2 Origin and domestication

Publisher Summary This chapter discusses the evolutionary origin of the members of Brassica coenospecies. Alloploid species— B. carinata , B. juncea and B. napus —originated multiple natural interspecific hybridizations. Natural hybridizations are always unidirectional as revealed by the studies on Fraction-I protein and cp DNA restriction patterns, which established that B. nigra and B. rapa are the cytoplasmic donors of B. carinata and B. juncea respectively, while in B. napus, there is a slightly altered B. oleracea cytoplasm. The close similarity in cp DNA of diploids and alloploids suggests that alloploids are of recent origin. A survey of rDNA of the allotetraploid species led to propose that B. juncea was the first to evolve and B. napus and B. carinata originated later. This agrees with the delayed entrance of B. oleracea in the agricultural world. Amount of DNA in tetraploids has not changed significantly since their origin, though there has been a reduction in nuclear size, probably because of the higher DNA density resulting from greater condensation of chromosome material. Nuclear DNA composition of alloploid species is closely related to the maternal cytoplasmic donors than to the male parents.