The hypothesis of three reassortments implies that the viruses have acquired exactly the same genome segments from the same group of parental viruses, which seems unlikely to occur by chance (probability = 0.0089, assuming panmixis and that exactly two genomic segments are swapped out). This probability might be increased if reassortments confer a selective advantage. We did detect a significantly stronger selection pressure on the M1 protein in the pre-emergence lineage of group 3 parental strain that led to the reassortant viruses (Table S5) . Previous reports suggested a few amino acid changes in M1 of influenza A and B viruses can confer a growth advantage in mouse lungs [27] [28] [29] . Although the M1 mutations identified in this pre-emergence lineage have not been functional characterized elsewhere in the authors' knowledge, one (residue 137, TRA) of them is close to a previously characterized mutation (residue 139, TRA) which controls the virulence in mouse model [27, 29] . Three of the inferred residue changes are located close to the electropositive surface of N-terminal domain of M1 protein ( Figure S7 ) that acts to bind viral RNA [30, 31] . The M1 matrix protein mediates encapsulation of viral RNAnucleoproteins into membrane envelope during packaging [31] , and has close contact with other viral proteins inside the viral particle. It seems possible that some of these changes may be involved in the adaptation of reassortant viruses, through promotion of structural interactions among viral proteins.