Homoeologous shuffling and chromosome compensation maintain genome balance in resynthesized allopolyploid Brassica napus

Abstract

Polyploidy has contributed to the evolution of eukaryotes, particularly flowering plants. The genomic consequences of polyploidy have been extensively studied, but the mechanisms for chromosome stability and diploidization in polyploids remain largely unknown. By using new cytogenetic tools to identify all of the homoeologous chromosomes, we conducted a cytological investigation of 50 resynthesized Brassica napus allopolyploids across generations S 0:1 to S 5:6 and in the S 10:11 generation. Changes in copy number of individual chromosomes were detected in the S 0:1 generation and increased in subsequent generations, despite the fact that the mean chromosome number among lines was approximately 38. The chromosome complement of individual plants (segregants) ranged from 36 to 42, with a bias toward the accumulation of extra chromosomes. Karyotype analysis of the S 10:11 generation detected aneuploidy and inter-and intragenomic rearrangements, chromosome breakage and fusion, rDNA changes, and loss of repeat sequences. Chromosome sets with extensive homoeology showed the greatest instability. Dosage balance requirements maintained chromosome numbers at or near the tetraploid level, and the loss and gain of chromosomes frequently involved homoeologous chromosome replacement and compensation. These data indicate that early generations of resynthesized B. napus involved aneuploidy and gross chromosomal rearrangements, and that dosage balance mechanisms enforced chromosome number stability. Seed yield and pollen viability were inversely correlated with increasing aneuploidy, and the greatest fertility was observed in two lines that were additive for parental chromosomes. These data on resynthesized B. napus and the correlation of fertility with additive karyotypes cast light on the origins and establishment of natural B. napus.

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). It is worth mentioning that none of the rDNA-bearing chromosomes were involved in recombination showing the A7/C6 translocation, because the number of rDNA loci is stable and A-genome-like chromosomes are not painted by a C-genome-specific BAC clone.…”

“…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.…”

“…It can be concluded that more detailed FISH analyses of mitotic chromosomes and their rearrangements will be required beyond study of ribosomal landmarks. This could be accomplished through the use of either chromosome-specific or even arm-specific sets of BAC clone-based probes for both the B. rapa and B. oleracea chromosomes, together with PCR-based disease resistance markers (Koo et al 2004;Xiong et al 2011;Fredua-Agyeman et al 2014).…”

“…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 shock of being united and symphiliosis

“…The list includes chromosome translocations, inversions, deletions, duplications, loss, amplification or reduction of repetitive sequences, extensive chromosome repatterning, horizontal transfer of genomic segments between the parental genomes, nonreciprocal homeologous recombination, repetitive sequences, transposon activation, and aneuploidy. 18,34,[36][37][38] At least some of the genomic rearrangements are not random and involve elimination of specific noncoding DNA sequences and intergenomic suppression of disease-resistant genes. 18 Like genomic aberrations in cancer, genomic rearrangements in allopolyploids can range from simple to highly complex, persist through generations, or re-emerge after being in remission for generations.…”