FIGURE 5.

A, schematic and consensual representation of metazoan phylogeny illustrating the presence/absence of Ngbs, other globins, the two other respiratory proteins (hemocyanin and hemerythin) and blood circulatory systems. The sporadic presence of globins in certain metazoan lineages can be explained by independent functional shifts from Ngb-like proteins (i.e. convergent evolution). Acoelomorphs and Xenoturbella are represented in two alternative phylogenetic positions, reflecting the ongoing debate as to their affiliations. B, unrooted molecular phylogeny based on multiple alignments of a subset of 84 sequences that comprise 138 amino acids of Ngbs, Lgb-related (Ngb-like), Hb, Mb, and Cygb sequences from diverse phyla. Dots indicate a possible myristoylated Ngb-like (green) or not (red). Vertebrate globins are in the yellow clusters. Hb, Mb, and Cygb appear to be an invention of vertebrates whereas vertebrate Ngbs and GbXs are embedded within the large green group where the functional Ngbs of S. roscoffensis and C. hemisphaerica occur (respective names are in bold red). The blue cluster that includes Ngb-like sequences from Choanoflagellates, Porifera (sponges), Placozoa, some Cnidaria, some protostome and deuterostomes including vertebrate Ngbs (yellow cluster) likely represents plesiomorphic Ngbs.