STRUCTURAL EVOLUTION OF WHEAT CHROMOSOMES 4A, 5A, AND 7B AND ITS IMPACT ON RECOMBINATION

The construction of comparative genetic maps of chromosomes 4A(m) and 5A(m) of Triticum monococcum and chromosomes of homoeologous groups 4, 5 and 7 of T. aestivum has provided insight into the evolution of the se chromosomes. The structures of chromosomes 4A, 5A and 7B of modern- day hexaploid bread wheat can be explained by a 4AL/5AL translocation that occurred at the diploid level and is present both in T. monococcu m and T. aestivum. Three further rearrangements, a 4AL/7BS translocati on, a pericentric inversion and a paracentric inversion, have taken pl ace in the tetraploid progenitor of hexaploid wheat. These structural rearrangements and the evolution of chromosomes 4A, 5A and 7B of bread wheat are discussed. The presence of the 4AL/5AL translocation in sev eral Triticeae genomes raises two questions - which state is the more primitive, and is the translocation of mono- or poly-phylogenetic orig in? The rearrangements that have occurred in chromosome 4A resulted in segments of both arms having different positions relative to the telo mere, compared to 4A(m) and to 4B and 4D. Comparisons of map length in these regions indicate that genetic length is a function of distance from the telomere, with the distal regions showing the highest recombi nation.