Although good correspondence between expected and observed synaptic configurations was found, there were two discordances. One heteromorphic bivalent showed centromere signals on its both ends (Fig. 3b, arrowhead). A trivalent comprising metacentric chromosome 1 of M. arvalis and twin acrocentrics of M. levis contained three misaligned centromeres. Lemskaya et al.34 proposed that chromosome 1 of M. arvalis resulted from tandem fusion of the proximal end of chromosome 3 and the distal end of chromosome 2 of M. levis. However, the synaptic configurations observed in these F1 hybrids indicate that chromosome 1 of M. arvalis was generated by fusion of the distal ends of the acrocentrics followed by inactivation of their centromeres, and re-activation of the centromere at the point of fusion.