Sphingolipids serve as stress second messengers and the genes involved in sphingolipid biosynthesis are conserved between humans and C. elegans. In mammals, ceramide mediates apoptosis, cell cycle arrest and differentiation, whereas sphingosine-1-phosphate (S1P) promotes proliferation and inhibits cell death. In this study, ionizing radiation (IR)-induced germ cell apoptosis was investigated in deletion mutants of twelve genes of the C. elegans sphingolipid metabolic pathway. We found that IR-induced germ cell apoptosis in N2 and lf ABL-1 mutant,
abl-1(
ok171), are suppressed in two lf ceramide synthase mutants,
hyl-1(
ok976) and
tag-160(
gk327). In contrast,
tag-274(
ok1097), a null allele for sphingosine kinase (SK), which generates S1P, displays increased baseline and IR-induced germ cell apoptosis. Microinjection of ceramide analogues C2 or C16 into N2 mimicked IR, inducing germ cell apoptosis. This event was blocked by inactivating the core apoptosis machinery using lf mutants in
ced-3,
ced-4 or a gf
ced-9. To delineate the interaction between sphingolipids and the established DNA damage cell death pathway, we generated double mutants of
hyl-1(
ok976) and
tag-160(
gk327) with
abl-1(
ok171). The increase in IR-induced germ cell apoptosis in
abl-1(
ok171) was abolished by lf of either ceramide synthase gene. Further, treatment of
abl-1(
ok171) worms with S1P, which in mammals blocks ceramide action rather than generation, abolished apoptosis after IR in
abl-1(
ok171). These studies indicate that ceramide is required for radiation-induced apoptosis in the C. elegans germline. Prior studies showed the
p53 homolog CEP-1 is also required for DNA damage-induced germ cell apoptosis via transcriptional up-regulation of
egl-1. As radiation-induced apoptosis is abrogated by lf CEP-1, and as enhanced apoptosis in the SK mutant
tag-274(
ok1097) or in ceramide-injected worms, is only partially inhibited, the data suggest that ceramide-mediated apoptosis is on a pathway parallel to and obligate for CEP-1 function. Consistent with this hypothesis, CEP-1-mediated induction of
egl-1 and
ced-13 after IR was unaltered in the two ceramide mutants
hyl-1(
ok976) and
tag-160(
gk327). Our study provides evidence that the sphingolipid pathway plays an essential role in regulating germ cell apoptosis in C. elegans. We propose a model in which ceramide and CEP-1/p53 respond in parallel to cellular stress, propagating signals that ultimately converge at the cell death core machinery.