Two evolutionarily distinct classes of paleopolyploidy.

Whole genome duplications (WGDs) occurred in the distant evolutionary history of many lineages and are particularly frequent in the flowering plant lineages. Following paleopolyploidization in plants, most duplicated genes are deleted by intrachromosomal recombination, a process referred to as fractionation. In the examples studied so far, genes are disproportionately lost from one of the parental subgenomes (biased fractionation) and the subgenome having lost the lowest number of genes is more expressed (genome dominance). In the present study, we analyzed the pattern of gene deletion and gene expression following the most recent WGD in banana (alpha event) and extended our analyses to seven other sequenced plant genomes: poplar, soybean, medicago, arabidopsis, sorghum, brassica, and maize. We propose a new class of ancient WGD, with Musa (alpha), poplar, and soybean as members, where genes are both deleted and expressed to an equal extent (unbiased fractionation and genome equivalence). We suggest that WGDs with genome dominance and biased fractionation (Class I) may result from ancient allotetraploidies, while WGDs without genome dominance or biased fractionation (Class II) may result from ancient autotetraploidies.