MAPPING THE BRASSICA GENOME

The six cultivated species of Brassica furnish a wide range of crop ty pes (including oilseed, vegetable and fodder crops) which seem quite d ifferent when observed under normal cultivation (Figure 1). However, B rassica species and a large number of other wild and cultivated specie s are all closely related (Figure 2) and genetic exchange through sexu al crosses is possible across most of this very extensive gene pool. T raditionally, the investigation of genome organization in plants has e mployed cytology to study chromosomes and genetic markers to define li nkage groups. Cytology is difficult in Brassica because the chromosome s are small, but the genus is very amenable to investigations using mo lecular-genetic markers because of the high degree of natural polymorp hism. Gene homology and the general structure of the genome seems to b e conserved between Brassica and related genera and modern marker tech nologies are freely interchangeable across this group. However, the co llinearity of related chromosomes in different Brassica species has be en disrupted frequently by chromosomal translocations. Thus Brassica s pecies have quite distinct genetic maps, in contrast to cereal species where collinear homoeologous chromosomes are the general rule. The ma pping of the Brassica genome will have a considerable impact on the br eeding of Brassica crops. In particular, it will facilitate the transf er of beneficial genes between species and the rapid introgression of genes from wild plants into useful cultivars. These improvements in br eeding should be translated into crops which are more easily adapted t o suit the needs of new agronomic practices and the demands of a chang ing environment.