OSBORN, T. C.*, M. E. SCHRANZ, P. A. QUIJADA, and L. N. LUKENS. Department of Agronomy, University of Wisconsin, Madison, WI 53706. - Polyploidy and genetic variation in Brassica species.
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