RAPID ELIMINATION OF LOW-COPY DNA-SEQUENCES IN POLYPLOID WHEAT - A POSSIBLE MECHANISM FOR DIFFERENTIATION OF HOMOEOLOGOUS CHROMOSOMES

To study genome evolution in allopolyploid plants, we analyzed polyplo id wheats and their diploid progenitors for the occurrence of 16 low-c opy chromosome- or genome-specific sequences isolated from hexaploid w heat. Based on their occurrence in the diploid species, we classified the sequences into two groups: group I, found in only one of the three diploid progenitors of hexaploid wheat, and group II, found in all th ree diploid progenitors. The absence of group II sequences from one ge nome of tetraploid wheat and from two genomes of hexaploid wheat indic ates their specific elimination from these genomes at the polyploid le vel. Analysis of a newly synthesized amphiploid, having a genomic cons titution analogous to that of hexaploid wheat, revealed a pattern of s equence elimination similar to the one found in hexaploid wheat. Appar ently, speciation through allopolyploidy is accompanied by a rapid, no nrandom elimination of specific, low-copy, probably noncoding DNA sequ ences at the early stages of allopolyploidization, resulting in furthe r divergence of homoeologous chromosomes (partially homologous chromos omes of different genomes carrying the same order of gene loci). We su ggest that such genomic changes may provide the physical basis for the diploid-like meiotic behavior of polyploid wheat.