Brassica Genomics
We are interested primarily in understanding the
genome structure and evolution of the Brassica
genomes and applying this information to crop genetic improvement.
Our comparative genomics
activities in the past includes development of linkage maps on the basis of DNA-based
markers 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. The data obtained from mapping the genes of these
complexes, as well as other RFLPs A genome map, B. genome and C genome grp 1-4 / grp 5-9 detected with the same
set of probes.
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
(transcriptone maps). Based on this technique we have
cloned two major glucosinolate genes and have
identified a third candidate gene, as explained below.
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 reiteration of chromosomes within the
genomes agrees with the hypothesis that the existing Brassica genomes derive from a
smaller ancestral genome. Mapping data from various laboratories indicate that
it is unlikely that the Brassica genomes originated by polysomy or
duplication of whole chromosomes. 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, such BoGSL-ALK
and BoGSL-ELONG and BoGSL-PRO. 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.
Last update: January 2006
