Brassica napus L. (genome AACC, 2n = 4x = 38) is a recent allopolyploid species, formed during the past ~7500 years by interspecific hybridization between B. rapa (AA, 2n = 2x = 20) and B. oleracea (CC, 2n = 2x = 18). The genus Brassica contains three basic genomes (A, B and C) that form three diploid species, namely, B. rapa (AA genome), B. nigra (BB genome), and B. oleracea (CC genome), which further hybridize to give rise to three allopolyploid species, namely, B. napus (AACC genome), B. juncea (AABB genome), and B. carinata (BBCC genome), as described by the triangle of U model. It is not known in the wild and probably originated in the eastern Mediterranean and West Asian region. It was introduced to western Europe where several types were developed. Rape kale is a minor leafy vegetable in western Europe and a rather important fodder during winter. It is of some importance in southern Africa, where it was introduced during colonial times. Swede (the thickened upper taproot and lower stem) is a minor vegetable in western Europe and in North America, on rare occasions found in southern Africa. In northern Mali and in oases in the Sahara an ancient introduction of swede is still grown. Fodder swede is occasionally grown in western Europe, but not recorded in tropical Africa. Oilseed rape was originally grown in India for its edible oil; later it became important as an industrial and lamp oil in Europe. Cultivars with a much improved quality edible oil have been developed from B. napus, B. juncea (L.) Czern. and B. rapa L. (collectively known as ‘canola’), and these have become important oil crops in Europe, North America, China, Japan and India. Oilseed rape is also grown in the cooler highlands of Kenya and Tanzania, and reported as a minor oil crop from Ethiopia. Here and there, B. napus is found as an escape from cultivation, but less common than B. rapa. Besides, diversification of selection has also affected various characteristics of B. napus, such as winter hardiness, vernalization requirement, and photoperiod-responsive flowering, thereby leading to the selection of three main types:
- a spring-sown type that is commonly grown in northern Canada and Scandinavia;
- a semi-winter-type that is common in subtropical areas of Asia, and southern and western Australia;
- a winter-type that is grown in Europe.
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