Grain trade between China and the United States shapes the spatial genetic pattern of common ragweed populations in East China cities

30 Common ragweed ( Ambrosia artemisiifolia L.) is an invasive alien species (IAS) that can cause 31 severe allergies among urban residents. Understanding its invasion pathways is critical for 32 designing effective control programs. However, studies on the invasion of common ragweed 33 normally skip urban areas. Results from a few studies based on analyzing occurrence records 34 contain high uncertainties. We attempted to address this knowledge gap through a case study in 35 East China cities by combining the population genetic method with the occurrence records. We 36 first collected leaf samples of 37 common ragweed populations from 15 cities. We then quantified 37 the spatial genetic pattern of common ragweed populations by analyzing genomic and chloroplast 38 DNA extracted from the leaf samples. Combined with the analysis of occurrence data and trade 39 data, we discovered that multiple introductions have impacted the spatial genetic pattern of 40 common ragweed populations in East China Cities. We inferred that the modern-day grain trade 41 between the United States and China could be the primary invasion source while the bridgehead 42 introduction passage through Japan played a minor role. Among the studied cities, Nanjing and 43 Shenyang dispersed more gene flows than other cities. The two cities' central roles in 44 transportation and grain importation might explain the observed pattern. Based on our findings, 45 we suggest that invasive species control programs should consider the potential role of cities as 46 landing points and dispersal hubs of common ragweed in invaded countries. 47 assignment our we


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Common ragweed (Ambrosia artemisiifolia) is native to North America but is now an important 52 invasive alien species (IAS) species in all continents except Antarctica 1 . Besides causing 53 significant loss in agricultural production, common ragweed pollen is a primary allergen for 54 seasonal allergic rhinitis and asthma, which result in substantial health costs and labor losses 1,2 .

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The negative health impact of common ragweed pollens is worsened in urban areas because the 56 production of pollens and allergenicity of pollens are both enhanced in urban environments 3,4 . 57 Therefore, control of the common ragweed has been given a high priority in many cities 58 worldwide 5 .

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Cities need information on the invasion pathway of common ragweed in order to control it 60 more effectively. So far, this information was primarily inferred from occurrence records. For 61 example, researchers found common ragweed populations in the urban area of Magdeburg,

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Germany and speculated that common ragweed was introduced through the seed trade 6 . Based on 63 the occurrence data, a study found that communication lines played a vital role in the dispersion of 64 common ragweed in urban areas of Croatia 6 . While those inferences are helpful, they also contain 4 interpret the invasion pathway. For example, genetic diversity showed that the populations of wild

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In this study, we intend to disclose the invasion pathways of common ragweed in urban areas 92 by using a combination of the population genetic method and occurrence records. We selected 93 cities in East China as the sites for this case study. China first recorded common ragweed in the 94 1930s. The common ragweed currently occur in more than 23 provinces, and most of the 95 occurrences were in East China 18,19 . East China accounts for 43% of China's land area but nearly 96 94% of China's population 20 . The highly urbanized region contains key port cities for 97 international trade and a well-developed transportation network 21 . Existing studies have shown

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Genetic patterns of common ragweed populations in East China cities 107 Six nuclear microsatellite loci have been identified from the leaf samples. The results of exact 108 tests for genotypic disequilibrium showed that none of the microsatellite loci presented significant 109 linkage equilibrium. Also, the estimates of null alleles were highly variable ( Supplementary Fig.   110 1). Therefore, these loci could be used for further analysis. Based on the nuclear microsatellite 111 loci, deviation from Hardy-Weinberg proportions was detected in most common ragweed 112 populations in East China cities (Table 1), which suggested heterozygote deficiency. Bottleneck 113 effects caused by the population size reduction were found in six populations.

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Ten cpDNA haplotypes were identified after considering indels and the seven 115 polymorphisms observed within the 802-bp concatenated alignment in chloroplast intergenic 116 spacers. The haplotype diversity (HD) ranged from 0 to 0.68, and the nucleotide diversity (π) 117 ranged from 0 to 0.00122 (Table 1). common haplotypes (e.g., Hap22 and Hap21) and rare

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The results from GENELAND and STRUCTURE jointly indicated that the common ragweed 124 populations in East China cities could be classified into seven genetic clusters (Fig. 2). Posterior 6 and E were moderate. Nevertheless, the two clusters were separable based on the result of 133 STRUCTURE.

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The migration rates estimated by BayesAss, i.e., the fraction of individuals in a population 135 that are migrants derived from another population per generation, indicated that the 136 metapopulations from Nanjing and Shenyang exhibited more current gene flows than the 137 metapopulations from other sampled cities (Fig. 3). The values of strength were also much higher 138 at Nanjing and Shenyang than those of the remaining cities. Migration rates between paired cities 139 can be found in Supplementary

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The trade data showed that the states in the modern North American cluster 2 exported more grain 168 products to China than those in the modern North American cluster 1 ( Table 3). Importation of 169 grains in the 15 cities showed that Nanjing and Qingdao received the most grain products from

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We also found that introductions occurred more frequently in coastal areas than in inland   China are processed into food in factories 44 . Many opportunities exist for common ragweed 233 seeds to escape into the urban environment in this process. Also, the dispersal of common 234 ragweed between cities is primarily dependent on transportation. While we could not entirely rule 235 out natural dispersal, the chance that natural dispersal serves as the primary dispersal means 236 among cities is low because the natural dispersal distance of common ragweed seed was less than Based on the occurrence data collected in rural areas, previous studies listed Wuhan, 240 Nanjing, and Shenyang as regions with a high dispersal potential of common ragweed 19 . Our 241 result did not show that Wuhan had a high dispersal potential of common ragweed. Except for the 242 difference in urban and rural environments, two potential reasons might cause the discrepancy.        Table 3).

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In-house ROX-labeled size standards were used for genotyping 53 . GENEMarker version 1.5 was