A crossover (CO) and its cytological signature, the chiasma, are major features of eukaryotic meiosis. The formation of at least one CO/chiasma between homologous chromosome pairs is essential for accurate chromosome segregation at the first meiotic division and genetic recombination. Polyploid organisms with multiple sets of homoeologous chromosomes have evolved additional mechanisms for the regulation of CO/chiasma. In hexaploid wheat (2n = 6× = 42), this is accomplished by pairing homoeologous (Ph) genes, with Ph1 having the strongest effect on suppressing homoeologous recombination and homoeologous COs. In this study, we observed homoeologous COs between chromosome 5Mg of Aegilops geniculata and 5D of wheat in plants where Ph1 was fully active, indicating that chromosome 5Mg harbors a homoeologous recombination promoter factor(s). Further cytogenetic analysis, with different 5Mg/5D recombinants, showed that the homoeologous recombination promoting factor(s) may be located in proximal regions of 5Mg. In addition, we observed a higher frequency of homoeologous COs in the pericentromeric region between chromosome combination of rec5Mg#2S·5Mg#2L and 5D compared to 5Mg#1/5D, which may be caused by a small terminal region of 5DL homology present in chromosome rec5Mg#2. The genetic stocks reported here will be useful for analyzing the mechanism of Ph1 action and the nature of homoeologous COs.
Keywords: Aegilops geniculata; Homoeologous recombination; Meiotic synapsis; Triticum aestivum; Wheat.