The wild Brassica juncea (L.) Czern. et Coss., systematically belonging to the genus Brassica L. in the Cruciferae family, has become a noxious weed for cropping systems nowadays. Here, simple sequence repeat (SSR) markers were applied to investigate the genetic diversity, genetic structure and migration routes of the wild B. juncea populations in China. The results showed that a total of 90 alleles, with extensive allelic diversity, were observed at the 11 SSR loci of the wild B. juncea. The STRUCTURE analysis indicated that all the 25 wild populations were best described as belonging to two lineages. High Fst value (0.568), together with the partitioning, provided significant evidence for lineage differentiation in wild B. juncea. The high differentiation between the two lineages was, perhaps, due to limited gene flow (Nm = 0.301) of this species. The analysis of molecular variance with distances among individuals corrected for the dominant nature of SSRs showed that most of the variation (59%) occurred within populations, and the remaining 41% variance was attributed to differences among populations. The distribution of diversity across China was significantly geographically dependent. NJ cluster analysis, based on genetic distance, grouped populations geographically, which further corroborated spatial pattern of two lineages. Based on these results, two routes were proposed for the migration of wild B. juncea in China after its origin from northwest China, heading east along the Yellow River or Yangtze River, respectively. We concluded that China, especially the northwest, is one of the primary origins of B. juncea.
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Brassica juncea varieties were kindly provided by Jinghua Yang in Zhejiang University. This research was financially supported by the Ability Improvement Project of Jiangsu Social Scientific Research Institutions (#BM2015019) granted to X. Q. Sun, the Youth Foundation of Jiangsu Province (BK20160602) and the National Natural Science Foundation of China (X.L.S, 31270579).
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Conflict of interests
The authors declare that they have no conflict of interest.
Fig. S1Photos of plants of wild B. juncea populations in China. a. JNT; b. ACH; c. ACZ; d. HLY; e. HZK; f. SZA; g. SHZ; h. QHL; i. QXN; j. ZJH; k. HSY; l. HJZ; m. GGY; n. GAS; o. JJP. Acronyms refer to Table 1 (JPEG 2160 kb)
Fig. S2Wild B. juncea usually grown alongside the farmland, accompanied by Brassica crops. Wild B. juncea (indicated by white arrowheads) in a. ZJH sampling site and b. JWY sampling site. Acronyms refer to Table 1 (JPEG 427 kb)
Fig. S3UPGMA tree based on RPS2 sequence of wild B. juncea and its relatives. The RPS2 sequence of Arabidopsis thaliana was used as outgroup (TIFF 232 kb)
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