Genome redundancy, generated by polyploidy, is a prominent feature of many plant species and may provide novel genetic variation for the evolution of phenotypic diversity. We have investigated the molecular basis of this phenomenon in the genus Brassica by analyzing both recent and ancient polyploid events. Brassica rapa and B. oleracea behave genetically as diploids, but they have replicated genomes probably due to ancient polyploid events. These species also contain genotypes with a wide range of flowering times, an important component of the vast morphological diversity within the species. Our results from comparative mapping suggest that much of the variation in flowering time within B. rapa is due to alleles at multiple copies of FLC, a key regulator of flowering time in Arabidopsis thaliana. Classical genetic and transgenic experiments in A. thaliana demonstrate that FLC inhibits flowering in a rheostat-like manner. In B. rapa, the effects of alleles within and between FLC loci are completely additive, suggesting that polyploidy has provided a mechanism for expanding the rheostat-like effects of FLC on flowering time. The Brassica genus also includes amphidiploid species that arose relatively recently through hybridization of the diploid species. These amphidiploids can be resynthesized by crossing diploids and treating with colchicine, and our molecular analyses of early generation progenies suggest that polyploid genomes can change rapidly after formation. Variation in flowering time also has evolved quickly among the progeny of resynthesized B. napus polyploids, and this variation is heritable in self- and out-cross progenies. Thus, polyploidy not only affects phenotypic diversity by providing additional loci for the accumulation and maintenance of allelic variation, but it also may trigger the generation of novel genetic variation. We are now studying the structure and function of genes in resynthesized polyploids to understand the molecular basis of new genetic variation.

Key words: Brassica species, flowering time, genetic variation, polyploidy