Structural rearrangement in chromosome 2M of Aegilops comosa has prevented the utilization of the Compair and related wheat-Ae. comosa translocations in wheat improvement

The genetic constitutions of chromosome 2M of Aegilops comosa and the derived wheat-Ae. comosa translocations were analyzed by molecular cytogenetic techniques. Hybridization of 15 RFLP markers covering the entire length of the group-2 chromosomes revealed that chromosome 2M was structurally rearranged compared to the homoeologous chromosomes of wheat by either a pericentric inversion or a terminal intrachromosomal translocation. The breakpoint of the rearrangement was located in a region between the loci Xpsr131 and Xcdo405, resulting in the translocation of 47% of 2MS to 2ML. This aberrant structure of 2M allowed homoeologous recombination between 2M and its wheat counterpart only in the translocated segment on 2ML. C-banding and genomic in situ hybridization analyses confirmed that all translocation chromosomes consisted of the complete 2MS arm, a large part of 2ML, and very small distal segments derived from 2AS or 2DS, as expected from the aberrant structure of chromosome 2M. Thus, the translocation in the line 2A-2M?4/2 can be described as T2AS-2M?1L???2M?1S and the translocations in the lines Compair and 2D-2M?3/8 as T2DS-2M?1L???2M?1S. RFLP analysis determined the breakpoints in these translocation chromosomes to be within the telomeric 16% of the wheat chromosome arms. The breakpoint of the 2A/2M translocation was between Xbcd348 and Xcdo783, and that of the 2D/2M translocation was between Xcdo783 and Xpsr666. Because the translocation chromosomes retain the structural aberration found in chromosome 2M, further exploitation of the wheat-Ae. comosa translocations for cultivar improvement is questionable.