Immediate unidirectional epigenetic reprogramming of NORs occurs independently of rDNA rearrangements in synthetic and natural forms of a polyploid species Brassica napus

Abstract

The dynamics of genome modification that occurred from the initial hybridization event to the stabilization of allopolyploid species remains largely unexplored. Here, we studied inheritance and expression of rDNA loci in the initial generations of Brassica napus allotetraploids (2n = 38, AACC) resynthesized from Brassica oleracea (2n = 18, CC) and B. rapa (2n = 20, AA) and compared the patterns to natural forms. Starting already from F1 generation, there was a strong uniparental silencing of B. oleracea genes. The epigenetic reprogramming was accompanied with immediate condensation of C-genome nucleolar organizer region (NOR) and progressive transgeneration hypermethylation of polymerase I promoters, mainly at CG sites. No such changes were observed in the A-genome NORs. Locus loss and gains affecting mainly non-NOR loci after the first allotetraploid meiosis did not influence established functional status of NORs. Collectively, epigenetic and genetic modifications in synthetic lines resemble events that accompanied formation of natural allopolyploid species.

Chinese cabbage (Brassica rapa ssp. pekinensis) – a valuable source of resistance to clubroot (Plasmodiophora brassicae)

“…The use of a species-specific B. oleracea BAC clone revealed the chromosome rearrangements between A-and C-genomes in the synthetic B. napus forms (Książczyk et al 2011), and our present work confirmed this observation presenting possible A7/C6 chromosome translocation in the B. napus genotype HL06. Similarly, an A7/C6 chromosomal translocation was observed in synthetic Brassica allotetraploids by Xiong et al (2011) andGrandont et al (2014), indicating known patterns of genome duplication within the Brassica napus genome (Parkin et al 2003).…”

“…The main sources of resistance used to date originate from different species of the genus Brassica, including B. rapa (Agenome), B. oleracea (C-genome) and B. napus (ACgenome). Different experimental approaches have been applied to study chromosome rearrangements in the Brassica allotetraploid and ancestral genomes, such as the production of synthetic allopolyploids relative to natural forms, using chromosome mapping and cytogenetic analysis including FISH (Leflon et al 2006;Książczyk et al 2011;Xiong et al 2011;FreduaAgyeman et al 2014;Grandont et al 2014). Physical mapping of 5S and 18S-5.8S-26S (35S) rRNA genes by FISH provides valuable chromosomal landmarks, and their characteristic positions enable chromosome identification allowing detection of chromosome variability (Maluszynska and Heslop-Harrison 1993;Hasterok et al 2006).…”

“…The presence of intergenomic chromosome translocations in the hybrid genome, indicating that the two parental genomes may have undergone some rearrangements following hybridization, was revealed in newly synthesized Brassica allopolyploids (Książczyk et al 2011;Xiong et al 2011), which can be a rapid response to formation of the allotetraploid genome. The use of a species-specific B. oleracea BAC clone revealed the chromosome rearrangements between A-and C-genomes in the synthetic B. napus forms (Książczyk et al 2011), and our present work confirmed this observation presenting possible A7/C6 chromosome translocation in the B. napus genotype HL06.…”

“…The species-specific BoB014O06 BAC clone from a B. oleracea BAC library was used as a probe for the C-genome (GISH-like technique; Książczyk et al 2011). The BoB014O06 clone was labelled by random priming with digoxigenin-11-dUTP (Roche).…”

Asymmetric Epigenetic Modification and Elimination of rDNA Sequences by Polyploidization in Wheat

“…rRNA genes are long tandem repeats that experienced drastic changes such as sequence loss, structural rearrangement, and modified expression during the course of plant evolution (Schubert and Kunzel, 1990;Wendel et al, 1995;Mishima et al, 2002;Pontes et al, 2004;Ksią _ zczyk et al, 2011). In six tetraploid Oryza species, the internal transcribed spacer (ITS) sequence of rRNA genes exhibits coexistence and maintenance of paralogs, generation of novel sequence variants, loss of arrays, or interarray sequence homogenization (Bao et al, 2010).…”