Mechanism for insertional dysploid reduction of ancestral chromosomes . The arms of the progenitors of Sb7 and Sb4 are indicated in blue and orange, respectively. Centromeres are shown as solid circles, and telomeres as grey rectangles. (A) original configuration. (B) intermediate state in which (1) chromosome 7 circularizes, and (2) 7 and 4 recombine at breaks that occur within centromeres. The order of occurrence of circularization and break/recombination are unspecified. (C) The resulting order and orientation matches that found in Ms7 (Figure 4B). Note that the original telomeres of 7 lie within the fused chromosome, and are presumably lost. Only one of the two centromeres (shown as mix of orange and blue) survives. An analogous event is proposed to have produced chromosome 4 of Aegilops tauschii .