Assessing viral connectivity of Iceland at the global scale
Global phylogenetic reconstruction of the PB2 segment demonstrated that viruses sampled in Iceland cluster within both the Eurasian and North American clades (Figure 1) and indicated multiple, independent introduction events rather than sustained endemic circulation.
The most significant geographic state transitions from global regions to Iceland occurred via mainland Europe and to a lesser degree North America (Figure 2a), with 18% of global Markov jumps between mainland Europe and Iceland (which are represented as the percent of global transitions between regional states along phylogenetic tree branches) and to a lesser degree between North America and Iceland (5% of global Markov jumps). Consistent with the stronger connectivity to Europe relative to North America, IAVs introduced from mainland Europe to Iceland occurred at a rate 3.6 times greater Markov jumps (which are represented as the percent of global transitions between regional states along phylogenetic tree branches) than from North America to Iceland (Figure 2b). Iceland primarily acted as a sink for viruses from other high latitude regions, rather than a source. Other global regions were significantly phylogeographically connected, highlighting ecologic and epidemiologic linkages from North America to South America (BF= 47475), Asia to Africa (BF= 1893.96), Asia to Europe (BF=494.52), Asia to North America (BF= 30.94), Europe to Africa (BF= 21.47), South America to North America (BF= 19.99) (Figure 2a, Supplementary table 2). Markov rewards, which represent the mean proportion of time that viruses spend in each geographic region, demonstrated that IAVs circulate longest in Asia, followed by North America, and that continuous circulation in mainland Europe and Iceland is far less than the larger global regions (Supplementary Table 3a).
Continuous diffusion analyses further revealed the interconnectedness of Iceland isolates with lineages in mainland Europe and North America. Given the strong evolutionary and migratory connectivity between viruses isolated in Iceland with those isolated in Europe and North America, we sought to characterize the strength of association and directional flow of IAVs between intracontinental regions within and between mainland Europe, North America, and Iceland. Discrete phylogeographic diffusion modeling demonstrated with very strong statistical support (BF= 110,622) that locations in mainland Europe (excluding Scandinavia) are the most significant sources of IAVs in Iceland (Markov jumps 5.37%), followed by the northeastern region of the United States (Markov jumps 1.25%). Iceland acted as a highly supported (BF= 127.22) source region of IAVs to northeastern Canada and Greenland (Markov jumps 0.82%) (Figure 3). Bayes factors and posterior probabilities for all intracontinental regional state transitions can be found in Supplementary Table 4.