Figure 3. Maps of inferred ancestries of the swine-origin A/H1N1 flu pandemic, based on 279

hemagglutinin (HA) DNA sequences available on Genbank as of the 26/06/2009. Arrows represent inferred ancestries, with colors indicating the statistical support (Equation 2). Local transmissions are shown as dots for single events, and sunflowers for multiple transmissions. The inset histogram displays the number of strains analyzed by collection date. Dates are provided as days since the most recent common ancestor (MRCA), estimated as the 21/01/2009 [22]. The three different panels display successive stages of the pandemic (corresponding time frame highlighted in the inset): a) transmissions from Mexico to the US, b) transmissions within the US, and beginning of world-wide spread, and c) further worldwide spread, likely from unobserved outbreaks outside the Americas.