Development of the vulva is a complex process regulated by several signaling pathways. Two of these regulatory inputs, Ras and Wnt, converge on
lin-39 , which is essential for proper regulation of cell fates and fusion during vulval development. We previously reported that the
elt-5 and
elt-6 genes may be involved in development of the vulva as well as in seam cell development (KK and JR, WCWM, '00). Here we report that
egl-18, a gene previously identified in genetic screens, is identical to
elt-5 , and that
egl-18/elt-5 and
elt-6 may function in the vulva in a manner similar to that of
lin-39 .
elt-5 and
elt-6 are adjacent genes encoding single-finger GATA factors that appear to be transcribed both monocistronically and as a dicistronic operon. Injection of concentrated
elt-5 dsRNA into adults results in fully penetrant late embryonic or L1 lethality. Seam cells in affected animals do not differentiate properly and often undergo inappropriate fusion with the epidermal syncytia. Injection of
elt-6 dsRNAs does not cause any observable phenotype, and co-injection of
elt-5 and
elt-6 dsRNAs does not enhance the
elt-5(RNAi) single mutant phenotype. These results can be explained by proposing that while
elt-5 dsRNA eliminates both monocistronic
elt-5 transcripts and dicistronic
elt-5 / 6 transcripts,
elt-6 dsRNAs interfere with only the dicistronic transcripts. Both
elt-5 and -6 are expressed in the developing vulva, seam cells, and many other cells. Their transcription is controlled by separable enhancer elements that direct expression to different groups of cells. When ELT-6 is driven by a partial promoter of
elt-5 that does not include the vulval enhancer region, the early lethality and seam cell defects resulting from
elt-5 dsRNA are rescued. However, the surviving animals show Vul, Pvl, and Egl phenotypes. These phenotypes are similar to those of mutations in
egl-18 , which maps to the vicinity of
elt-5 . We found that
egl-18 mutant phenotypes can be rescued by a transgenic
elt-5 gene. Moreover, three
egl-18 alleles (
n162,
n474, and
n475 ) are nonsense mutations in the
elt-5 sequence, all of which are predicted to terminate translation prior to the DNA binding domain. Thus,
egl-18 encodes the ELT-5 GATA factor.
elt-6(RNAi) in
egl-18 chromosomal mutants cause penetrant late embryonic/early larval lethality. Collectively, these results imply that
egl-18/elt-5 and
elt-6 are functionally redundant and that only
egl-18 /
elt-5 activity, but not
elt-6 activity, is affected in
egl-18 chromosomal mutants. Preliminary results suggest that
egl-18/elt-5 and
elt-6 are expressed in the vulval precursor cells in a graded pattern: i.e., most strongly in P6.p, at an intermediate level in P5.p and P7.p, and weakly in P3-4.p and P8.p. This expression pattern is similar to that of
lin-39 , suggesting that like
lin-39 ,
egl-18/elt-5 and
elt-6 may be upregulated upon Ras signaling. In addition, in
egl-18/elt-5(RNAi) animals, vulval precursor cells often fuse in the late-L2/early-L3 stage or undergo only one or two cell divisions, similar to the defects seen in
lin-39(rf) mutants. We are currently performing experiments to assess the relationship between
egl-18/elt-5 ,
elt-6 , and
lin-39 in the pathway for vulval development.