Molecular mechanisms underpinning the multiallelic inheritance of MS5 in Brassica napus

The Brassica-specific gene MS5 mediates early meiotic progression, and its allelic variants contribute to a valuable genic male sterility three-line hybrid production system in rapeseed (Brassica napus L.). However, the underlying mechanisms of its triallelic inheritance are poorly understood. Herein, we show that the restorer allele MS5^a and the maintainer allele MS5^c are both necessary for male fertility in B. napus. The functional divergence of MS5^a and MS5^c is strongly related to sequence variations in their coding regions and less strongly to their promoter regions. The male-sterile allele MS5^b encodes a chimeric protein containing only the complete MS5 coiled-coil (CC) domain, having lost the MS5 superfamily domain. Both MS5^a and MS5^c can form homodimers in the nucleus via the CC domain. MS5^b can interact competitively with MS5^a or MS5^c to form non-functional heterodimers. Owing to the close transcript levels of MS5^b and MS5^c in MS5^b MS5^c , these heterodimers induced a dominant-negative effect of MS5^b on MS5^c , resulting in a male-sterile phenotype. The extremely high transcript abundance of MS5^a maintains sufficient MS5^a homodimers in MS5^a MS5^b , causing the recovery of male sterility. These findings provide substantial genetic and molecular evidence to improve our understanding of the mechanisms underlying the multiallelic inheritance of MS5, and enable the construction of a solid foundation for improved use of the MS5-controlled GMS system in Brassica species.

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.

Stable progeny production of the amphidiploid resynthesized Brassica napus cv. Hanakkori, a newly bred vegetable

Resynthesized Brassica napus cv. Hanakkori (AACC, 2n = 38) was produced by cross-hybridization between B. rapa (AA, 2n = 20) and B. oleracea (CC, 2n = 18) as a new vegetative crop. Many studies have provided evidences for the instability and close relationship between A and C genome in the resynthesized B. napus cultivars. In fact, seed produced to obtain progeny in Hanakkori had unstable morphological characters and generated many off-type plants. In this study, we investigated the pollen fertility, chromosome number, structure, and behavior linked to various Hanakkori phenotypes to define factors of unstable phenotypic expression in the progeny. Hanakkori phenotypes were categorized into five types. The results of pollen fertility, chromosome number, and fluorescence in situ hybridization analysis for somatic mitosis cells indicated that the off-type plants had lower pollen fertility, aberrant chromosome number, and structures with small chromosome fragments. Observation of chromosomes at meiosis showed that the meiotic division in off-type plants led to appreciably higher abnormalities than in on-type plants. However, polyvalent chromosomes were observed frequently in both on- and off-type plants in diplotene stage of meiosis. We assume that the unstable morphological characters in resynthesized progeny were the result of abnormal division in meiosis. It results as important that the plants of normal phenotype, chromosome structure and minimized abnormal meiosis are selected to stabilize progeny.