Background: The introgression of a transgene conferring glyphosate resistance from Brassica napus (rapeseed, canola) to Brassica rapa weeds (bird rape) was documented at a single location in 2007. In 2015, several cases of glyphosate resistant mustard were reported by growers in areas where rapeseed was seldom grown.Results: Survey result indicated glyphosate resistant bird rape mustard is present in areas where glyphosate tolerant corn and soybean are often grown in rotation. Genetic analyses reveal that hybridization followed by introgression and progressive loss of chromosome is the likely mechanism for the horizontal gene transfer (HGT) of glyphosate resistance.Conclusion: Introgression of the glyphosate-resistance conferring transgene in the populations studied appears to have occurred several times, consistent with the ease for B. rapa to form hybrids with B. napus. The introduction of a transgene into a crop should therefore take into account the weediness of the species that share a common genome and their ability to form hybrids. We provide here such an example between B. napus and B. rapa, and potentially between B. napus and Raphanistrum raphanistrum.

The full spectrum of herbicide resistance in a weed can vary according to the mechanistic basis and cannot be implied from the selective pressure. Common ragweed (Ambrosia atemisiifolia L.) is an important weed species of horticultural crops that has developed resistance to linuron based on either target site or non target site resistance mechanisms. The objective of the study is to characterize the cross resistance to metribuzin of linuron selected biotypes of A. artemisiifolia with target site and non target site resistance and determine its genetic basis. Crosses were made between two types of linuron resistant biotype and linuron susceptible biotypes and the progeny were further backcrossed with susceptible plants to third backcross generation to determine their responses to both herbicides compared to parental lines. The target site based linuron resistant biotype was cross resistant to metribuzin and resistance to both herbicides was maintained at the same level in the BC3 line. In contrast, the linuron selected biotype with non target site resistance mechanism was not cross resistant to metribuzin. In addition, the BC3 lines deriving from the non target site resistant parents had very low-level resistance. While the target site resistance trait is maintained through multiple crosses, non target site-based resistance would be lost over time when selection is absent or insufficient to retain all genes involved in resistance as a complex trait. This would imply A. artemisiifolia biotypes with different mechanisms would need to be managed differently over time.

Roadsides are vectors of spread for invasive and other non-native plants. Therefore, fields located along roadsides could harbour more weeds and less native species compared to more isolated fields. To determine if field edges that are close to roadsides have different floras compared to more isolated fields, we surveyed 26 field pairs (52 fields) located in the province of Québec, Canada. For each pair, one field could be directly accessed by a major paved road (AD fields) while the other field, located on the same farm, was more isolated (IS fields) and could only be accessed via a secondary farm road. Two borders of these fields (IS) were sampled as well as the parallel (AD-pa) and the perpendicular border (AD-pe) of A fields and the adjacent roadside (RO). Plant species present along these field borders were recorded and classified (e.g. non-native, native, monocot, dicot, annual, perennial) in 0.5 m2 quadrats located every 20 m. The number of common ragweed (Ambrosia artemisiifolia) plants was also noted. Analyses included linear mixed models, generalized estimating equations models and multiple correspondence analyses. All border types had equivalent species richness. Roadsides had higher densities of common ragweed and lower occurrences of native and perennial species compared to field edges. All analyses indicate roadside floras are different from field edge floras but field edges close to roadsides were similar to those of more isolated fields. Results do not support a simple diffusive spread of roadside plants into field crop edges since field edges located along roadsides did not harbour more common ragweed plants or more roadside species.