Brassica Genomics



 Our primary interest has been to understand the structure and evolution of the Brassica genomes and applying this information to crop genetic improvement, glucosinolate and isothiocyanate content in particular.

Our comparative genomics activities in the past includes development of linkage maps on the basis of DNA-based markers and gene sequences for the species containing the A ( B. rapa , 2n=2x=20), B ( B. nigra 2n=2x=16), and C ( B. oleracea 2n=2x=18) genomes. 

As part of this work we constructed cytogenetic stocks, such as alien addition lines and used them for physical mapping and assignment of C and B genomes linkage groups to their respective chromosomes.

We have consolidated Brassica oleracea maps developed independently by four different laboratories, which allows a more efficient utilization of this information to breeding and genetics of cole crops.

The distribution of the gene complexes on various chromosomes in the three Brassica genomes allows detection of intra and intergenomic chromosomal homology.

We developed a new marker system called sequence related amplified polymorphism (SRAP), simpler that AFLPs but with similar efficiency, which allow us to construct rapidly and effectively linkage maps based on both genomic DNA and cDNA. Based on this technique we have cloned several major glucosinolate genes and have used them for marker-assisted selection for glucosinolate content.

    Based on the mapping data disclosing extensive loci duplication, it is clear that the three diploid Brassica cultivated species are indeed ancient polyploids. It is evident that extensive chromosome re-patterning has taken place during the evolution of Brassica species, even though there is considerable conservation among certain chromosome regions within and among the three genomes.  This results in complex intra- and inter-genomic chromosomal relationships where gene and marker colinearity is maintained for some segments, but broken up for other chromosomal regions.  The complexity of the existing chromosomal relationships discards the possibility of autopolyploidy as an explanation for the higher chromosome numbers observed today in the cultivated genomes. Data gathered from various laboratories point to chromosomal rearrangements, and hybridization followed by aneuploidy as the main events involved in the origin of the Brassica genomes. There is however, disagreement on whether the number founder species constituting each these genomes was two or three species. We have proposed the hypothesis called "cyclic amphiploidy" that is based on the hybridization of different pairs of species containing x=4 and/or 5 chromosomes derived from an ancestral Brassica genome of similar chromosome number but unknown ancestry. Each pair of species originated the different Brassica A, B and C genomes. The ancestral cytotypes originating the cultivated genomes likely arose as a result of chromosomal structural modifications due to differential evolutionary forces caused by spatial isolation of the species containing the ancestral genome. Because of their ancestral common origin, the three cultivated genomes have conserved chromosome segments and extensive duplications. After genomic stabilization, the species containing these genomes have generated by another cycle of hybridization the cultivated allotetraploid species we know today. 

Comparative genomics of Brassica and Arabidopsis: We have sequenced three complete BAC clones of B. oleracea B21H13,  B19N3,   B21F5  and compared them to their corresponding regions in A. thaliana. As a general rule, there is less gene density in Brassica due to larger spacer caused by the insertion of transposable elements. However, the gene content in both genomes seems to be similar.

As an application of our work, we are cloning the major genes involved in the aliphatic glucosinolate pathway in B. oleracea, and B. rapa. The objective is to manipulate the content and type of glucosinolates in Brassica. This will allow to maximize content of glucoraphanin, which is the precursor of sulforaphane, a potent anticarcinogenic found in some varieties of broccoli and other cole crops. Identification and isolation of genes involved in the aliphatic glucosinolate pathway will allow engineering varieties containing optimal amounts of this compound. We used B. macrocarpa, a close relative from B. oleracea to develop broccoli lines with hi sulforaphane content.  

 


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Last update: July 2011