Figure 4. HGT of ice-binding domain sequences between Arctic algae.

Consensus best scoring tree topology obtained with RAxML under JTT and WAG substitution models for a 4,862-branch × 193 aa alignment of all ice-binding domains (PF11999) sampled from UniRef, JGI algal genomes, MMETSP, and Tara Oceans. Branches are shaded by evolutionary origin and leaf nodes by biogeography (either isolation location of cultured accessions where recorded or on oceanic region for which >70% total abundance of each Tara unigene could be recorded). One Tara unigene (asterisked) shows bipolar distributions (>35% total abundance in both the Arctic Ocean and the Antarctic Southern Ocean). Thick branches indicate the presence of a clade in both best scoring tree outputs. The upper tree schematic shows an overview of the global topology obtained, four clades of algal IBPs with probable within-Arctic transfer histories and two clades of algal IBPs with probable within-Antarctic IBPs (indicated as Arctic A, B, C, and D and Antarctic 1 and 2). Numbers in parentheses identify the number of non-identical branches (i.e., gene sequences) identified in each named species. The earliest diverging branch in each clade, relative to the remaining global tree topology, is marked with an arrow. From these rooting points, probable horizontal transfer events can be inferred, for example, from monophyletic groups of sequences positioned within paraphyletic groups of sequences between sister groups of species with different phylogenetic derivations.