Molecular markers have been used to examine the population biology and dynamics of hybridisation in Britain, of some invasive weeds from the genus Fallopia: F. japonica var. japonica and F. sachalinensis. Both species are gynodioecious, but in Britain, only female plants of F. japonica var. japonica are present. Molecular studies using RAPDs (randomly amplified polymorphic DNA) suggest that the entire British population of this taxon consists of a single genet. The same genotype has also been recovered in all samples examined from Europe and North America. F. sachalinensis. on the other hand, showed higher levels of clonal diversity consistent with sexual reproduction.\ud The two species hybridise in Britain, and the hybrid (Fallopia x bohemica) shows partial to full fertility. High levels of clonal diversity were detected using RAPDs and inter-SSRs (inter-simple sequence repeats), probably reflecting a combination of multiple origins and hybrid fertility.\ud A study of chloroplast RFLPs (restriction fragment length polymorphisms) has shown that the mode of chloroplast inheritance appears to be predominantly maternal, and Fallopia japonica var. japonica is the female parent of all of its hybrids.\ud Based on RAPD data, evidence for backcrossing between Fallopia x bohemica and F. japonica var. japonica was detected at two sites. Flowering material at one site showed the putative backcross to be male fertile. Such plants are of interest as they are possible intermediate steps in the acquisition of genes restoring male fertility to F. japonica var. japonica.\ud The ability of an already invasive species to reproduce and disperse by seed as well as by vegetative propagation, may serve to enhance its invasive capabilities.\ud Molecular data based on chloroplast trnL intron sequences and RAPDs support previous morphological evidence that introduced material of at least Fallopia japonica var. japonica is different from Asiatic stock
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