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Allopolyploidy, which involves genome doubling of an interspecific hybrid is an important mechanism of abrupt speciation in flowering plants [1–6]. Recent studies show that allopolyploid formation is accompanied by extensive changes to patterns of parental gene expression ("transcriptome shock") [7–15] and that this is likely the consequence of interspecific hybridization rather than polyploidization [16]. To investigate the relative impacts of hybridization and polyploidization on transcription, we compared floral gene expression in allohexaploid Senecio cambrensis with that in its parent species, S. vulgaris (tetraploid) and S. squalidus (diploid), and their triploid F1 hybrid, S. x baxteri[17]. Major changes to parental gene expression were associated principally with S. x baxteri, suggesting that the polyploidization event responsible for the formation of S. cambrensis had a widespread calming effect on altered gene …