Fig. 6.

Divergent ectodermal and mesodermal GRN architecture in the early development of echinoids. (A) Truncated Sp GRN showing ectodermal and mesodermal regulatory interactions investigated in this study. The solid black lines indicate conserved interactions, the dashed black lines indicate interactions that are likely conserved, and the solid red lines indicate divergent regulatory interactions. Embryonic domains are represented by boxes: pink, ANE; yellow, OE; blue, CB; green, AE; brown, oral NSM; light brown, aboral NSM; white, SM. (B) Restriction of ectodermal CB in three echinoderm taxa. Red bars indicate the presence of gene restriction. Color scheme is the same as in A. The top row of the schema represents the unperturbed state; the bottom row, alternative patterning in perturbation background. (C) Mechanisms of polarization of NSM in the same echinoderm taxa. In both cidaroids and euechinoids, Nodal signaling up-regulates Not, which segregates O-A NSM cell types by restricting genes to the aboral side. Whereas asteroids show no indication of NSM O-A polarity, cidaroid and euechinoid lineages have undergone heterochronic shifts since their divergence.