In polyploids, one plus one does not equal two

Polyploidization has occurred often in the evolutionary history of eukaryotes, yet we are only now learning about the genetic and epigenetic events that occur in newly formed polyploids. A long-standing assumption has been that each member of a gene pair would contribute equally to expression levels after polyploidization. This assumption has now been shown to be false in cottons. Adams et al. studied expression levels in different plant tissues from a newly formed allotetraploid cotton (bearing two copies of the so-called ‘A’ genome and two copies of the ‘D’ genome). The authors found that the proportion of mRNA levels transcribed from the A and D genomes varied dramatically among tissues. This result implies that duplicated genes can undergo an immediate divergence in function as a pleiotropic effect of hybridization or polyploidization, providing a novel explanation for why selection often preserves duplicated genes.