Toward unraveling the structure of Brassica rapa genome

Genomic research in any organism encompasses understanding structure of the target genome and genes, their function, and evolution. Brassica rapa, which is phylogenetically related to Arabidopsis thaliana, is an important species with respect to its uses as vegetable, oil, and fodder. The availability of suitable genetic and genomic resources is a prerequisite to undertake genomic research in B. rapa. We have developed reference mapping populations of Chinese cabbage (B. rapa ssp. pekinensis) comprising 78 doubled haploid lines and over 250 recombinant inbred lines. Two Bacterial Artificial Chromosome (BAC) libraries, generated by restriction enzymes HindIII (KBrH) and BamHI (KBrB), comprise 56 592 and 50 688 clones, respectively. We have also constructed 22 cDNA libraries from different plant tissues consisting of 104 914 clones with an average length of 575 bp. Initial BAC-end sequence analysis of 1473 clones of the KBrH library led us to understand the structure of B. rapa genome with respect to extent of genic sequences and their annotation, and relative abundance of different types of repetitive DNAs. Full-length sequence analysis of BAC clones revealed extensive triplication of B. rapa DNA segments coupled with variable gene losses within the segments. The formulation of the ‘Multinational Brassica Genome Project’ has laid the foundation to sequence the complete genome of B. rapa ssp. pekinensis by the international Brassica research community. It has been proposed to undertake BAC-to-BAC sequencing of genetically mapped seed BACs. In recent years, development of bioinformatics tools in Brassica has given a boost to structural genomics research in Brassica species. The research undertaken with the availability of various genomic resources in the public domain has added to our understanding of the structure of B. rapa.