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.