The mechanism of origin of Robertsonian translocations was investigated in plants monosomic for chromosome 1A of wheat and 1H^t of Elymus trachycaulus by GISH. Chromosomes 1A and 1H^t stayed univalent in all metaphase I cells analyzed, suggesting that Robertsonian translocations do not originate from meiotic recombination in centromeric regions with shared DNA sequence homology. At ana-/telophase I, the 1H^t and 1A univalents underwent either chromosome or chromatid segregation and misdivided in 6–7% of the pollen mother cells. None of the ana-/telophases I analyzed had Robertsonian translocations, which were only observed in 2% of the “half tetrads” at ana-/telophase II. The frequency of Robertsonian translocations observed at ana-/telophase II corresponds well with the number of Robertsonian translocations (1–4%) detected in progenies derived from plants monosomic for group-1 chromosomes of wheat (1A, 1B, and 1D) and 1H^t of E. trachycaulus. Our data suggest that Robertsonian translocations arise from centric misdivision of univalents at ana-/telophase I, followed by segregation of the derived telocentric chromosomes to the same nucleus, and fusion of the broken ends during the ensuing interkinesis.

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