RAPID GENOME CHANGE IN SYNTHETIC POLYPLOIDS OF BRASSICA AND ITS IMPLICATIONS FOR POLYPLOID EVOLUTION

Although the evolutionary success of polyploidy in higher plants has b een widely recognized, there is virtually no information on how polypl oid genomes have evolved after their formation, In this report, we use d synthetic polyploids of Brassica as a model system to study genome e volution in the early generations after polyploidization. The initial polyploids we developed were completely homozygous, and thus, no nucle ar genome changes were expected in self-fertilized progenies. However, extensive genome change was detected by 89 nuclear DNA clones used as probes, Most genome changes involved loss and/or gain of parental res triction fragments and appearance of novel fragments, Genome changes o ccurred in each generation from F-2 to F-5, and the frequency of chang e was associated with divergence of the diploid parental genomes, Gene tic divergence among the derivatives of synthetic polyploids was evide nt from variation in genome composition and phenotypes. Directional ge nome changes, possibly influenced by cytoplasmic-nuclear interactions, were observed in one pair of reciprocal synthetics, Our results demon strate that polyploid species can generate extensive genetic diversity in a short period of time. The occurrence and impact of this process in the evolution of natural polyploids is unknown, but it may have con tributed to the success and diversification of many polyploid lineages in both plants and animals.