Evolution of the BBAA Component of Bread Wheat during Its History at the Allohexaploid Level

Abstract

Subgenome integrity in bread wheat (Triticum aestivum; BBAADD) makes possible the extraction of its BBAA component to restitute a novel plant type. The availability of such a ploidy-reversed wheat (extracted tetraploid wheat [ETW]) provides a unique opportunity to address whether and to what extent the BBAA component of bread wheat has been modified in phenotype, karyotype, and gene expression during its evolutionary history at the allohexaploid level. We report here that ETW was anomalous in multiple phenotypic traits but maintained a stable karyotype. Microarray-based transcriptome profiling identified a large number of differentially expressed genes between ETW and natural tetraploid wheat (Triticum turgidum), and the ETW-downregulated genes were enriched for distinct Gene Ontology categories. Quantitative RT-PCR analysis showed that gene expression differences between ETW and a set of diverse durum wheat (T. turgidum subsp durum) cultivars were distinct from those characterizing tetraploid cultivars per se. Pyrosequencing revealed that the expression alterations may occur to either only one or both of the B and A homoeolog transcripts in ETW. A majority of the genes showed additive expression in a resynthesized allohexaploid wheat. Analysis of a synthetic allohexaploid wheat and diverse bread wheat cultivars revealed the rapid occurrence of expression changes to the BBAA subgenomes subsequent to allohexaploidization and their evolutionary persistence.

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

“…The presence of the Ph1 locus on the 5B chromosome, which prevented homeologous exchanges, likely contributed to the high karyotype stability of the restituted wheat (Zhang et al 2014). Theoretically, the extracted genome should be .99.8% identical to the A a A a B a B a subgenomes of its bread wheat donor after the ninth backcrossing (Zhang et al 2014). Although the extracted AABB genomes would largely represent those in bread wheat, the minor part of the genomic composition of the extant tetraploid still retained could make them diverge from the original situation to certain extents.…”

“…The normal growth and vigor of resynthesized B. napus parented by RBR (data not shown) further proved the correction of the altered functions of the AA subgenome by the CC subgenome. Similarly, the extracted tetraploid wheat (A a A a B a B a ) with a stable karyotype exhibited the wide-ranging phenotypic abnormalities (Kerber 1964;Zhang et al 2014), and many (rather than a few) genes were likely affected in its genome. But the resynthesized allohexaploid wheat parented by extracted tetraploid wheat fully restored normal phenotypes and high fertility, which also demonstrated that the DD subgenome compensated for the compromised functionality of the BBAA subgenomes of bread wheat (Kerber 1964;Zhang et al 2014).…”

“…durum) and hexaploid wheat (A a A a B a B a D a D a ) (T. aestivum) was carried out, followed by the nine cycles of backcrossing of the hybrids (A at A at B at B at D a ) to the hexaploid wheat to recover the genome (A a A a B a B a ) of the restituted wheat (Kerber 1964). The presence of the Ph1 locus on the 5B chromosome, which prevented homeologous exchanges, likely contributed to the high karyotype stability of the restituted wheat (Zhang et al 2014). Theoretically, the extracted genome should be .99.8% identical to the A a A a B a B a subgenomes of its bread wheat donor after the ninth backcrossing (Zhang et al 2014).…”

“…Then, the extraction of the constituent subgenome(s) of a given allopolyploid to restitute an independent organism through certain experimental procedures provides the unique opportunity to investigate the genome evolution and interplay. The study of the AABB component extracted from bread wheat (AABBDD) provided novel clues for the modifications in phenotype, karyotype, and gene expression due to a history at the allohexaploid level (Kerber 1964;Zhang et al 2014).…”

“…Then, the extraction of the constituent subgenome(s) of a given allopolyploid to restitute an independent organism through certain experimental procedures provides the unique opportunity to investigate the genome evolution and interplay. The study of the AABB component extracted from bread wheat (AABBDD) provided novel clues for the modifications in phenotype, karyotype, and gene expression due to a history at the allohexaploid level (Kerber 1964;Zhang et al 2014).The A subgenome was previously extracted from one B. napus cultivar "Oro" and the ancestral B. rapa was restituted (RBR Oro) by inducing the preferential loss of the C-genome chromosomes in intertribal crosses with Isatis indigotica (2n = 14) (Tu et al 2010). The cultivar was likely representative of one origin event for B. napus, as it was bred from the landrace "Liho" in Germany and kept the commonest type of plastid haplotypes among a wide range of B. napus accessions (Allender and King 2010).…”