Members of the Pax family of transcription factors are well characterized as regulators with important roles in the specification of various tissues and organs. Pax-6 is a key regulator of eye development in several species, and is important for the specification of other sensory tissues as well (Gehring, 2002). We study the role of the C. elegans Pax-6 gene
vab-3 in sensory organ development. Two sensory structures important for male mating are the copulatory spicules and the postcloacal sensillum. These structures develop from two of four post-embryonic blast cells in the male hindgut: F, U, B, and Y. The anterior daughter of B, B.a, gives rise to cells of the copulatory spicules, while the posterior daughter of Y, Y.p, produces the postcloacal sensillum (Sulston et al., 1980).
vab-3 is expressed in the B.a and Y.p lineages, and in
vab-3 mutant males, these blast cells fail to produce normal sensory structures (Zhang et al., 1998; Chamberlin and Sternberg, 1995). To define the regulatory inputs important for
vab-3 expression, we have utilized a transcriptional GFP reporter transgene. We have found that
vab-3 expression within the hindgut is restricted to the male sensory structure lineages by converging genetic pathways. One pathway corresponds to a transcriptional cascade involving the Pax-2,5,8 factor EGL-38 and the OVO-like zinc-finger protein LIN-48. In
lin-48 mutant males, the U cell lineage is transformed to a B.a-like lineage, and produces ectopic spicule cell types (Chamberlin et al., 1999; Jiang and Sternberg, 1999). EGL-38 is known to directly activate
lin-48 in the hindgut, and
egl-38 mutants also exhibit aberrant U lineage development (Johnson et al., 2001; Chamberlin et al, 1997). Consistently, we find
vab-3 is ectopically expressed in the U cell lineage of both
lin-48 and
egl-38 mutants. Thus, one function of these two genes is to negatively regulate
vab-3 in the U lineage. Additionally, the
egl-38 and
lin-48 genes display distinct developmental repression of
vab-3 , indicating that
egl-38 -dependent repression of
vab-3 is not mediated solely by
lin-48 . A second mechanism restricting
vab-3 expression within the male tail involves a Wnt signal transduction pathway. Reception of the LIN-44/Wnt signal by LIN-17/Frizzled is essential for the asymmetry and orientation of the first division of B. In wild type males, B divides to produces the larger anterior daughter B.a, and a smaller posterior daughter, B.p. In
lin-44 mutant males, B often divides with a reversed asymmetry, and produces a corresponding reversal of cell fates between daughter cells (Herman and Horvitz, 1994).
lin-17 mutant males frequently exhibit a symmetric division of B, and both daughters develop with a presumptive B.a-like cell fate (Sternberg and Horvitz, 1988). We have found that
vab-3 transcription correlates with the B.a cell fate in each of these mutants. The transcription factor of canonical Wnt signaling is encoded by the Tcf-1 ortholog
pop-1 , and
pop-1 mutants display independent defects in B cell behavior and
vab-3 expression. We conclude that Wnt signaling acts as a negative regulator of
vab-3 transcription in the B.p cell. In order to identify cis regulatory elements important for
vab-3 expression, we have initiated a promoter analysis. We have uncovered elements in the proximal upstream region and the first intron necessary for early transcriptional repression in B.p, resembling the activities of the Wnt pathway genes. In addition, this analysis has revealed other cell-specific, timing-specific, and sex-specific regulatory features of
vab-3 . Chamberlin and Sternberg. Dev Biol (1995) 170 : 679-89. Chamberlin et al. Dev (1997) 124 : 3919-27. Chamberlin et al. Genetics (1999) 153 : 731-42. Herman and Horvitz. Dev (1994) 120 : 1035-47. Gehring. Int J Dev Biol (2002) 46 : 65-73. Jiang and Sternberg. Dev Biol (1999) 211 : 88. Johnson et al. Dev (2001) 128 : 2857-65. Sternberg and Horvitz. Dev Biol (1988) 130 : 67-73. Sulston et al. Dev Biol (1980) 78 : 542-76. Zhang et al. Mech of Dev (1998) 78 : 179-87.