A Common Origin for the NT Superfamily Underlies Nervous System Structure and Function

<div><p>(A) NT superfamily members identified in protostomes (<i>Drosophila</i>) and deuterostomes (vertebrates, <i>Amphioxus</i>, sea urchin, and acorn worm) imply that NTs were present in <i>Urbilateria</i>, their common ancestor. A protostomian <i>NT</i> gene would have duplicated to give rise to <i>DNT1</i>, <i>DNT2</i>, and <i>spz</i> in insects or perhaps earlier. A chordate <i>NT</i> duplicated twice to give rise to the four vertebrate NTs, and the <i>NGF</i> ortholog duplicated again in fish to result in <i>NT6/7.</i> Identified Trk and p75 receptors are also shown; Trk-like receptors lack some extracellular domains. NT receptors and signalling mechanisms may have diversified through evolution. Annelids, flatworms, nematodes, and tunicates are not shown, see <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0060284#pbio-0060284-sg001" target="_blank">Figure S1</a>.</p> <p>(B) Hypothesis that the NTs are required in all centralised nervous systems to link structure and function. NTs are also present at least in acorn worm with a nerve net, a diffuse nervous system, where NT may play a subset of functions, suggesting that NTs could also be present in lower animals (e.g., Cnidarians). <i>Drosophila</i> can be used as a model system for NT-related studies.</p></div>