page_title: Inversion (evolutionary biology)
section_title: Evidence for inversion
hierarchical_section_title: Inversion (evolutionary biology) / Evidence for inversion
caption_attribution_description: English: A drawing of an acorn worm (Ptychodera flava) from New Caledonia.
context_page_description: In evolutionary developmental biology, inversion refers to the hypothesis that during the course of animal evolution, the structures along the dorsoventral axis have taken on an orientation opposite that of the ancestral form.
  Inversion was first noted in 1822 by the French zoologist Étienne Geoffroy Saint-Hilaire, when he dissected a crayfish and compared it with the vertebrate body plan. The idea was heavily criticised, but periodically resurfaced, and is now supported by some molecular embryologists.
context_section_description: In addition to the simple observation that the dorsoventral axes of protostomes and chordates appear to be inverted with respect to each other, molecular biology provides some support for the inversion hypothesis. The most notable piece of evidence comes from analysis of the genes involved in establishing the DV axis in these two groups. In the fruit fly Drosophila melanogaster, as well as in other protostomes, the β-type transforming growth factor (TGF-β) family member decapentaplegic (dpp) is expressed dorsally and is thought to suppress neural fate. On the ventral side of the embryo, a dpp inhibitor, short gastrulation (sog), is expressed, thus allowing nervous tissue to form ventrally. In chordates, the dpp homolog BMP-4 is expressed in the prospective ventral (non-neural) part of the embryo while several sog-like BMP inhibitors (Chordin, Noggin, Follistatin) are expressed dorsally.
  Other patterning genes also show conserved domains of expression. The neural patterning genes vnd, ind, msh, and netrin are expressed in the Drosophila ventral nerve cells and midline mesectoderm. The chordate homologs of these genes, NK2, Gsh1/2, Msx1/3, and Netrin, are expressed in the dorsal neural tube. Furthermore, the tinman/Nkx2-5 gene is expressed very early in cells that will become the heart in both Drosophila (dorsally) and chordates (ventrally).
  Additional support comes from work on the development of the polychaete annelid Platynereis dumerilii, another protostome. Even more so than Drosophila, its pattern of central-nervous-system development is strikingly similar to that of vertebrates, but inverted. There is also evidence from left-right asymmetry. Vertebrates have a highly conserved Nodal signaling pathway that acts on the left side of the body, determining left-right asymmetries of internal organs. Sea urchins have the same signaling pathway, but it acts on the right side of the body. It was even shown that an opposing right-sided signal for regulating left-right asymmetry in vertebrates, i.e. BMP signaling pathway, is activated on the left side of the sea urchin larva, suggesting an axial inversion during evolution from basal deuterostome to chordate such like amphioxus. Because in amphioxus, Nodal signaling pathway is on the left side of the embryo, which is the same situation as vertebrates.  Sea urchins, like other echinoderms, have radially-symmetric adults, but bilaterally-symmetric larvae. Since sea urchins are deuterostomes, this suggests that the ancestral deuterostome shared its orientation with protostomes, and that dorsoventral inversion originated in some ancestral chordate.
  There is evidence that invertebrate chordates are also inverted. Ascidian larvae have a dorsal mouth, as one would expect from inversion. The amphioxus has an odd feature: its mouth appears on the left side and migrates to the ventral side. Biologist Thurston Lacalli speculates that this may be a recapitulation of the migration of the mouth from the dorsal to the ventral side in a protochordate.
caption: Balanoglossus, an example of a Hemichordata, represents an "evolutionary link" between invertebrates and vertebrates.
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