Evolutionary implications of genome and karyotype restructuring in nicotiana tabacum L

Abstract

Nicotiana tabacum is an allopolyploid that formed within the last 200,000 years from relatives of the extant diploids N. sylvestris and N. tomentosiformis, the donors of the S- and T-genomes, respectively. Here we review progress in our understanding of the divergence of N. tabacum subsequent to its formation, by comparing the N. tabacum genome with those of its diploid progenitors. We also review the data from synthetic N. tabacum, where there is evidence for much genetic change in early generations, including various chromosomal translocations, allopolyploid-induced retroelement mobility and loss, and reductions in the copy numbers of some tandem repeats. These observations are similar to patterns found in natural N. tabacum, suggesting that rapid genetic divergence is induced by allopolyploidy. The T-genome of N. tabacum shows the greatest number of genetic changes and appears to be less stable than the S-genome. We describe possible mechanisms that may have stimulated these genetic changes and propose that these can lead to enhanced fertility, more regular chromosome pairing, and the evolution of disomic inheritance.