Distribution and evolution of DANGER genes in metazoa.

<p>(A) The tree on the left summarizes the phylogenetic relationships of the informative species used. Numbers at nodes represent cell types adopted from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0000204#pone.0000204-Carroll2" target="_blank">[13]</a>. Yellow boxes indicate the presence of a DANGER gene in a particular taxon. Numbers correspond to the number of genes found in each taxon. “X” indicates the absence of a DANGER gene in a particular taxon (blue boxes). Uncertain orthologous relationships are indicated by question marks (green boxes). “UN” indicates the unclassified DANGER proteins or the ones that could not be unconditionally assigned to a specific group. The choanoflagellate sequence from <i>M. ovata</i> assumes outgroup position in all phylogenetic analyses. Divergence times in million years ago (MYA) were taken from references <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0000204#pone.0000204-Ayala1" target="_blank">[58]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0000204#pone.0000204-Peterson1" target="_blank">[59]</a>. (B–C) Two alternative evolutionary scenarios explain the evolution of DANGER families in metazoa. Both scenarios presume that the genome sequence of the vertebrate ancestor encoded at least six DANGER lineages, namely D1–D6. (B) According to the first scenario, anthozoa contain two (D2, D6) of the six vertebrate DANGER lineages, while the remaining DANGER lineages have evolved by repeated cycles of gene birth (+) and death (−). (C) In the second scenario anthozoa species contain sequences orthologous to all vertebrate DANGER groups. This scenario presupposes that in ecdysozoa, hemichordates, and urochordates four DANGER groups were lost. Dotted lines correspond to ancestral stages. The order of events is presented according to the species tree.