Figure 4 Patterning of equivalence groups in C. elegans (A) Model for EGF, Wnt and TGF-b signaling during Bg/d specification Patterning of equivalence groups in C. elegans (A) Model for EGF, Wnt and TGF-b signaling during Bg/d specification. The EGF and TGF-b pathways specify Bg fate by regulating the transcription of target genes such as
ceh-13/hox1. Wnt controls the axis of division of Bg, possibly by orienting the mitotic spindle. POPTOP expression suggests Wnt may play a role in Bg fate specification. (B) A comparison of the HCG, VPCs, P11/12 and Bg/d groups. EGF and Wnt signaling have different requirements relative to each other during the patterning of each equivalence group. This difference may account for the specificity of fate by both pathways induced in each group. In addition, Wnt signaling is required for Bg division along the correct axis. Such a role for Wnt signaling has not been observed in the other equivalence groups. Another factor that may contribute to fate specification in each equivalence group is the use of a third pathway during patterning. TGF-b signaling by
dbl-1/dpp is required to specify Bg fate and does not appear to act during VPC and P12 specification, equivalence groups in which EGF signaling is the major inductive signal. Finally, downstream of the EGF and Wnt pathways, a different Hox gene is expressed in each equivalence group and required to specify fate within that group. One exception is
ceh-13/Hox1 for which a functional role in Bg fate specification has not been identified.