Genome sequences of five Sitopsis species of Aegilops and the origin of polyploid wheat B subgenome.

Common wheat (Triticum aestivum, BBAADD) is a major staple food crop worldwide. The diploid progenitors of the A and D subgenomes have been unequivocally identified; that of B, however, remains ambiguous and controversial but is suspected to be related to species of Aegilops, section Sitopsis. Here, we report the assembly of chromosome-level genome sequences of all five Sitopsis species, namely Aegilops bicornis, Ae. longissima, Ae. searsii, Ae. sharonensis, and Ae. speltoides, as well as the partial assembly of the Amblyopyrum muticum (synonym Aegilops mutica) genome for phylogenetic analysis. Our results reveal that the donor of the common wheat B subgenome is a distinct, and most probably extinct, diploid species that diverged from an ancestral progenitor of the B lineage to which the still extant Ae. speltoides and Am. muticum belong. In addition, we identified interspecific genetic introgressions throughout the evolution of the Triticum/Aegilops species complex. The five Sitopsis species have various assembled genome sizes (4.11-5.89 Gb) with high proportions of repetitive sequences (85.99%-89.81%); nonetheless, they retain high collinearity with other genomes or subgenomes of species in the Triticum/Aegilops complex. Differences in genome size were primarily due to independent post-speciation amplification of transposons. We also identified a set of Sitopsis genes pertinent to important agronomic traits that can be harnessed for wheat breeding. These newly assembled genome resources provide a new roadmap for evolutionary and genetic studies of the Triticum/Aegilops complex, as well as for wheat improvement.