Genome evolution due to allopolyploidization in wheat

Abstract

The wheat group has evolved through allopolyploidization, namely, through hybridization among species from the plant genera Aegilops and Triticum followed by genome doubling. This speciation process has been associated with ecogeographical expansion and with domestication. In the past few decades, we have searched for explanations for this impressive success. Our studies attempted to probe the bases for the wide genetic variation characterizing these species, which accounts for their great adaptability and colonizing ability. Central to our work was the investigation of how allopolyploidization alters genome structure and expression. We found in wheat that allopolyploidy accelerated genome evolution in two ways: (1) it triggered rapid genome alterations through the instantaneous generation of a variety of cardinal genetic and epigenetic changes (which we termed "revolutionary" changes), and (2) it facilitated sporadic genomic changes throughout the species' evolution (i.e., evolutionary changes), which are not attainable at the diploid level. Our major findings in natural and synthetic allopolyploid wheat indicate that these alterations have led to the cytological and genetic diploidization of the allopolyploids. These genetic and epigenetic changes reflect the dynamic structural and functional plasticity of the allopolyploid wheat genome. The significance of this plasticity for the successful establishment of wheat allopolyploids, in nature and under domestication, is discussed.

Figure 1

Schematic of the wheat karyotype. The wheat karyotype is arranged into genomes A, B, and D and into seven homeologous groups (e.g., group 1 consists of chromosomes 1A, 1B, and 1D). This arrangement is after Sears (1954), who classified homeologous chromosomes based on their ability to compensate for each other’s absence. Examples of the different types of sequences are drawn on top of the chromosomes, namely: group-specific sequences that are present in only one homeologous group, chromosome-specific sequences (CSSs) that are present in only one chromosome pair, genome-specific sequences (GSSs) that can be on more than one chromosome pair but only in one of the genomes, and nonspecific sequences that are present on both homeologous and nonhomeologous chromosomes. The nonspecific sequences are mainly dispersed repeats (adapted from Levy and Feldman 2004).