Animal left–right asymmetry Martin Blum, Tim Ott  Current Biology  Volume 28, Issue 7, Pages R301-R304 (April 2018) DOI: 10.1016/j.cub.2018.02.073 Copyright © 2018 Elsevier Ltd Terms and Conditions

Figure 1 Human organ situs. (A) Normal arrangement (situs solitus). (B) Historical preparation by Meckel the Elder of a human torso displaying heterotaxia, with normal position of the heart (h) and inverted stomach (s). From the Meckelsche Sammlungen of the Institute of Anatomy and Cell Biology, Martin-Luther University Halle-Wittenberg (Germany). Photograph by Janos Stekovics. Artwork by Bernd Schmid (University of Hohenheim). Current Biology 2018 28, R301-R304DOI: (10.1016/j.cub.2018.02.073) Copyright © 2018 Elsevier Ltd Terms and Conditions

Figure 2 Animal asymmetry determinants. Distribution of the known asymmetry determinants Nodal, Myosin 1d/PCP and cilia across the animal tree of life (depicted using the examples of the freshwater polyp H. vulgaris, the marine annelid worm P. dumerilii, the great pond snail L. stagnalis, the fruit fly D. melanogaster, the purple sea urchin P. lividus and the South African clawed frog X. laevis). In lophotrochozoans, spiral cleavage determines Nodal asymmetry, while ecdysozoans lack Nodal but depend on myo1d and PCP. Deuterostomes (echinoderms and chordates) use a ciliated left–right organizer for asymmetric Nodal induction, which in case of the frog depends on myo1d as well. We propose that urbilateria, the hypothetical common ancestor of proto- and deuterostomes, used Myo1d and asymmetric Nodal to position a long gut in the body cavity. Artwork by Bernd Schmid. Current Biology 2018 28, R301-R304DOI: (10.1016/j.cub.2018.02.073) Copyright © 2018 Elsevier Ltd Terms and Conditions

Figure 3 The vertebrate left–right organizer. Schematic of a vertebrate left–right organizer, as it appears for example in amphibians and mammals. The left–right organizer is embedded in the gut endoderm (large cells on either side). Central cells are characterized by flow-generating polarized cilia; flow is sensed by lateral cells harboring immotile and central cilia. Flow-sensing cells co-express Nodal (blue) and the Nodal-inhibitor Dand5 (magenta), which is downregulated through flow on the left side. Release of Nodal-repression results in asymmetric Nodal cascade gene activation in the left lateral plate mesoderm (not depicted). Artwork by Bernd Schmid. Current Biology 2018 28, R301-R304DOI: (10.1016/j.cub.2018.02.073) Copyright © 2018 Elsevier Ltd Terms and Conditions