The genome of the mesopolyploid crop species Brassica rapaThe Brassica rapa Genome Sequencing Project Consortium 1 Abstract:The Brassicaceae family which includes Arabidopsis thaliana, is a natural priority for reaching beyond botanical models to more deeply sample angiosperm genomic and functional diversity. Here we report the draft genome sequence and its annoation of Brassica rapa, one of the two ancestral species of oilseed rape. We modeled 41,174 protein-coding genes in the B. rapa genome. B. rapa has experienced only the second genome triplication reported to date, with its close relationship to A. thaliana providing a useful outgroup for investigating many consequences of triplication for its structural and functional evolution. The extent of gene loss (fractionation) among triplicated genome segments varies, with one copy containing a greater proportion of genes expected to have been present in its ancestor (70%) than the remaining two (46% and 36%). Both a generally rapid evolutionary rate, and specific copy number amplifications of particular gene families, may contribute to the remarkable propensity of Brassica species for the development of new morphological variants. The B. rapa genome provides a new resource for comparative and evolutionary analysis of the Brassicaceae genomes and also a platform for genetic improvement of Brassica oil and vegetable crops.2

*These authors contributed equally to this work DatabaseThe following have been deposited to the GenBank database. Accession numbers are shown in parenthesis:

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BackgroundThe genus Brassica includes the most extensively cultivated vegetable crops worldwide. Investigation of the Brassica genome presents excellent challenges to study plant genome evolution and divergence of gene function associated with polyploidy and genome hybridization. A physical map of the B. rapa genome is a fundamental tool for analysis of Brassica "A" genome structure. Integration of a physical map with an existing genetic map by linking genetic markers and BAC clones in the sequencing pipeline provides a crucial resource for the ongoing genome sequencing effort and assembly of whole genome sequences.ResultsA genome-wide physical map of the B. rapa genome was constructed by the capillary electrophoresis-based fingerprinting of 67,468 Bacterial Artificial Chromosome (BAC) clones using the five restriction enzyme SNaPshot technique. The clones were assembled into contigs by means of FPC v8.5.3. After contig validation and manual editing, the resulting contig assembly consists of 1,428 contigs and is estimated to span 717 Mb in physical length. This map provides 242 anchored contigs on 10 linkage groups to be served as seed points from which to continue bidirectional chromosome extension for genome sequencing.ConclusionThe map reported here is the first physical map for Brassica "A" genome based on the High Information Content Fingerprinting (HICF) technique. This physical map will serve as a fundamental genomic resource for accelerating genome sequencing, assembly of BAC sequences, and comparative genomics between Brassica genomes. The current build of the B. rapa physical map is available at the B. rapa Genome Project website for the user community.