Genomic asymmetry of theBrassica napusseed: Epigenetic contributions of DNA methylation and small RNAs to subgenome bias

Polyploidy has predominated the genetic history of the angiosperms, and allopolyploidy is known to have contributed to the vast speciation of flowering plants. Brassica napus , one of the world’s most important oilseeds, is one such polyploid species originating from the interspecific hybridization of Brassica rapa (A n ) and Brassica oleracea (C n ). Nascent amphidiploids must balance progenitor genomes during reproduction, though the role of epigenetic regulation in subgenome maintenance is unknown. The seed is the pivotal developmental transition into the new sporophytic generation and as such undergoes substantial epigenetic modifications. We investigated subgenome bias between the A n and C n subgenomes as well as across syntenic regions by profiling DNA methylation and siRNAs characteristic of B. napus seed development. DNA methylation and siRNA accumulation were prevalent in the C n subgenome and most pronounced early during seed morphogenesis. Hypermethylation during seed maturation was most pronounced on non-coding elements, including promoters, repetitive elements, and siRNAs. Methylation on siRNA clusters was more prevalent in syntenic regions of the C n subgenome and implies selective silencing of genomic loci of the seed. Together, we find compelling evidence for the asymmetrical epigenetic regulation of the A n and C n subgenomes of Brassica napus across seed development.