MAPPING THE BRASSICA GENOME

The six cultivated species of Brassica furnish a wide range of crop types (including oilseed, vegetable and fodder crops) which seem quite different when observed under normal cultivation (Figure 1). However, Brassica species and a large number of other wild and cultivated species are all closely related (Figure 2) and genetic exchange through sexual crosses is possible across most of this very extensive gene pool. Traditionally, the investigation of genome organization in plants has employed cytology to study chromosomes and genetic markers to define linkage groups. Cytology is difficult in Brassica because the chromosomes are small, but the genus is very amenable to investigations using molecular-genetic markers because of the high degree of natural polymorphism. Gene homology and the general structure of the genome seems to be conserved between Brassica and related genera and modern marker technologies are freely interchangeable across this group. However, the collinearity of related chromosomes in different Brassica species has been disrupted frequently by chromosomal translocations. Thus Brassica species have quite distinct genetic maps, in contrast to cereal species where collinear homoeologous chromosomes are the general rule. The mapping of the Brassica genome will have a considerable impact on the breeding of Brassica crops. In particular, it will facilitate the transfer of beneficial genes between species and the rapid introgression of genes from wild plants into useful cultivars. These improvements in breeding should be translated into crops which are more easily adapted to suit the needs of new agronomic practices and the demands of a changing environment.