We have been studying how
lin-31 acts to specify vulval cell fates.
lin-31 mutants display a deregulated phenotype in vulval development; any of the six Pn.p cells (P3.p-P8.p) may adopt any of the three vulval cell fates (1 , 2 , or 3 ). For instance, in one animal P7 .pwas observed to express a 3 fate though it was in close proximity to the anchor cell while P8 .padopted a 2 cell fate even though it was farther away from the anchor cell. In addition, adjacent Pn.p cells can each adopt a 1 cell fate, suggesting that the lateral signalling pathway, whereby a 1 Pn.p cell prevents its neighbors from also adopting a 1 cell fate, is defective. Therefore,
lin-31 is an integral component in the specification of all three cell fates and is required for the determination of all six Pn.p cells. Our proposal that
lin-31 has distinct functions in specifying three alternative cell fates is different from models explaining the function of other vulval determination genes, such as those with a vulvaless loss-of-function phenotype (Vul genes) or a multivulva loss-of-function phenotype (Muv genes). The Vul and Muv genes are thought to act as on/off switches that either activate or repress the anchor cell and hypodermal signalling pathways that control Pn.p cell fate. In
lin-31 mutants, Pn.p cell fates may become deregulated because all Pn.p cells lack proper
lin-31 regulatory functions. Our studies indicate that
lin-31 appears to act at a late step in the anchor cell signalling pathway. First, the anchor cell was ablated with a laser in
lin-31 mutants and 1 and 2 cell fates were still expressed. This result indicates that
lin-31 cannot act only in the anchor cell, and suggests that
lin-31 is required at a step following the production of the anchor cell signal. Second,
lin-31 has been placed in the vulval developmental pathway using genetic epistasis experiments. Double mutants were constructed with
lin-31 (
n1053)and any one of five Vul mutants:
lin-2 (
e1309);
lin-7 (
e1413);
lin-10 (
n1390);
let-23 (
n1045);and
let-60 (
n1531dn)/+.All five double mutants display the
lin-31 deregulated phenotype. Pn.p cells can express 1 and 2 cell lineages in a variable way in the
lin-31 ;Vul double mutants as determined using direct observation with Nomarski microscopy or by staining with FITC-labelled concanavalin A, a marker for 1 and 2 cell lineages (G. Freyd, personal communication). Because we feel we are studying a regulatory pathway, we place
lin-31 downstream of
lin-2 ,
lin-7 ,
lin-10 ,
let-23 ,and
let-60 in the anchor cell signalling pathway. It is particularly interesting that
lin-31 is downstream of
let-60 ,a ras homologue. The
lin-31 gene product could be a substrate of
let-60 or could be acting farther downstream.