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.