Conservation of the microstructure of genome segments in Brassica napus and its diploid relatives.

The cultivated Brassica species are the group of crops most closely related to Arabidopsis thaliana (Arabidopsis). They represent models for the application in crops of genomic information gained in Arabidopsis and provide an opportunity for the investigation of polyploid genome formation and evolution. The scientific literature contains contradictory evidence for the dynamics of the evolution of polyploid genomes. We aimed at overcoming the inherent complexity of Brassica genomes and clarify the effects of polyploidy on the evolution of genome microstructure in specific segments of the genome. To do this, we have constructed bacterial artificial chromosome (BAC) libraries from genomic DNA of B. rapa subspecies trilocularis (JBr) and B. napus var Tapidor (JBnB) to supplement an existing BAC library from B. oleracea. These allowed us to analyse both recent polyploidization (under 10,000 years in B. napus) and more ancient polyploidization events (ca. 20 Myr for B. rapa and B. oleracea relative to Arabidopsis), with an analysis of the events occurring on an intermediate time scale (over the ca. 4 Myr since the divergence of the B. rapa and B. oleracea lineages). Using the Arabidopsis genome sequence and clones from the JBr library, we have analysed aspects of gene conservation and microsynteny between six regions of the genome of B. rapa with the homoeologous regions of the genomes of B. oleracea and Arabidopsis. Extensive divergence of gene content was observed between the B. rapa paralogous segments and their homoeologous segments within the genome of Arabidopsis. A pattern of interspersed gene loss was identified that is similar, but not identical, to that observed in B. oleracea. The conserved genes show highly conserved collinearity with their orthologues across genomes, but a small number of species-specific rearrangements were identified. Thus the evolution of genome microstructure is an ongoing process. Brassica napus is a recently formed polyploid resulting from the hybridization of B. rapa (containing the Brassica A genome) and B. oleracea (containing the Brassica C genome). Using clones from the JBnB library, we have analysed the microstructure of the corresponding segments of the B. napus genome. The results show that there has been little or no change to the microstructure of the analysed segments of the Brassica A and C genomes as a consequence of the hybridization event forming natural B. napus. The observations indicate that, upon polyploid formation, these segments of the genome did not undergo a burst of evolution discernible at the scale of microstructure.

Rana D ，van den Boogaart T ，O'Neill CM ，Hynes L ，Bent E ，Macpherson L ，Park JY ，Lim YP ，Bancroft I ... -  《PLANT JOURNAL》

Sequence-level comparative analysis of the Brassica napus genome around two stearoyl-ACP desaturase loci.

Cho K ，O'Neill CM ，Kwon SJ ，Yang TJ ，Smooker AM ，Fraser F ，Bancroft I ... -  《-》

Genome evolution in Arabidopsis/Brassica: conservation and divergence of ancient rearranged segments and their breakpoints.

Since the tetraploidization of the Arabidopsis thaliana ancestor 30-35 million years ago (Mya), a wave of chromosomal rearrangements have modified its genome architecture. The dynamics of this process is unknown, as it has so far been impossible to date individual rearrangement events. In this paper, we present evidence demonstrating that the majority of rearrangements occurred before the Arabidopsis-Brassica split 20-24 Mya, and that the segmental architecture of the A. thaliana genome is predominantly conserved in Brassica. This finding is based on the conservation of four rearrangement breakpoints analysed by fluorescence in situ hybridization (FISH) and RFLP mapping of three A. thaliana chromosomal regions. For this purpose, 95 Arabidopsis bacterial artificial chromosomes (BACs) spanning a total of 8.25 Mb and 81 genetic loci for 36 marker genes were studied in the Brassica oleracea genome. All the regions under study were triplicated in the B. oleracea genome, confirming the hypothesis of Brassica ancestral genome triplication. However, whilst one of the breakpoints was conserved at one locus, it was not at the two others. Further comparison of their organization may indicate that the evolution of the hexaploid Brassica progenitor proceeded by several events, separated in time. Genetic mapping and reprobing with rDNA allowed assignment of the regions to particular Brassica chromosomes. Based on this study of regional organization and evolution, a new insight into polyploidization/diploidization cycles is proposed.

Ziolkowski PA ，Kaczmarek M ，Babula D ，Sadowski J ... -  《PLANT JOURNAL》

Comparative physical mapping of segments of the genome of Brassica oleracea var. alboglabra that are homoeologous to sequenced regions of chromosomes 4 and 5 of Arabidopsis thaliana.

O'Neill CM ，Bancroft I 《PLANT JOURNAL》

Identifying the chromosomes of the A- and C-genome diploid Brassica species B. rapa (syn. campestris) and B. oleracea in their amphidiploid B. napus.

Snowdon RJ ，Friedrich T ，Friedt W ，Köhler W ... -  《-》