Editors' ChoiceEVOLUTION

It Runs in the Family

Science Signaling  20 Jul 2010:
Vol. 3, Issue 131, pp. ec223
DOI: 10.1126/scisignal.3131ec223

Three of the major groups of animals—the lophotrochozoans (molluscs, segmented worms), the arthropods (insects, crustaceans, chelicerates), and the deuterostomes (chordates, hemichordates, echinoderms)—have a segmented body plan, and whether segmentation in these groups shares a common evolutionary origin is unclear. Even whether or not annelids and arthropods, which are more closely related to one another than either is to the deuterostomes, share a common segmented ancestor has long been a subject of vigorous debate within the evolutionary and developmental biology communities. In arthropod embryos, hedgehog (Hh) signaling plays an important role in patterning segments along the anteroposterior (AP) axis and defining segmental boundaries. Dray et al. report that Hh signaling also participates in patterning segments in the annelid Platynereis dumerilii, a marine polychaete worm. Genes encoding homologs of Hh signaling components—the ligand Hh, the receptor Patched (Ptc), and the transcription factor Cubitus interruptus (also known as Gli)—were expressed in developing segments of Platynereis larvae in patterns consistent with a role in segmental patterning and analogous to their expression patterns in insects. Treating larvae with cyclopamine or SANT-1, small-molecule inhibitors of Hh signaling, resulted in malformed segments and disrupted late, but not early, expression of other segmental AP patterning genes such as engrailed and Wnt1. These results suggest that Hh signaling is not required for segment formation in Platynereis but is required for maintenance of segment polarity, which is similar to its role in arthropod segmentation. These results suggest a molecular link between the segmentation systems of arthropods and annelids, implying that the two groups may have shared a common segmented ancestor.

N. Dray, K. Tessmar-Raible, M. Le Gouar, L. Vibert, F. Christodoulou, K. Schipany, A. Guillou, J. Zantke, H. Snyman, J. Béhague, M. Vervoort, D. Arendt, G. Balavoine, Hedgehog signaling regulates segment formation in the annelid Platynereis. Science 329, 339–342 (2010). [Abstract] [Full Text]