Homoeologous Recombination: A Novel and Efficient System for Broadening the Genetic Variability in Wheat

Gene transfer from wild wheat relatives to bread wheat is restricted to homologous recombination. The presence of the Pairing homoeologous 1 (<i>Ph1</i>) gene in the long arm of wheat chromosome 5B allows only homologous chromosomes to pair and recombine, resulting in diploid inheritance of polyploid wheat. Previously, we identified a potent homoeologous pairing promotor gene(s) (<i>Hpp-5M^g</i>); its carrier chromosome 5M^g derived from <i>Aegilops geniculata</i> and its wheat homoeologous chromosome 5D freely recombined in the presence of the <i>Ph1</i> gene. In this study, we investigated the effect of <i>Hpp-5M^g</i> on homoeologous recombination in the absence of <i>Ph1</i>. In <i>Hpp-5M^g/ph1bph1b</i> plants, we observed a vast genome-wide increase in homoeologous recombination and multiple crossovers (CO), including CO breakpoints in proximal regions of the chromosomes where recombination is known to be suppressed. We tested the efficacy of <i>Hpp-5M^g/ph1bph1b</i>-induced homoeologous recombination by producing new recombinants for the wheat streak mosaic virus resistance gene, <i>Wsm3</i>, present in the wheat-<i>Thinopyrum intermedium</i> Robertsonian translocation (RobT T7BS.7S#3L). A recombination frequency of 6.5% was detected by screening the progenies double monosomic for T7BS.7S#3L and 7B by genomic in situ hybridization. This recombination frequency was about 100-fold higher compared with the recombinant frequency of 0.06% observed by using <i>ph1b</i>-induced homoeologous recombination alone. Our results indicate that chromosome 5M^g promotes homoeologous recombination between wheat and wild wheat relative chromosomes, which helps in the generation of pre-breeding materials thereby accelerating wheat crop improvement.