Triticum monococcum ssp. aegilopoides (Link) Thell. is a wild relative of common wheat, which is diploid of the Am genome. Since T. m. ssp. aegilopoides has agronomically beneficial traits, it provides useful genetic resources in wheat breeding. We developed AuAuBBAmAm synthetic hexaploid lines by crossing Triticum turgidum L. ssp. durum cv. ‘Langdon’ (AuAuBB genome) with T. m. ssp. aegilopoides (AmAm genome).

When the synthetic hexaploids were crossed with T. aestivum L. cv. ‘Chinese Spring’ (CS), 70% of the tested AuAuBBAmAm synthetic hexaploids showed growth inhibition with dead leaves called hybrid necrosis under low temperatures in F1 plants (AuAuBBAmD). When one AuAuBBAmAm synthetic hexaploid line was crossed with T. aestivum, all the tested accessions of T. aestivum showed the hybrid necrosis phenotypes. Therefore, this hybrid necrosis potentially prevents the use of Am genome in wheat breeding.

In this study, we aim to characterize the expression of hybrid necrosis and to identify genomic regions responsible for the hybrid necrosis. In the F1 plants (AuAuBBAmD), hybrid necrosis gradually proceeded. Plant growth was arrested around 30 days after germination. Differences in the number of leaves and tillers between the necrotic line and the wild-type line were obvious around 50 days after germination. By linkage analyses with AuAuBBAmAm F2 population between necrotic line and wild-type line based on MIG-seq markers and deletion mapping with a gametocidal gene, a causal region in the synthetic hexaploid was mapped near the centromere of chromosome 1Am.

Another causal region in common wheat was located at the short arm of chromosome 5D by cross experiments using CS nulli-tetrasomic lines and CS 5D chromosome deletion lines. These results indicated that the hybrid necrosis was caused by the interaction between genes located at 1Am and 5D chromosomes. Genome sequencing of CS 5D chromosome deletion lines narrowed down to 25.4 Mbp.

Currently, we have performed simultaneous CRISPR-Cas9 knockout of candidate genes using the bread wheat cultivar ‘Fielder’ and successfully obtained genome-edited plants with mutations in the multiple candidate genes, some of which were crossed with the AuAuBBAmAm synthetic hexaploid line. Identification of the causal genes could contribute to expanding the use of Am genome and understanding post-reproductive isolation by genome-genome interaction.

This study was supported by Cabinet Office, Government of Japan, Moonshot R&D Program for Agriculture, Forestry and Fisheries (funding agency: Bio-oriented Technology Research Advancement Institution).