fate. However, if the anchor cell is killed, in the absence of an inductive signal, then all of the VPCs assume the default 3° lineage. Molecular Mechanism The anchor cell directly induces the vulva fates by secreting the epidermal growth factor (EGF)-like ligand LIN-3. The P6.p cell receives the LIN-3 signal via the receptor tyrosine kinase LET-23 (P5.p and P7.p also receive LIN-3 but to a lesser extent). Activation of LET-23 in P6.p results in the activation of LIN-12 (Notch) in P5.p and P7.p. Experimental evidence shows that LIN-12 is necessary and sufficient for the formation of the 2° fate. Through lateral inhibition LIN-12 prevents the P5.p and P7.p cells from adopting the 1° lineage. Thus, in this example both inductive EGF signaling and lateral Notch activation patterns the VPC equivalence group. Ascidian Pigment Precursor Equivalence Group Introduction The larvae of ascidians (sea squirts) contain a pair of sensory pigment cells known as the otolith and ocellus. The otolith is used to sense gravity, whereas the ocellus responds to light. During embryogenesis the otolith and ocellus develop from two bilateral equivalent precursors. Either the left or right pigment precursor cell has equal probability of developing into the otolith or ocellus. 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. During normal development, after neural tube closure, the pigment precursors align dorsally along the anterior-posterior axis of the neural tube. Whichever cell aligns anteriorly will become the otolith, while the posterior cell will form the ocellus. In the absence of cell-cell interactions both cells develop into ocelli, which is the default fate. Experimental Methods for Studying Equivalence in Halocynthia roretzi 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. Cells that are labeled with fluorescein isothiocyanate-dextran (FDX) can be selectively photoablated by fluorescent excitation. When one FDX labeled pigment precursor cells is photoablated during the mid-neurula stage (15 hrs) the other will almost always develop into an ocellus. 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. Inhibiting cell division and morphogenesis with cytochalasin B is another