Investigation of the origin and transmission of linear mitochondrial plasmid based on phylogenetic analysis in Japanese rapeseed varieties

A linear mitochondrial plasmid is present in some varieties of rapeseed. To elucidate its origin and transmission the author investigated types of mitochondrial genome and the presence of plasmid in 78 rapeseed varieties and landraces in Japan and carried out a comparative analysis using the breeding history of Japanese rapeseed varieties. The mitochondrial genome of rapeseed was classified roughly into 2 types, type I (nap) and type II (cam). Type II rapeseed mitochondria closely resembles that of Brassica rapa , which is a related species of rapeseed. In this study, the author found that all varieties with type II mitochondria originated from interspecific crosses between rapeseed ( B. napus ) and B. rapa. This indicates that type II cytoplasm was introduced to rapeseed through a breeding program. The presence of plasmid was limited to B. rapa landraces and rapeseed varieties that arose by interspecific crosses between B. napus and B. rapa. The results suggest that mitochondrial plasmid is of B. rapa origin and that it has been introduced into rapeseed by interspecific crosses in a modern breeding program, as in the case of the mitochondrial genome. Phylogenetic study of Japanese rapeseed varieties suggests the participation not of the mitochondrial genome but, rather, the nuclear genome for the perpetuation of plasmid in progeny varieties.