The reproductive system of C. elegans regulates the lifespan of the rest of the organism. Germ cell removal {gc(-)} results in a 60% extension in lifespan. The presence of the somatic gonad is required for this increase in longevity, since removal of the whole gonad {somatic gonad/sg(-); gc(-)}, does not extend lifespan. Removal of the germ cells causes the FOXO transcription factor DAF-16 to accumulate in intestinal nuclei (Lin et al, 2001; Berman & Kenyon, 2006), where it functions to extend lifespan (Libina et al, 2003). We are exploring how the reproductive system modulates the lifespan of C. elegans. In principle, the presence of the germ cells could shorten lifespan by acting upstream of the somatic gonad to prevent the somatic gonad from producing signals that extend lifespan. In this case, removal of the somatic gonad {sg(-); gc (-)} should reverse every effect of germ cell ablation {sg(+); gc(-)}. Surprisingly, our experiments reveal that this is not the case, since animals missing the whole gonad also have DAF-16 in intestinal nuclei. Therefore, the germ cells probably act independently of the somatic gonad. Significantly, animals lacking the whole gonad have decreased transcription of the superoxide dismutase
sod-3, a DAF-16 target, relative to animals missing only the germ cells. This suggests that directly or indirectly, the somatic gonad promotes the activity of nuclear DAF-16 protein. Insulin-like signaling can also control the activity of DAF-16. In long-lived
daf-2 (insulin/IGF-1-receptor) mutants, lifespan is extended with germ-cell removal {gc(-)}. We find that when the level of DAF-2 activity is lowered below a certain threshold, removing the whole gonad {sg(-); gc(-)} does not shorten the lifespan of germline-ablated animals {sg(+); gc(-)}. One possibility is that germline-ablated animals {sg(+); gc(-)} live long because the presence of the somatic gonad lowers the activity of DAF-2 which normally functions to shorten lifespan. Our findings suggest that this is not the case. Instead, DAF-2 and the somatic gonad appear to function in parallel. In
daf-2(+) animals, the presence of the somatic gonad promotes longevity without affecting DAF-2. Curiously, the function of the somatic gonad can only be replaced by lowering DAF-2 signaling only when DAF-2 activity is reduced below an amount required simply to extend lifespan in otherwise normal animals. .