MOLECULAR ANALYSIS OF EVOLUTIONARY PATTERNS IN U-GENOME WILD WHEATS

The theory of pivotal-differential evolution states that one genome of polyploid wheats remains stable (i.e., pivotal) during evolution, whi le the other genome or genomes may become modified (i.e., differential ). A proposed mechanism for apparent modification of the differential genome is that different polyploid species with only one genome in com mon may exchange genetic material. In this study, we analyzed a set of sympatric and allopatric accessions of tetraploid wheats with the gen omic constitutions UM and UC. The U genome of these species is from Tr iticum umbellulatum and is considered to be the pivotal genome. The M and C genomes, from T. comosum and T. dichasians, respectively, are co nsidered to be the differential genomes. Low copy DNA was analyzed usi ng ''sequence tagged site'' primer sets in the polymerase chain reacti on, followed by digestion with restriction enzymes. Genetic similarity matrices based on shared restriction fragments showed that sympatric accessions of different U genome tetraploid species did not tend to sh are more restriction fragments than did allopatric accessions. Thus, n o evidence for introgression was found. Analysis of the diploid progen itor species showed that the U genome was less variable than the M and C genomes. Additionally, comparison of diploid and polyploid species using genome-specific primer sets suggests a possible polyphyletic ori gin for T. triunciale and T. machrochaetum. Thus, our results suggest that the differential nature of the M and C genomes may be the result of variability introduced by the diploid progenitors and not the resul t of frequent introgression events after formation of the polyploid.