Figure 4.

The U content evolution of segment 1, PB2. Blue asterisks are Human H1N1 strains, purple squares are H5N1 found in humans, and red pluses are the remaining human strains available from the NCBI database. Green crosses are all the avian strains available in the NCBI database at the time of analysis. The blue dashed line is the predicted evolutionary curve for U content change computed using the U, C block diagonal component of the substitution matrices. This matrix was derived from the nucleotide content of the 1918 H1N1 and 1933 H1N1 Wilson-Smith strains (Rabadan, Levine and Robins, 2006).