Equivalence group

[3] In many examples of equivalence groups, both induction and lateral inhibition are used to define patterns of distinct cell types.

This organ develops from a subset of cell of an equivalence group consisting of six VPCs, P3.p-P8.p, which lie ventrally along the anterior-posterior axis.

VPCs that are not induced form the 3° lineage (P3.p, P4.p and P8.p), which make epidermal cells that fuse to a large syncytial epidermis (see image).

[7] The anchor cell directly induces the vulva fates by secreting the epidermal growth factor (EGF)-like ligand LIN-3.

The decision to adopt either fate is determined after neural tube closure during the early tailbud stage (see image), via a poorly defined mechanism of induction.

[3][8] To elucidate whether the fates of the otolith and ocellus are determined in the early embryo or after the precursors align during neural tube closure, ablation and drug treatment techniques were used in the ascidian species Halocythia roretzi.

However, if the ablations are performed during the late tailbud stage (22.5 hrs) then the remaining cell has an equal likelihood of becoming an otolith or ocellus.

[1] Inhibiting cell division and morphogenesis with cytochalasin B is another method used to determine when the pigment precursor equivalence group is specified.

[1] Both experiments suggest that fates of the pigment precursor cells are irreversibly determined by approximately the mid-tailbud stage (21 hrs).

Vulva Precursor Cell Equivalence Group
Model for the determination of the VPCs (cells P3.p-P8.p) in C. elegans. An inductive signal from the anchor cell via LIN-3 determines P6.p to adopt the 1°. Then, the lateral signal LIN-12 causes P5.p and P5.7 to become 2°. The other VPCs do not receive an inductive signal and are fated as 3°. Anterior is to the left.
Otolith and Ocellus
Two bilateral pigment precursor cells form an equivalence group that eventually interacts to seal mutually exclusive fates (otolith and ocellus). Shown is a timeline of the developing Halocythia roretzi embryo (anterior is to the left). Pigment precursor cells (gray circles) remain equivalent until they align after neurulation. At 21 hours post fertilization the ocellus (white circle) and otolith (black circle) are determined.