Origin of these plants

This lab illustrates the phenotypic variation that has arisen by (artificial) selection and hybridization within a group of related species. According to the triangle of U (see figure below) cultivated Brassicas have arisen from three ancestral species, B. nigra, B. oleracea, and B. rapa. The hybrids between these are sterile but, after chromosome doubling, the allotetraploids, B. juncea, B. carinataand B. napus are fertile. Also it is possible to hybridize radish, Raphanus sativus with B. oleracea to produce Raphanobrassica; once again chromosome doubling results in a fertile allotetraploid (which finds some use as a fodder crop).

The cytogenetic interrelationships among six Brassica species and Raphanus sativus. Intergeneric crosses between R. sativus and other Brassica species are also possible. Cytoplasmic genome is designated by capitals. Nuclear genome is designated by lower case letters, where a=10 chromosomes; b=8 chromosomes; c and r=9 chromosomes.

Present day varieties

B. rapa and B. oleracea include many familiar and not so familiar vegetables which are represented in the lab today. B. juncea includes the common yellow seed mustard as well as mustard greens; B. nigra is black mustard and B. carinata is Ethiopian mustard. Oil-seed rape and canola are forms of B. napus. The economic value of most of the plants is as edible crops (for humans or animals) or sources of oil; however, there are a few ornamental types among the cabbages and kales. The rapid cycling B. rapa which we have used in several labs was obtained by selection and breeding from a worldwide collection of 2000 forms held by the United States Department of Agriculture.

In lab today

Try to figure out how the ancestral plants have been modified to derive the vegetables on display in the following terms:

Stem: normal (long internodes), compressed (short internodes - rosette form)

Leaves: normal (flat lamina), undulate (wavy lamina), entire, dissected (pinnatifid)

Edible part: what has been enlarged? - root, hypocotyl, stem, axillary or terminal bud, petiole, lamina, flower buds.