A triallelic genetic male sterility locus in Brassica napus: an integrative strategy for its physical mapping and possible local chromosome evolution around it.

BACKGROUND AND AIMS

Spontaneous male sterility is an advantageous trait for both constructing efficient pollination control systems and for understanding the developmental process of the male reproductive unit in many crops. A triallelic genetic male-sterile locus (BnMs5) has been identified in Brassica napus; however, its complicated genome structure has greatly hampered the isolation of this locus. The aim of this study was to physically map BnMs5 through an integrated map-based cloning strategy and analyse the local chromosomal evolution around BnMs5.

METHODS

A large F(2) population was used to integrate the existing genetic maps around BnMs5. A map-based cloning strategy in combination with comparative mapping among B. napus, Arabidopsis, Brassica rapa and Brassica oleracea was employed to facilitate the identification of a target bacterial artificial chromosome (BAC) clone covering the BnMs5 locus. The genomic sequences from the Brassica species were analysed to reveal the regional chromosomal evolution around BnMs5.

KEY RESULTS

BnMs5 was finally delimited to a 0·3-cM genetic fragment from an integrated local genetic map, and was anchored on the B. napus A8 chromosome. Screening of a B. napus BAC clone library and identification of the positive clones validated that JBnB034L06 was the target BAC clone. The closest flanking markers restrict BnMs5 to a 21-kb region on JBnB034L06 containing six predicted functional genes. Good collinearity relationship around BnMs5 between several Brassica species was observed, while violent chromosomal evolutionary events including insertions/deletions, duplications and single nucleotide mutations were also found to have extensively occurred during their divergence.

CONCLUSIONS

This work represents major progress towards the molecular cloning of BnMs5, as well as presenting a powerful, integrative method to mapping loci in plants with complex genomic architecture, such as the amphidiploid B. napus.