THE HOX GENE LIN-39 IS REQUIRED DURING CAENORHABDITIS-ELEGANS VULVAL INDUCTION TO SELECT THE OUTCOME OF RAS SIGNALING

The Ras signaling pathway specifies a variety of cell fates in many or ganisms. However, little is known about the genes that function downst ream of the conserved signaling cassette, or what imparts the specific ity necessary to cause Ras activation to trigger different responses i n different tissues, In C. elegans, activation of the Ras pathway indu ces cells in the central body region to generate the vulva, Vulval ind uction takes place in the domain of the Hox gene lin-39. We have found that lin-39 is absolutely required for Ras signaling to induce vulval development, During vulval induction, the Ras pathway, together with basal lin-39 activity, up-regulates lin-39 expression in vulval precur sor cells. We find that if lin-39 function is absent at this time, no vulval cell divisions occur, Furthermore, if lin-39 is replaced with t he posterior Hox gene mab-5, then posterior structures are induced ins tead of a vulva, Our findings suggest that in addition to permitting v ulval cell divisions to occur, lin-39 is also required to specify the outcome of Ras signaling by selectively activating vulva-specific gene s.