Polyploidization has provided much genetic variation for plant adaptive evolution, but the mechanisms by which the molecular evolution of polyploid genomes establishes genetic architecture underlying species differentiation are unclear. Brassica is an ideal model to increase knowledge of polyploid evolution. Here we describe a draft genome sequence of Brassica oleracea, comparing it with that of its sister species B. rapa to reveal numerous chromosome rearrangements and asymmetrical gene loss in duplicated genomic blocks, asymmetrical amplification of transposable elements, differential gene co-retention for specific pathways and variation in gene expression, including alternative splicing, among a large number of paralogous and orthologous genes. Genes related to the production of anticancer phytochemicals and morphological variations illustrate consequences of genome duplication and gene divergence, imparting biochemical and morphological variation to B. oleracea. This study provides insights into Brassica genome evolution and will underpin research into the many important crops in this genus.

BackgroundPlant disease resistance (R) genes with the nucleotide binding site (NBS) play an important role in offering resistance to pathogens. The availability of complete genome sequences of Brassica oleracea and Brassica rapa provides an important opportunity for researchers to identify and characterize NBS-encoding R genes in Brassica species and to compare with analogues in Arabidopsis thaliana based on a comparative genomics approach. However, little is known about the evolutionary fate of NBS-encoding genes in the Brassica lineage after split from A. thaliana.ResultsHere we present genome-wide analysis of NBS-encoding genes in B. oleracea, B. rapa and A. thaliana. Through the employment of HMM search and manual curation, we identified 157, 206 and 167 NBS-encoding genes in B. oleracea, B. rapa and A. thaliana genomes, respectively. Phylogenetic analysis among 3 species classified NBS-encoding genes into 6 subgroups. Tandem duplication and whole genome triplication (WGT) analyses revealed that after WGT of the Brassica ancestor, NBS-encoding homologous gene pairs on triplicated regions in Brassica ancestor were deleted or lost quickly, but NBS-encoding genes in Brassica species experienced species-specific gene amplification by tandem duplication after divergence of B. rapa and B. oleracea. Expression profiling of NBS-encoding orthologous gene pairs indicated the differential expression pattern of retained orthologous gene copies in B. oleracea and B. rapa. Furthermore, evolutionary analysis of CNL type NBS-encoding orthologous gene pairs among 3 species suggested that orthologous genes in B. rapa species have undergone stronger negative selection than those in B .oleracea species. But for TNL type, there are no significant differences in the orthologous gene pairs between the two species.ConclusionThis study is first identification and characterization of NBS-encoding genes in B. rapa and B. oleracea based on whole genome sequences. Through tandem duplication and whole genome triplication analysis in B. oleracea, B. rapa and A. thaliana genomes, our study provides insight into the evolutionary history of NBS-encoding genes after divergence of A. thaliana and the Brassica lineage. These results together with expression pattern analysis of NBS-encoding orthologous genes provide useful resource for functional characterization of these genes and genetic improvement of relevant crops.Electronic supplementary materialThe online version of this article (doi:10.1186/1471-2164-15-3) contains supplementary material, which is available to authorized users.

BackgroundThe species Brassica rapa (2n=20, AA) is an important vegetable and oilseed crop, and serves as an excellent model for genomic and evolutionary research in Brassica species. With the availability of whole genome sequence of B. rapa, it is essential to further determine the activity of all functional elements of the B. rapa genome and explore the transcriptome on a genome-wide scale. Here, RNA-seq data was employed to provide a genome-wide transcriptional landscape and characterization of the annotated and novel transcripts and alternative splicing events across tissues.ResultsRNA-seq reads were generated using the Illumina platform from six different tissues (root, stem, leaf, flower, silique and callus) of the B. rapa accession Chiifu-401-42, the same line used for whole genome sequencing. First, these data detected the widespread transcription of the B. rapa genome, leading to the identification of numerous novel transcripts and definition of 5'/3' UTRs of known genes. Second, 78.8% of the total annotated genes were detected as expressed and 45.8% were constitutively expressed across all tissues. We further defined several groups of genes: housekeeping genes, tissue-specific expressed genes and co-expressed genes across tissues, which will serve as a valuable repository for future crop functional genomics research. Third, alternative splicing (AS) is estimated to occur in more than 29.4% of intron-containing B. rapa genes, and 65% of them were commonly detected in more than two tissues. Interestingly, genes with high rate of AS were over-represented in GO categories relating to transcriptional regulation and signal transduction, suggesting potential importance of AS for playing regulatory role in these genes. Further, we observed that intron retention (IR) is predominant in the AS events and seems to preferentially occurred in genes with short introns.ConclusionsThe high-resolution RNA-seq analysis provides a global transcriptional landscape as a complement to the B. rapa genome sequence, which will advance our understanding of the dynamics and complexity of the B. rapa transcriptome. The atlas of gene expression in different tissues will be useful for accelerating research on functional genomics and genome evolution in Brassica species.