AbstractThe bottleneck restricting introgression of useful genes directly from diploid into hexaploid wheats is the low number of BC1F1seeds obtained. In crosses between hexaploid wheat (Triticum aestivumL.; AABBDD) andAegilops squarrosaL. (DD) orT. urartuThum. (AA), this bottleneck may be overcome simply by pollinating a sufficient number of F1spikes. However, hybrids between hexaploid wheat cultivars (T. aestivum) andT. monococcumL. (AA) generally are highly female‐sterile, often having no pistils. OneT. monococcumaccession, PI 355520, when crossed withT. aestivum, produced hybrids with female fertility in the same range as that ofT. aestivum/A. squarrosaorT. aestivum/T. urartuhybrids, ca. 0.5 to 1.0 backcross seed per spike. We found that female fertility was controlled by two duplicate genes in PI 355520, and that this accession can be used as a bridging parent to introgress genes from otherT. monococcumaccessions into hexaploid wheat. Pairing of homologous chromosomes was less frequent and weaker in such crosses than inT. aestivum/A. squarrosacrosses, but homoeologous bivalents occurred at a rate of almost 0.5 II per cell. Restitution division was detected in crosses involving all three diploid species and was confirmed cytologically in crosses with PI 355520. Chromosome numbers of BC1F1plants ranged from 35 to 67; plants with 49 or more chromosomes occurred at frequencies of 0.09 to 0.21 among progeny ofA. squarrosaandT. urartuand 0.29 in progeny ofT. aestivum/T. monococcumcrosses involving PI 355520. These results are consistent with those of previous studies, demonstrating the potential of direct Hexaploid/diploid crosses for rapidly introgressing useful genes into Hexaploid wheat with minimum disturbance of the background genotyp
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