Genomic evolution of wheat and barley

Abstract

Wheat (Triticum aestivum) and barley (Hordeum vulgare) are closely related cereal crops that were both domesticated in the Fertile Crescent but differ markedly in their evolutionary histories. Wheat evolved through repeated interspecific hybridization and allopolyploidization, resulting in the formation of tetraploid and hexaploid crop genomes, whereas cultivated barley remained diploid despite belonging to a genus that contains numerous polyploid species. This comparative review integrates evidence from recent genome sequencing, pangenome analyses, and evolutionary studies to examine the mechanisms underlying these contrasting evolutionary trajectories. We synthesize findings from phylogeny, genome structure, transposable element dynamics, reproductive barriers, and domestication studies. We show that wheat evolution was facilitated by extensive hybridization networks among partially compatible ancestral lineages, chromosome-pairing control by the Ph1 locus and related meiotic stabilization mechanisms, and genome diversification following polyploidization. In contrast, barley evolved from a single diploid lineage characterized by strong reproductive isolation, limited opportunities for interspecific gene flow, and a highly conserved genome structure. We propose a dual-constraints model in which both restricted hybridization and insufficient meiotic stabilization constrained allopolyploid formation in barley. Understanding these contrasting evolutionary pathways provides a framework for comparative genomics and may facilitate the transfer of genetic knowledge between wheat and barley.