In Caenorhabditis elegans and Pristionchus pacificus the vulva equivalence group is specified by the Hox gene
lin-39. C. elegans
lin-39 mutants are vulvaless and the VPCs fuse with the surrounding hypodermis, whereas in P. pacificus
lin-39 mutants the VPCs die of apoptosis. Mechanistically, LIN-39 might inhibit non-vulval fate (cell fusion in C. elegans, apoptosis in P. pacificus), promote vulval fate or do both. To study the mechanism of
lin-39 function, we isolated cell death mutations in P. pacificus and made double mutants between
lin-39 and a cell death defective mutation. To isolate cell death defective mutations in P. pacificus, we made use of the fact that 7 of the 12 Pn.p cells in the ventral epidermis die of PCD. In cell death defective mutations, no PCD should occur in the ventral epidermis, a region of the worm body that can easily be scored using Nomarski optics. We mutagenized wild-type animals with EMS and analyzed the progeny of F2 clonal lines. The progeny of approximately 3500 F2 hermaphrodites were examined and three recessive mutations were isolated in which the ventral epidermal cells P(1-4).p and P(9-11).p survived. The three mutations fall into two complementation groups, that have been originally named
Ppa-ipa-1 and
Ppa-ipa-2 for inhibitor of P-ectoblast apoptosis. However, further phenotypic characterization indicated that the absence of cell death is not restricted to the ventral epidermal region. To clone the P. pacificus homolog of
ced-3, we performed PCR using primers to the highly conserved C-terminal portion of
Cel-ced-3. To determine whether
Ppa-ipa-1(
tu79) or
Ppa-ipa-2(
tu80,
tu81) correspond to mutations in
Ppa-ced-3, we sequenced the coding region of
Ppa-ced-3 in the mutants. This sequence analysis showed that the strain
tu79 contains a point mutation at Pro417 resulting in a missense mutation, indicating that the
Ppa-ipa-1 mutant phenotye results from mutations in
Ppa-ced-3. Therefore, we rename
Ppa-ipa-1 as
Ppa-ced-3. Surprisingly, P. pacificus
ced-3;
lin-39 double mutants form a functional vulva in the absence of LIN-39 activity. Thus, in P. pacificus
lin-39 specifies the vulva equivalence group by inhibiting programmed cell death. Furthermore, these data reveal an important difference in a later function of
lin-39 between the two species. In C. elegans, LIN-39 specifies vulval cell fates in response to inductive RAS signaling, and in P. pacificus LIN-39 is not required for vulval induction.