Extraction of the Constituent Subgenomes of the Natural Allopolyploid Rapeseed (Brassica napus L.)

Abstract

As the dynamic nature of progenitor genomes accompanies the speciation by interspecific hybridization, the extraction of the constituent subgenome(s) from a natural allopolyploid species of long history and then restitution of the progenitor(s) provides the unique opportunity to study the genome evolution and interplay. Herein, the A subgenome from the allotetraploid oilseed rape (Brassica napus L., AACC) was extracted through inducing the preferential elimination of C-subgenome chromosomes in intertribal crosses and the progenitor B. rapa was restituted (RBR). Then by crossing and backcrossing RBR with B. napus donor, the C subgenome was in situ dissected by adding each of its nine chromosomes to the extracted A subgenome and establishing the whole set of monosonic alien addition lines (MAALs). RBR from spring-type B. napus genotype "Oro" expressed a phenotype resembling some type of B. rapa never observed before, but showed a winter-type flowering habit. This RBR had weaker growth vigor and suffered more seriously from biotic and abiotic stresses compared with Oro. The phenotypes specific for these MAALs showed the location of the related genes on the particular C-subgenome chromosomes. These MAALs exhibited obviously different frequencies in homeologous pairing and transmission of additional C-subgenome chromosomes, which were associated with the distinct degrees of their relatedness, and even with the possible genetic regulation for meiotic pairing evolved in B. napus. Finally, large scaffolds undetermined for sequence assembly of B. napus were anchored to specific C-subgenome chromosomes using MAALs.KEYWORDS allopolyploid; Brassica napus; B. rapa; alien additional lines; aneuploid M ANY angiosperms including important crops (bread wheat, cotton, oilseed rape, tobacco, etc.) are allopolyploid species that originated from interspecific hybridizations between two or more diploid ancestors followed by chromosome doubling (Ramsey and Schemske 1998;Levin 2003;Otto 2007;Doyle et al. 2008;Soltis and Soltis 2012). These allopolyploid crops outcompete their progenitors by their adaptability to a wide range of climatic conditions and higher yield and better quality of targeted products, under human domestication and improvement. To unravel the origin and structure of the component genomes in these crops has been the goal of the extensive genetic research over a long period. The hybridization events leading to the evolutionary origin of these allopolyploids were traditionally elucidated by the artificial synthesis of their counterparts from the crosses between the extant relatives of the presumed progenitors available or by observing the meiotic chromosome pairing in the hybrids between the natural allopolyploid and the possible progenitors (Kihara 1924;Nagaharu 1935). Alternatively, the genome of one progenitor (AA) was readily dissected by separating and adding its own chromosome to the genome of another progenitor (BB) (namely the alien addition lines), from successively backcrossing the natural or synthesi...

Gene Expression Changes During the Allo-/Deallopolyploidization Process of Brassica napus

“…RBR manifested many significant phenotypes at multiple growth and developmental stages that differed from those of their parental B. napus lines, such as small leaf lobes, short siliques, and greater susceptibility to diseases during and after flowering in humid and high-temperature environments. Particularly, as with many other B. rapa lines, RBR showed severe self-incompatibility that resulted in poor seed set by self-pollination, although with a high pollen stainability (Zhu et al, 2016). Because its parent ZS11 is self-compatible, this result indicated that the selfincompatibility of RBR might result from the loss of C genome interaction when the A genome was extracted.…”

“…A B. rapa line (designated as RBR, restituted B. rapa) with an "extracted" genome (A e A e ) of the allotetraploid B. napus (cultivar Zhongshuang 11; designated as ZS11) was produced previously (Zhu et al, 2016). The A and C subgenome within nucleus of B. napus was named as A n and C n , respectively.…”

“…Despite the exact process, C genome chromosomes were lost step-wise during the backcrosses. Using a similar method, another restituted B. rapa (designated as RBR, A e A e , 2n = 20) containing the A n sub-genome of B. napus ZS11 (A n A n C n C n , 2n = 38) was also developed (Zhu et al, 2016).…”

“…Stepwise cytological investigations have revealed the complete retained A genome chromosome and the lost C genome chromosome (Zhu et al, 2016). RBR manifested many significant phenotypes at multiple growth and developmental stages that differed from those of their parental B. napus lines, such as small leaf lobes, short siliques, and greater susceptibility to diseases during and after flowering in humid and high-temperature environments.…”

Genome-Wide Gene Expression Disturbance by Single A1/C1 Chromosome Substitution in Brassica rapa Restituted From Natural B. napus

“…Increasing narrow genetic diversity due to selection pressure excessively focusing on quality and yield in the crop’s breeding pools has been a big challenge for breeders to enhance current levels of resistance to biotic and abiotic stresses ( Chang and de Jong, 2005 ; Gupta et al, 2016 ). Therefore, plant breeders and geneticists are greatly interested in enhancing genetic variants of crops via interspecific and somatic hybridization to transfer desirable traits from wild species ( Quiros et al, 1987 ; Du et al, 2009 ; Heneen et al, 2012 ; Chen et al, 2014 ; Kang et al, 2014 ; Zhu et al, 2016 ). As intermediate crossing products of interspecific hybridization, alien chromosome addition lines, CS lines and other aneuploidy lines not only offer the ideal opportunity to produce introgression lines, but provide a unique avenue to check heterologous gene expression and interaction between recipient genome and donor chromosome in plants ( Barthes and Ricroch, 2001 ; Zhu et al, 2016 ).…”