There is a need to understand whether weed genetic diversity is the same among different populations, especially between those exposed to herbicide selection and other without exposure history. Inter-simple sequence repeat (ISSR) were used to assess level and patterns of genetic diversity in wild Brassica juncea (L.) Czern. et Coss. populations. A total of 93 plants from 24 wild populations in China were analysed by eight primers resulting in 86 highly reproducible ISSR bands. The analysis of molecular variance (AMOVA) with distances among individuals corrected for the dominant nature of ISSRs showed that most of the variation (54.09%) occurred among populations, and the remaining 45.91% variance was attributed to differences among individuals within populations. The high differentiation was, perhaps, due to limited gene flow (Nm \ 1.0) of this species. Though highest gene diversity was observed in resistant B. juncea population, the overall distribution of diversity across China was not geographic dependent. High F ST value (0.541) corroborated AMOVA partitioning and provided significant evidence for population differentiation in wild B. juncea. UPGMA cluster analyses, based on Nei's genetic distance, revealed grouping pattern geographically. Based on these results, the factors affect weed population genetic diversity and implication for herbicide resistance evolution were discussed in the context of transgenic crops advent and increasing herbicide usage in China.

Four successive reciprocal backcrosses between F(1) (obtained from wild Brassica juncea as maternal plants and transgenic glyphosate- or glufosinate-tolerant oilseed rape, B. napus, as paternal plants) or subsequent herbicide-tolerant backcross progenies and wild B. juncea were achieved by hand pollination to assess potential transgene flow. The third and forth reciprocal backcrosses produced a number of seeds per silique similar to that of self-pollinated wild B. juncea, except in plants with glufosinate-tolerant backcross progeny used as maternal plants and wild B. juncea as paternal plants, which produced fewer seeds per silique than did self-pollinated wild B. juncea. Germination percentages of reciprocal backcross progenies were high and equivalent to those of wild B. juncea. The herbicide-tolerant first reciprocal backcross progenies produced fewer siliques per plant than did wild B. juncea, but the herbicide-tolerant second or third reciprocal backcross progenies did not differ from the wild B. juncea in siliques per plant. The herbicide-tolerant second and third reciprocal backcross progenies produced an amount of seeds per silique similar to that of wild B. juncea except for with the glufosinate-tolerant first and second backcross progeny used as maternal plants and wild B. juncea as paternal plants. In the presence of herbicide selection pressure, inheritance of the glyphosate-tolerant transgene was stable across the second and third backcross generation, whereas the glufosinate-tolerant transgene was maintained, despite a lack of stabilized introgression. The occurrence of fertile, transgenic weed-like plants after only three crosses (F(1), first backcross, second backcross) suggests a potential rapid spread of transgenes from oilseed rape into its wild relative wild B. juncea. Transgene flow from glyphosate-tolerant oilseed rape might be easier than that from glufosinate-tolerant oilseed rape to wild B. juncea. The original insertion site of the transgene could affect introgression.

In the present article, we investigated the epidermal and anatomical features of 10 wild Brassica juncea populations with different susceptibilities to glyphosate in detail. The characteristics of the leaf surface and anatomy could affect the efficacy of glyphosate uptake in weeds through the influence of its adherence and penetration. Our results showed that herbicide-resistant wild populations generally had greater leaf thickness, adaxial cutin thickness, and trichome density in the upper epidermis, but lower spongy tissue thickness and vascular bundle numbers, as compared with the susceptible populations, suggesting that differences in morphological traits among populations might contribute to their variable response to glyphosate. The possible morphological involvement in the differential susceptibility of wild B. juncea populations to glyphosate also was discussed.

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The results obtained suggest that the populations responded differentially to glyphosate, and this variability may provide the genetic basis for evolution of individuals with increased resistance to glyphosate, with important implications for herbicide resistance management, especially in the context of risk assessment of glyphosate-tolerant crops.

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