Genetic diversity and phylogeography of Japanese brome (Bromus japonicus Thunb. ex Murr.) populations in China

Japanese brome (Bromus japonicus Thunb. ex Murr.) is a pervasive annual weed with wide distribution in winter wheat fields across the North China Plain. In this study, we researched the genetic diversity and phylogeography of 24 B. japonicus populations in China based on total genomic DNA and chloroplast DNA. The results showed that 106 fragments were scored using 12 inter-simple sequence repeat (ISSR) primers, and 101 fragments (95.28%) were polymorphic. The findings revealed substantial genetic diversity and differentiation among populations from different locations (Ht = 0.2125, Hs = 0.0730, Gst = 0.6562 and Nm = 0.2619). Mean values of Nei's genetic diversity (H) and Shannon index of diversity (I) were 0.0731 and 0.1068, respectively. In addition, 15 haplotypes were identified based on combined cpDNA regions among the 24 populations exhibiting abundant haplotype (gene) diversity and nucleotide diversity. The AMOVA based on ISSR and cpDNA both showed that genetic variation mainly exists among populations rather than within them. The STRUCTURE analysises of ISSR and cpDNA indicated that geographical location and genetic relationship had no significant correlation. The haplotype network also illustrated that the widespread haplotypes (H1, H2) might represent ancient polymorphism. The results obtained in this study demonstrate the presence of extensive genetic variability among and within the Chinese populations of B. japonicus, which is likely to contribute significantly to its adaptability and infestation as a weed species.