer up choices, but no Arabidopsis-like model species exists in the world of weeds. We outline the roadmap for creating a powerful synergy of weed science and genomics, given well-placed effort and resources.
Determining the frequency of crop-wild transgene flow under field conditions is a necessity for the development of regulatory strategies to manage transgenic hybrids. Gene flow of green fluorescent protein (GFP) and Bacillus thuringiensis (Bt) transgenes was quantified in three field experiments using eleven independent transformed Brassica napus L. lines and the wild relatives, B. rapa L. and Raphanus raphanistrum L. Under a high crop to wild relative ratio (600:1), hybridization frequency with B. rapa differed among the individual transformed B. napus lines (ranging from ca. 4% to 22%), however, this difference could be caused by the insertion events or other factors, e.g., differences in the hybridization frequencies among the B. rapa plants. The average hybridization frequency over all transformed lines was close to 10%. No hybridization with R. raphanistrum was detected. Under a lower crop to wild relative ratio (180:1), hybridization frequency with B. rapa was consistent among the transformed B. napus lines at ca. 2%. Interspecific hybridization was higher when B. rapa occurred within the B. napus plot (ca. 37.2%) compared with plot margins (ca. 5.2%). No significant differences were detected among marginal plants grown at 1, 2, and 3 m from the field plot. Transgene backcrossing frequency between B. rapa and transgenic hybrids was determined in two field experiments in which the wild relative to transgenic hybrid ratio was 5–15 plants of B. rapa to 1 transgenic hybrid. As expected, ca. 50% of the seeds produced were transgenic backcrosses when the transgenic hybrid plants served as the maternal parent. When B. rapa plants served as the maternal parent, transgene backcrossing frequencies were 0.088% and 0.060%. Results show that transgene flow from many independent transformed lines of B. napus to B. rapa can occur under a range of field conditions, and that transgenic hybrids have a high potential to produce transgenic seeds in backcrosses.
Transgenes from transgenic oilseed rape, Brassica napus (AACC genome), can introgress into populations of wild B. rapa (AA genome), but little is known about the long-term persistence of transgenes from different transformation events. For example, transgenes that are located on the crop’s C chromosomes may be lost during the process of introgression. We investigated the genetic behavior of transgenes in backcross generations of wild B. rapa after nine GFP (green fluorescent protein)-Bt (Bacillus thuringiensis) B. napus lines, named GT lines, were hybridized with three wild B. rapa accessions, respectively. Each backcross generation involved crosses between hemizygous GT plants and non-GT B. rapa pollen recipients. In some cases, sample sizes were too small to allow the detection of major deviations from Mendelian segregation ratios, but the segregation of GT:non-GT was consistent with an expected ratio of 1:1 in all crosses in the BC1 generation. Starting with the BC2 generation, significantly different genetic behavior of the transgenes was observed among the nine GT B. napus lines. In some lines, the segregation of GT:non-GT showed a ratio of 1:1 in the BC2, BC3, and BC4 generations. However, in other GT B. napus lines the segregation ratio of GT:non-GT significantly deviated from 1:1 in the BC2 and BC3 generations, which had fewer transgenic progeny than expected, but not in the BC4 generation. Most importantly, in two GT B. napus lines the segregation of GT:non-GT did not fit into a ratio of 1:1 in the BC2, BC3 or BC4 generations due to a deficiency of transgenic progeny. For these lines, a strong reduction of transgene introgression was observed in all three B. rapa accessions. These findings imply that the genomic location of transgenes in B. napus may affect the long-term persistence of transgenes in B. rapa after hybridization has occurred.
Release of transgenic insect-resistant crops creates the potential not only for the insect pest to evolve resistance
but for the escape of transgenes that may confer novel or enhanced fitness-related traits through hybridization
with their wild relatives. The differential response of diamondback moth (Plutella xylostella) populations in
eastern and western Canada to Bt-producing (GT) Brassica napus and the potential for enhanced fitness of GT
B. napus and weedy GT Brassica rapa × B. napus hybrid populations (F1, BC1, BC2) were studied. Comparative bioassays using neonates and 4th instars showed that GT B. napus and GT B. rapa × B. napus hybrids are lethal to larvae from both populations. No measurable plant fitness advantage (reproductive dry weight) was observed
for GT B. napus (crop) and GT B. rapa × B. napus hybrid populations at low insect pressure (1 larva per leaf). At high insect densities (>10 larvae per leaf), vegetative plant weight was not significantly different for GT B. napus
and non-GT B. napus, whereas reproductive plant weight and proportion of reproductive material were
significantly higher in GT B. napus. Establishment of the Bt trait in wild B. rapa populations may also increase
its competitive advantage under high insect pressure.
The movement of transgenes from crops to weeds and the resulting consequences are
concerns of modern agriculture. The possible generation of “superweeds” from the
escape of fitness-enhancing transgenes into wild populations is a risk that is
often discussed, but rarely studied. Oilseed rape, Brassica napus (L.), is a crop
with sexually compatible weedy relatives, such as birdseed rape (Brassica rapa (L.)).
Hybridization of this crop with weedy relatives is an extant risk and an excellent
interspecific gene flow model system. In laboratory crosses, T3 lines of seven
independent transformation events of Bacillus thuringiensis (Bt) oilseed rape were
hybridized with two weedy accessions of B. rapa. Transgenic hybrids were generated
from six of these oilseed rape lines, and the hybrids exhibited an intermediate
morphology between the parental species. The Bt transgene was present in the hybrids,
and the protein was synthesized at similar levels to the corresponding independent
oilseed rape lines. Insect bioassays were performed and confirmed that the hybrid
material was insecticidal. The hybrids were backcrossed with the weedy parent,
and only half the oilseed rape lines were able to produce transgenic backcrosses.
After two backcrosses, the ploidy level and morphology of the resultant plants were
indistinguishable from B. rapa. Hybridization was monitored under field conditions
(Tifton, GA, USA) with four independent lines of Bt oilseed rape with a crop to
wild relative ratio of 1200:1. When B. rapa was used as the female parent,
hybridization frequency varied among oilseed rape lines and ranged from 16.9%
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