According to the simplest model for pattern formation during vulval development, the inductive signal from the anchor cell is graded and directly stimulates Pn.p cells to be 1 or 2 , depending on their distance from the anchor cell [Cell 44:761(1986)]. In this view, anchor cell-proximal Pn.p cells prevent more distal Pn.p cells from receiving a high level of signal simply by being in the way. I have now obtained evidence that 1 cells prevent adjacent cells from also becoming 1 . In
lin-15 Multivulva hermaphrodites, all six Pn.p cells are 1 or 2 . In no case are adjacent cells 1 (0/43 pairs), while adjacent cells often are 2 (14/43 pairs). These observations suggested that a 1 cell prevents its neighbors from being 1 . To test this hypothesis, I destroyed by irradiation from a laser microbeam the gonad and P(3,4,5,6,8).p from six
lin-15 hermaphrodites. In all six animals, P7.p was 1 . By contrast, in four animals lacking a gonad but with all six Pn.p cells intact, P7.p was 2 . I conclude that the 'ground state' of a Pn.p cell in a
lin-15 animal is to be 1 , and that if a neighboring cell is 1 , a Pn.p cell will become 2 . This result holds so far for at least some other Pn.p cells in
lin-15 animals, and in
lin-8; by genetic ablation. [See Figure 1] I have also examined
lin-15 animals with two Pn.p cells: if two Pn.p cells are touching, one is 1 and one is 2 ; if two Pn.p cells are not touching, both are 1 . Thus, the inhibitory signal from a 1 cell to its neighbors might be mediated by direct cell-cell contact. Since
lin-12 is necessary for 2 cell fates in
lin-15 animals (all six Pn.p cells are 1 in a
lin-12(0); utant),
lin-12 may play a central role in this cell-cell contact inhibition, a role not inconsistent with the structure inferred by Greenwald et al. [1987 CSH Mtg].. The precision of the pattern formation process during vulval development may result from the combined action of two intercellular signals: a graded signal from the anchor cell differentially affecting 1 and 2 cells, and a signal from 1 cells to 2 cells inhibiting them from becoming 1 and/or stimulating them to become 2 . [See Figure 2]