Lifespan-extending downregulation of insulin signalling reduces germline mutation load

Reduced insulin/IGF-1 signalling (IIS) robustly extends lifespan and enhances somatic stress resistance across taxa, yet its consequences for germline genome integrity remain unclear. Here we combine multigenerational mutation accumulation with whole-genome sequencing in <i>C. elegans</i> to test whether adulthood-only IIS downregulation can simultaneously promote somatic maintenance and limit germline mutational burden. We reduced IIS by adult-onset <i>daf-2</i> RNAi in wild-type and heritable RNAi-deficient ( <i>hrde-1</i> ) backgrounds, allowing either spontaneous or UV-induced germline mutations to accumulate over multiple generations. In wild-type animals, reduced IIS lowered germline single-nucleotide mutation rates by up to ∼50% and prevented the UV-induced elevation in mutation rate, without detectable costs to fecundity or lineage persistence. By contrast, in <i>hrde-1</i> mutants the same intervention increased both point mutations and transposable-element-driven insertions under UV exposure, accelerating lineage extinction. Thus, the genome-protective effect of reduced IIS critically requires the germline nuclear Argonaute HRDE-1, which mediates small-RNA-guided epigenetic silencing. Functional annotation of germline variants revealed enrichment in pathways linked to development, cellular maintenance and conserved longevity regulators, including IIS and mTOR, and identified high-impact mutations in genes with human orthologs implicated in neurodegeneration and cancer. Our findings show that IIS can coordinate somatic and germline maintenance in concert, rather than in competition, through an HRDE-1-dependent epigenetic pathway. This work positions nutrient-sensing IIS as a central regulator of germline genome stability and suggests that IIS downregulation can reduce germline mutation load while extending lifespan, with broad implications for biogerontology and evolutionary biology.