My lab has been studying the vital role of LET-60/RAS in specifying the excretory duct cell fate. The excretory duct cell is one component of the worm?s primitive renal system that functions in osmoregulation and is essential for viability (Nelson and Riddle, 1984). Early ablation experiments (Sulston et al., 1983) suggested that the excretory duct cell (ABplpaaaapa) and the embryonic excretory pore cell G1 (ABprpaaaapa) form an equivalence group: ablation of the duct cell precursor caused G1 to adopt the duct fate. Those data suggested the possible presence of a lateral inhibition mechanism such as that typically mediated by Notch proteins. Alternatively (or in addition), the duct and G1 precursors could compete for access to a third cell that is the source of a duct-promoting inductive signal. The identity of this signaling cell is unknown. (STOP)
Specification of the excretory duct cell fate requires both Ras and Notch signaling (Lambie et al., 1991; Yochem et al., 1997).
let-60 ras is required cell autonomously for the excretory duct cell fate, and in
let-60 ras mutants, the duct cell precursor appears to adopt a G1 fate (Yochem et al., 1997). Furthermore, a duct precursor that lacks
let-60 ras can no longer prevent a
let-60+ G1 precursor from adopting the duct cell fate (Yochem et al., 1997). Therefore, Ras seems to be involved in receiving a duct-inducing signal and producing a duct-inhibiting signal. The role of Notch is unclear but, given the duct-less mutant phenotype, it cannot be exclusively to receive the duct-inhibiting signal. (STOP)
I expect the duct-inducing signal to be LIN-3/EGF since
lin-3 mutants lack the excretory duct cell at high frequency. Therefore I have asked where in embryos
lin-3::GFP is expressed. I see that the excretory cell strongly expresses
lin-3::GFP at an appropriate time, suggesting it may be the key duct-inducing cell. If so, this could explain why ABplpaaaapa normally becomes the duct (it reaches the left-of-midline excretory cell sooner than ABprpaaaapa does) and why
lin-12 glp-1 double mutants lack an excretory duct cell (they also lack the excretory cell and hence would have no inductive signal). Laser ablation and genetic experiments are planned to test this hypothesis.