Cell fate specification is critical for developmental patterning, and is well illustrated by the C. elegans male-specific postembryonic blast cell B. The B cell's first division is asymmetric, producing a large anterior daughter B.a, and a small posterior daughter B.p. B.a gives rise to all cells responsible for synthesis of the male copulatory spicules, whereas B.p produces no spicule cells. Genes known to be important for spicule formation include the Pax-6 homolog
vab-3, which is specifically necessary for B.a cell development. In
vab-3 mutant animals, the B.a cell or its daughters behave similar to the B.p cell. We have made a
vab-3::gfp transcriptional reporter that is expressed in B.a and its descendents, recapitulating the reported VAB-3 expression pattern(1). Through mutant analysis, we find that two distinct genetic pathways act to restrict
vab-3 expression to the B.a cell and its descendents. One pathway, mediated by the Wnt gene
lin-44, restricts
vab-3 expression within the B lineage. LIN-44 signals to coordinate the asymmetric division of B through a Frizzled-type receptor LIN-17. In
lin-44 mutants, division of B often shows a reversed asymmetry, with only the posterior daughter dividing like a normal B.a(2). In
lin-17 mutants, B cell division is commonly symmetric, and both daughters divide like a normal B.a(3). In these mutants, we find that
vab-3::gfp is expressed only in cells exhibiting the B.a fate, suggesting differential
vab-3 transcription is a key response to Wnt signaling in B. Experiments focused on the Tcf-1 homolog
pop-1 are underway to further elucidate the regulation of
vab-3 transcription in this lineage. A second pathway, mediated by the ovo-related gene
lin-48, restricts
vab-3 expression to the B cell. In
lin-48 mutants, the blast cell U can inappropriately produce spicule cell types. We observe that
vab-3::gfp is ectopically expressed in the daughters of the presumptive U cell in
lin-48 mutants. This indicates one function of
lin-48 may be to distinguish U from B by restricting
vab-3 expression to B. Further analysis is planned to test the roles of other known transcriptional regulators to better understand how different developmental inputs converge to influence
vab-3 expression. 1. Zhang, Y. et al. (1998) Mech. Dev. 78: 179-87; 2. Herman, M.A., and Horvitz, H.R. (1994) Development 120: 1035-1047; 3. Sternberg, P.W., and Horvitz, H.R. (1988). Dev. Biol. 130: 67-73.