Fig. 5: Summary of anthozoan muscle transcriptome and model of muscle cell evolution.

a The left side shows key regulatory genes and their association with the four muscle types shown in the center panel. The dendrogram on the left indicates the developmental relationships between the cell types. Individual genes within the cell type boxed in the center panel are proposed here as cell-type-specific regulatory factors, whereas those positioned on the dendrogram are shared between descendants of these branches. On the right side, the described effector modules, or aponemes, are indicated. Note that the composition of this list is identical, but the individual paralogs are specific to each effector module. b Hierarchical clustering with expressed DNA-binding proteins vs. structural proteins shows noncongruence between regulatory gene profile and effector gene profile among the four muscle cell types in Nematostella. c Hypothesized events underlying the evolution of myocytes in the sea anemone. The fast-contracting effector module is hypothesized to have evolved within the endo(meso)dermal muscle populations and was then co-opted by the ectodermal epithelium after the radiation of PaTH-related bHLH proteins (Paraxis, Twist and Hand (see Supplementary Fig S1.5), red) and recruitment of one bHLH TF paralog, nem64, to the tentacle ectoderm within the sea anemone lineage.