Canola, Rapeseed, and Mustard: For Biofuels and Bioproducts - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 135

Fig. 7.1 Triangle of U (1935) (Reprinted from Triangle of U Simple1. Wikipedia. http://en.
wikipedia.org/wiki/File:Triangle_of_U_Simple1.PNG . Last accessed on February 26, 2013. With
permission from Creative Commons License)

10,000 years or less in areas where the natural range of distribution for the parent
species overlapped in the wild or agricultural settings [ 24 , 25 ].
Studies of cytoplasmic genetic diversity indicate that the amphidiploid culti-
vated species originated several times from independent interspecific crosses [ 26 ,
27 ]. Further, the cytoplasmic diversity studies suggest that the interspecific hybrid-
ization events giving rise to the amphidiploids occurred mostly unidirectionally
[ 27 ]. Brassica juncea has the cytoplasm of B. rapa , while B. carinata has the
cytoplasm of B. nigra [ 28 ]. Brassica napus has a cytoplasm which is most similar to
that found in B. oleracea ssp. r obertiana , a wild species of B. oleracea found in the
Mediterranean region [ 27 ].
The evolutionary origins of Brassica species are complex and uncertain. Some
cytological studies of Brassica species suggest a common ancestor with a base
chromosome number of six once existed [ 29 , 30 ]. Other cytological and/or molec-
ular studies of Brassica species suggest an ancestor with a base chromosome
number of seven or eight [ 31 , 32 ]. More recent molecular studies suggest that the
Brassica species may share a common ancestor with the weedy species Arabidopsis
thaliana (L.) Heynh. ( n 5) and that several chromosome rearrangements and

ploidy level changes have occurred during the evolution of the Brassica species

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