SUPPLEMENTARY INFORMATION:
Supplementary Methods
Association between genetic and geographical distances
Sequences used in this study were for the 18S rRNA gene from Sicyoidochytrium minutum isolates of known locations around the globe. They were retrieved from publicly available source (GenBank; Altschul, 1990, Benson et al., 2014;  Table S1). The sequences were aligned using MAFFT with the L-INS-i strategy (Katoh et al., 2005). Pair wise genetic distances (p-distances; Nei and Kumar, 2000), were calculated between the sequences of using the software MEGA6 (Tamura et al., 2013). Geographical distances were estimated by using the great circle distance method (Sinnott, 1984) spanning from the shortest sea route from Iceland via the Bering Strait to the Pacific Ocean. To test a hypothesis of isolation by distance under the stepping stone model (Kimura and Weiss, 1964), a Mantel test (Mantel, 1967) was carried out on both pair wise genetic distances and geographical distances. MRM regression analysis (multiple regression on distance matrices) was used to estimate regression statistics between the resulting genetic and geographical distances (Lichstein, 2007).
To examine the influence of taxonomic “lumping” (Green and Bohannan, 2006) on the cosmopolitanism of Labyrinthulomycetes species, a sample of 16 isolates throughout the world (Table S1),were tested for a signal of isolation by distance as outlined above.