Broadening the bread wheat D genome

Although Ae. tauschii has been extensively utilised for wheat breeding, the D-genome-containing allopolyploids have largely remained unexploited. In this review, we discuss approaches that can be used to exploit the D genomes of the different Aegilops species for the improvement of bread wheat. The D genome of allohexaploid bread wheat (Triticum aestivum, 2n = AABBDD) is the least diverse of the three wheat genomes and is unarguably less diverse than that of diploid progenitor Aegilops tauschii (2n = DD). Useful genetic variation and phenotypic traits also exist within each of the wheat group species containing a copy of the D genome: allopolyploid Aegilops species Ae. cylindrica (2n = D^cD^cC^cC^c), Ae. crassa 4x (2n = D¹D¹X^crX^cr), Ae. crassa 6x (2n = D¹D¹X^crX^crD^crD^cr), Ae. ventricosa (2n = D^vD^vN^vN^v), Ae. vavilovii (2n = D¹D¹X^crX^crS^vS^v) and Ae. juvenalis (2n = D¹D¹X^crX^crU^jU^j). Although Ae. tauschii has been extensively utilised for wheat breeding, the D-genome-containing allopolyploids have largely remained unexploited. Some of these D genomes appear to be modified relative to the bread wheat and Ae. tauschii D genomes, and others present in the allopolyploids may also contain useful variation as a result of adaptation to an allopolyploid, multi-genome environment. We summarise the genetic relationships, karyotypic variation and phenotypic traits known to be present in each of the D genome species that could be of relevance for bread wheat improvement and discuss approaches that can be used to exploit the D genomes of the different Aegilops species for the improvement of bread wheat. Better understanding of factors controlling chromosome inheritance and recombination in wheat group interspecific hybrids, as well as effective utilisation of new and developing genetics and genomics technologies, have great potential to improve the agronomic potential of the bread wheat D genome.