Fig. 5

Conserved and divergent mechanisms of secondary muscle lineage induction. Schematic of the currently proposed models of secondary lineage muscle specification compared between Ciona (top row) and Halocynthia (bottom). In Ciona, Nodal from the b6.5 lineage at the 64-cell stage is required for neuromesodermal potential (represented by blue fill) in A7.8, and for expression of Delta ligand in A7.6. At the 110-cell stage, Delta signals from A7.6 and b6.5 line cells are also required for neuromesodermal potential in A8.16, a daughter cell of A7.8. In Halocynthia, unknown signal (“X”) from b6.5 line cells is required for muscle (but not neural) potential in A7.8, while unknown signal “Y” is required for Wnt5.a ligand expression in A7.6. At the 110-cell stage, Wnt5.a from A7.6 is likely required for secondary muscle specification in the A8.16 lineage. Diagram based on illustrations by Hudson and Yasuo [147] and data from Hudson and Yasuo [146], Hudson et al. [145], and Tokuoka et al. [329]