Structural and Functional Evolution of Resynthesized Polyploids

Abstract

Polyploidy is widespread among the flowering plants. While many extant plant species show evidence of polyploidy in their genomes, there is still much to be learned regarding the role it has played in phenotypic evolution and speciation. The Brassica genus has polyploidy at multiple levels: the genomes of diploid species show evidence of repeated rounds of ancient polyploidization, and the agronomically important diploid species (Brassica rapa, Brassica oleracea, and Brassica nigra) may hybridize to form allopolyploids (Brassica napus, Brassica juncea, and Brassica carinata). The phenotypic diversity among these six domesticated species is spectacular. Research has provided evidence that gene and genome redundancy contribute significantly to the variation observed among Brassica species. The relative ease by which Brassica allopolyploids can be resynthesized has allowed them to emerge as an efficient model for studying the consequences of polyploidization. Studies on resynthesized B. napus polyploids have reported on homoeologous genome rearrangements and epigenetic changes, as well as changes in gene expression, protein expression, and phenotypic variation in the early generations following hybridization and polyploidization. In contrast, studies in resynthesized B. juncea polyploids show little evidence for rapid changes, although a recent report indicated changes within the organelle genomes. The differences between these two resynthesized allopolyploids may be attributed to the differing degrees of similarity between their diploid progenitor genomes (those of B. napus being more similar to each other than those of B. juncea). It may also be due to differences in genetic variation for homoeologous pairing control in these two species. These studies demonstrate that polyploidization may have immediate genetic and phenotypic consequences, particularly in B. napus. The variation that results from a polyploidization event may be critical in the early establishment and evolution of new polyploids; however, selection must have played a critical role in the establishment of natural Brassica polyploids, as their genomes appear relatively stable. Future studies of resynthesized Brassica allopolyploids should include tests for the effects of progenitor genotypes, selection for fertility under field-style conditions, and use genomic approaches that have up to now been limited to studies of polyploid Arabidopsis suecica.