Considerable genetic diversity is present within the three amphidiploid species (Song et al., 1996). Based upon studies of genetic diversity, B. napus may be considered as the most ancient amphidiploid, succeeded by B. juncea and B. carinata. Two major factors are responsible for general diversity within amphidiploids: multiple hybridizations with different diploid parents and genome modifications following polyploidization. A good example of multiple hybridizations was found in B. napus. Four cytoplasmic types developed in a group of B. napus accessions; these matched with different parental diploid
cytoplasms. Direct evidence for genome changes after polyploidization was obtained by studying synthetic amphidiploids (Song et al., 1993).
Artificial synthesis of three analogues of B. napus, B. juncea and B. carinata permitted their analysis for morphological traits, chromosome number and restriction fragment length polymorphisms (RFLPs) in chloroplast, mitochondrial and nuclear DNA clones, and comparison with the natural amphidiploids. This showed that the synthetic hybrids were closer to their diploid parents as compared with the polyploids. Genome changes, mainly involving either loss or gain of parental fragments and novel fragments, were seen in the early generations of the four synthetic amphidiploids. The frequency of genome change and the direction of its evolution in the synthetic hybrids were associated with divergence between the parental diploid species. Rapid genome changes resulted in faster divergence among the derivatives of synthetic amphidiploids which, by providing raw materials for selection, played an important role in the evolutionary success of Brassica amphidiploids and many polyploid lineages. Based on RFLP data from Brassica, Song et al. (1988) generated a phylogenetic tree that permitted the quantitative analysis of the relationships between Brassica species. Results from this study suggest the following:
This research has generated a hypothetical scheme for the possible evolution of Brassica species and their subsequent domesticated variants (Fig. 1.3).
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