Making the Bread： Insights from Newly Synthesized AIIohexaploid Wheat

Bread wheat （or common wheat, Triticum aestivum） is an allohexaploid （AABBDD, 2n = 6x = 42） that arose by hybridization between a cultivated tetraploid wheat T. turgidum （AABB, 2n = 4x = 28） and the wild goatgrass Aegilops tauschfi （DD, 2n = 2x = 14）. Polyploidization provided niches for rigorous genome modification at cytogenetic, genetic, and epigenetic levels, rendering a broader spread than its progenitors. This review summarizes the latest advances in understanding gene regulation mechanisms in newly synthesized allo- hexaploid wheat and possible correlation with polyploid growth vigor and adaptation. Cytogenetic studies reveal persistent association of whole-chromosome aneuploidy with nascent allopolyploids, in contrast to the genetic stability in common wheat. Transcriptome analysis of the euploid wheat shows that small RNAs are driving forces for homoeo-allele expression regulation via genetic and epigenetic mechanisms. The ensuing non-additively expressed genes and those with expression level dominance to the respective pro- genitor may play distinct functions in growth vigor and adaptation in nascent allohexaploid wheat. Further genetic diploidization of allohexaploid wheat is not random. Regional asymmetrical gene distribution, rather than subgenome dominance, is observed in both synthetic and natural allohexaploid wheats. The combinatorial effects of diverged genomes, subsequent selection of specific gene categories, and subgenome-specific traits are essential for the successful establishment of common wheat.