Introduction of agriculturally beneficial traits from the gene pool of wild relatives into wheat cultivars can contribute to wheat breeding. Although wild relatives of wheat, Aegilops spp., are used as genetic resources of abiotic stress tolerance and disease resistance, the use of Aegilops ssp. in wheat breeding is still limited. Introgression from wild relatives into cultivated wheat is achieved using a mutant of Pairing homoeologous (Ph) gene, which promotes homoeologous recombination between different genomes. Ph1 and Ph2 are known as Ph genes.

The ph1 mutant is commonly used for introgression of alien chromosomes, whereas the ph2 mutant is not. In this study, to evaluate introgression by homoeologous recombination caused by ph2 mutant, we used the synthetic polyploid lines generated by crossing Triticum turgidum L. subsp. durum cv. ‘Langdon’ (AABB genome) as female and four Aegilops species: Ae. umbellulata Zhuk. (UU genome), five lines of Ae. geniculata Roth. (UUMM genome), Ae. biuncialis Vis. (UUMM genome), Ae. peregrina (Hackel in J. Fraser) Maire & Weiller. (UUSS genome).

These synthetic polyploid lines were crossed with the ph2 mutant of T. aestivum ‘Chinese Spring’. The F1 hybrids of the synthetic polyploids and the ph2 mutant were repeatedly backcrossed with the wild type ‘Chinese Spring’ to generate backcrossed lines. We confirmed chromosomal rearrangements including translocations of Aegilops chromosomal fragments, intragenomic recombination, and aneuploidy in the backcrossed lines by multiplexed inter-simple sequence repeats genotyping by sequencing and chromosome karyotyping with fluorescence in situ hybridization.

Intragenomic recombination and translocation were more frequently observed between the wheat D-genome and Aegilops chromosomes than between the wheat A- or B-genome and Aegilops chromosomes. Aneuploidy with deletions or additions of whole chromosomes was also frequently observed. In addition, since synthetic hexaploid with U-genome showed salt stress tolerance at the time of seed germination, we screened the U-genome chromosome introgression lines harboring salt stress tolerance.

We identified introgression lines with U-genome chromosomes exhibiting more salt tolerance than the control wheat lines. These results suggest that the ph2 mutant can be used for Aegilops chromosome introgression into wheat cultivars, but several backcrosses with wheat cultivars will be necessary to avoid aneuploidy and to obtain euploid plants with agriculturally beneficial traits derived from the wild relatives.

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).