Proc. Natl. Acad. Sci. USA 92: 7719-7723 (Aug 1995)
Rapid genome change in synthetic polyploids of Brassica and its implications for polyploid evolution
Keming Song, Ping Lu, Keliang Tang, and Thomas C. Osborn

ABSTRACT Although the evolutionary success of polyploidy in higher plants has been widely recognized, there is virtually no information on how polyploid genomes have evolved after their formation. In this report, we used synthetic polyploids of Brassica as a model system to study genome evolution in the early generations after polyploidization. The initial polyploids we developed were completely homozygous, and thus, no nuclear 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 restriction fragments and appearance of novel fragments. Genome changes occurred in each generation from F2 to F5, and the frequency of change was associated with divergence of the diploid parental genomes. Genetic divergence among the derivatives of synthetic polyploids was evident from variation in genome composition and phenotypes. Directional genome changes, possibly influenced by cytoplasmic-nuclear interactions, were observed in one pair of reciprocal synthetics. Our results demonstrate 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 contributed to the success and diversification of many polyploid lineages in both plants and animals.

Implications of Rapid Genome Change for Polyploid Evolution. Using synthetic polyploids, we have demonstrated that extensive genome change can occur in the early generations of Brassica polyploids. Genetic diversity accumulated among self-fertilized progenies, even when the starting materials were completely homozygous. We do not know whether these types of changes or this extent of change has occurred in the early generations of natural Brassica or other polyploid species. However, our molecular results, when combined with variation in fertility and other morphological traits observed in our synthetic polyploids and in previous studies (26, 27), suggest that rapid genome change in newly formed polyploids can produce many novel genotypes that would provide new genetic variation for selection. Thus, rapid genome change could accelerate evolutionary processes among progenies of newly formed polyploids, and this may, in part, account for the success and diversification of many ancient polyploid lineages in both plants and animals.

26. Olson, G. (1960) Species crosses within the genus Brassica I. Artificial  Brassica juncea cross. Hereditas 46, 171-223.
27. Olson, G. (1960) Species crosses within the genus Brassica II. Artificial Brassica napus L. Hereditas 46, 351-386.