VARYING CHROMOSOME COMPOSITION OF 56-CHROMOSOME WHEAT X THINOPYRUM-INTERMEDIUM PARTIAL AMPHIPLOIDS

Thinopyrum intermedium (2n = 42) is a source of many potentially usefu l genes for wheat improvement. Many partial amphiploids have been prod uced between Th. intermedium and Triticum aestivum that are fertile an d stable. These partial amphiploids all have 56 chromosomes, including seven pairs of chromosomes from Th. intermedium. To explore the genom ic composition of these lines, meiotic analysis was conducted on 32 hy brid combinations between eight different partial amphiploids. All but two of the chosen parents were distinguishable on the basis of perenn iality, head morphology, and reactions to leaf, stripe, and stem rusts and to barley yellow dwarf virus. Chromosome pairing in the hybrids c learly indicated that all but two of the partial amphiploids differed in their composition of Thinopyrum chromosomes. The differences varied from one to five chromosomes. This confirms molecular evidence that t he extra genome of the octoploid partial amphiploids is a variable syn thetic genome combining chromosomes of the three Thinopyrum genomes E, J, and X. Though the extra synthetic genomes vary widely between diff erent octoploids, they are nevertheless stable once formed. It is argu ed that the failure to establish these octoploid amphiploids as a new crop is a consequence of their differing chromosome complements, which makes it impractical to interbreed them.