Antennapedia class Hox genes are best known for their role in specifying regional fates and segmental identity in early development. Nevertheless, these genes are expressed in intricate patterns throughout development in most metazoans. Recent studies have shown that this later expression is critically important for correct development; however, relatively little is known about the control or function of later Hox gene expression. The C. elegans Hox gene
lin-39 is required for specifying a number of mid-body cell fates; we have been investigating its role and expression during development of the vulva. The anchor cell (AC) induces vulval development by sending a signal to a subset of the Vulval Precursor Cells (VPCs). The AC signal is an EGF-like ligand, which activates a conserved Ras/Map-kinase pathway in the VPCs. We have found that the Hox gene
lin-39 is expressed in a striking graded pattern in the VPCs shortly before vulval development begins. Expression is strongest in P6.p, the presumptive primary cell, weaker in the presumptive secondary cells, and weaker still in the presumptive tertiary cells. Gonad ablated animals (which have no AC) show a much weaker gradient of
lin-39 expression in the VPCs. In addition, mutations which reduce function of the signalling pathway reduce
lin-39 expression in the VPCs, and mutations which increase signalling increase
lin-39 expression. Combined, these results demonstrate that
lin-39 expression can be controlled by AC signaling and the Ras pathway. Our staining experiments suggested that
lin-39 might play a role at the time of vulval induction. However, in
lin-39 mutants the VPCs fuse with the hypodermal syncytium long before vulval induction takes place. To determine if
lin-39 is required at the time of induction, we used a heat-shock-
lin-39 fusion construct to prevent the early VPC fusion and found that
lin-39 is also required later for the VPCs to respond to Ras signalling. Additional results suggest that
lin-39 acts downstream of
lin-1, the ETS-domain protein at the end of the known, conserved Ras/MapK pathway. We have found that
lin-39 levels do not determine which of the alternative vulval fates is adopted; instead
lin-39 helps determine organ identity. In males, posterior homologs of the VPCs make mating structures known as the hook and preanal ganglion (hook/PAG). We have found that ectopic
lin-39 can partially transform the hook/PAG lineages into vulval lineages, suggesting that
lin-39 plays an important role in specifying vulval fates. This result further suggests that
lin-39 plays an important role in determining the specificity of Ras signalling.