Sequence of events leading to near-complete genome turnover in allopolyploid Nicotiana within five million years
o Analyses of selected bacterial artificial chromosomes (BACs) clones suggest that the retrotransposon component of angiosperm genomes can be amplified or deleted, leading to genome turnover. Here, Nicotiana allopolyploids were used to characterize the nature of sequence turnover across the whole genome in allopolyploids known to be of different ages. Using molecular-clock analyses, the likely age of Nicotiana allopolyploids was estimated. Genomic in situ hybridization (GISH) and tandem repeat characterization were used to determine how the parental genomic compartments of these allopolyploids have diverged over time. Paternal genome sequence losses, retroelement activity and intergenomic translocation have been reported in early Nicotiana tabacum evolution (up to 200 000 yr divergence). Here it is shown that within 1 million years of allopolyploid divergence there is considerable exchange of repeats between parental chromosome sets. After c. 5 million years of divergence GISH fails. This GISH failure may represent near-complete genome turnover, probably involving the replacement of nongenic sequences with new, or previously rare sequence types, all occurring within a conserved karyotype structure. This mode of evolution may influence or be influenced by long-term diploidization processes that characterize angiosperm polyploidy-diploid evolutionary cycles.