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II.3. BRASSICA CROPS (BRASSICA SPP.) – 155

The difficulty in clearly separating the genera and species among the Brassica and

their close relatives has arisen because similar plant forms and morphological traits occur

in more than one genus or species. The difficulties encountered by early taxonomists in

separating and classifying the various species and forms within the Brassicaceae family

are well documented by Hedge (1976) and Prakash and Hinata (1980). As a result, there

have been numerous changes and modifications to Schulz’s (1919; 1936) original species

names and arrangement. The cytological studies by Morinaga (1928; 1929; 1931; 1933;

1934a; 1934b) and his student, U (1935) clarified the broad relationships among the

economically important Brassica species in which chromosome pairing clearly showed

the three species with the higher chromosome number, B. napus, B. juncea and

B. carinata are amphidiploids derived from the monogenomic or basic species, B. nigra,

B. rapa and B. oleracea (Figure 3.1).

Figure 3.1. Genome relationships of Brassica species and allied genera

Notes: A, B, C... are the genome symbols. The number in brackets following the haploid chromosome number
(n) indicates the maximum possible number of autosyndetic^2 chromosome pairs. The numbers within lines
connecting two genomes give the maximum allosyndesis, i.e. the number of bivalents possible between the
respective interspecific hybrids (Downey and Röbbelen, 1989).

Source: Modified from Mizushima (1980).

The genome relationships among the amphidiploids were confirmed by resynthesis of

the three species from their diploid parents (Frandsen, 1947, 1943; Ramanujam and

Srinivasachar, 1943). Further verification of these species’ relationships were obtained

from studies on phenolic compounds (Dass and Nybom, 1967), protein patterns

(Vaughan, 1977), isozymes (Coulthart and Denford, 1982; Chen, Heneen and Simonsen,

1989) and nuclear DNA, restriction fragment length polymorphisms (RFLP; Song,

Osborn and Williams, 1988a; 1988b). Additional verification has been achieved through