Brassica taxonomy based on nuclear restriction fragment length polymorphisms (RFLPs)

Summary Restriction fragment length polymorphisms (RFLPs) of nuclear DNAs have been used to explore the origin and evolution of the six cultivated Brassica species. Extensive RFLP variation was found at the species, subspecies and variety levels. Based on RFLP data from Brassica and related genera, a detailed phylogenetic tree was generated using the PAUP microcomputer program, which permits a quantitative analysis of the interrelationships among Brassica species. The results suggested that 1) B. nigra originated from one evolutionary pathway with Sinapis arvensis or a close relative as the likely progenitor, whereas B. campestris and B. oleracea came from another pathway with a possible common ancestor in wild B. oleracea or a closely related nine chromosome species; 2) the amphidiploid species B. napus and B. juncea have evolved through different combinations of the diploid morphotypes and thus polyphyletic origins may be a common mechanism for the natural occurrence of amphidiploids in Brassica; 3) the cytoplasm has played an important role in the nuclear genome evolution of amphidiploid species when the parental diploid species contain highly differentiated cytoplasms. A scheme for the origins of diploid and amphidiploid species is depicted based on evidence gathered from nuclear RFLP analysis, cpDNA RFLP analysis, cytogenetic studies and classical taxonomy.