Figure 9

Hypothetical scenario of the role of Pitx in LR asymmetry and mouth development across metazoans. Green text marks LR asymmetric expression; magenta text marks median expression. In protostomes, echinoderms and hemichordates, the Nodal-Pitx pathway regulates determination of the right side. In cephalochordates, tunicates and vertebrates, on the other hand, this pathway determines the left side due to inversion of the body axes, an event that supposedly occurred at the base of chordate phylogeny. Concomitantly, new Pitx expression downstream of BMP signaling occurred at the anterior neural boundary (this interaction remains to be confirmed in amphioxus). Mouth development seems to be regulated differently between different deuterostome lineages. BMP signaling suppresses oral development in echinoderms and hemichordates, while it promotes oral development through Pitx in tunicates and vertebrates. In the cephalochordate amphioxus, the Nodal-Pitx pathway regulates LR asymmetry and is also responsible for development of the mouth, whose relation to the mouth of other chordates is contested.