Figure 1.

lin-39 inhibits cell fusion during vulva formation. lin-39 is expressed and active at the first larval stage (L1) in six epidermal cells (P3.p-P8.p; black circles), also known as the vulva precursor cells (VPCs). This activity (black box above cells) is necessary and sufficient to inhibit the VPCs from adopting the fate of their neighboring cells [P(1,2,9–11).p]—fusion to the surrounding syncytium hyp7 (dashed circles). At L3, the VPCs are subjected to various signaling pathways. P(3,4,8).p undergo one cycle of division, followed by fusion of their daughter cells to hyp7 (dashed circles). In about half of the cases, P3.p does not divide before cell fusion. P(5–7).p escape cell fusion and continue to divide, yielding a 22-cell vulval primordium. It was proposed that basal activity of LIN-39 in P(3,4,8).p is not sufficient for these cells to escape cell fusion, and only elevated activity of LIN-39 induced by the Ras pathway in P(5–7).p in wild-type (Maloof and Kenyon 1998) or by ectopic Wnt signaling in all VPCs in some mutants (Gleason et al. 2002), allows these cells to escape cell fusion (black and shaded boxes above cells). Migration, cell fusion (dashed lines), ring formation, and invagination of the primordial cells at L4 lead to a tube-shaped adult vulva (Sharma-Kishore et al. 1999).