The research was designed to probe the developmental theory of ageing, which holds that senescence is a by-product of developmental processes that are finely tuned by strong selection early in life but left poorly optimised for adulthood. Because the force of natural selection declines with age, the theory predicts that gene expression in adults drifts away from fitness optima, producing either too much of some activities, termed hyperfunction, or too little of others, termed hypofunction, as organisms grow old. Crucially, it also predicts that interventions made during adulthood, but not during development, should be able to slow ageing without fitness costs, since developmental regulation has been honed by intense selection. The IIS pathway, which integrates environmental cues into decisions about growth, reproduction and stress resistance, is an ideal test case because of its broad pleiotropic effects across the life course.

In C. elegans, downregulating the IIS pathway by knocking out or silencing the daf-2 insulin receptor is famous for roughly doubling lifespan. Knockout mutants, however, pay for their longevity with compromised development, delayed reproduction and reduced offspring number, and earlier studies of RNA interference against daf-2 during larval development likewise reported reduced early fitness. The new experiments used RNAi feeding, in which bacteria expressing double-stranded RNA against daf-2 silence the receptor gene, at precise life stages, comparing lifelong treatment with brief 24-hour exposures during the first larval stage or the first day of adulthood, and with knockdown spanning the whole developmental period from hatching to sexual maturity. All assays were blinded with respect to treatment.

The lifespan results were striking. Even a transient 24-hour exposure to daf-2 RNAi, whether in early development or at the onset of adulthood, extended lifespan by up to 40 percent, while lifelong knockdown approximately doubled it, consistent with previous work. When knockdown ran throughout development, the magnitude of lifespan extension depended strongly on temperature: median lifespan rose by almost 35 percent at 15 degrees Celsius, but the effect vanished entirely at 25 degrees. Quantitative PCR showed that daf-2 expression returned to control levels within 48 hours of treatment ending, yet the longevity phenotype persisted long afterwards. The authors attribute this durability to downstream genes regulated by the IIS pathway that are implicated in stress response, antimicrobial resistance and protein degradation, meaning that changes in proteostasis, stress resistance and immunity, rather than the receptor itself, carry the long-lasting signal.

Contrary to the prevailing paradigm, developmental knockdown of daf-2 accelerated larval development at all three temperatures tested, 15, 20 and 25 degrees, and improved reproduction at the standard laboratory temperature of 20 degrees. Treated worms produced more offspring early in life, achieved higher lifetime reproductive success and showed higher individual fitness, a rate-sensitive measure calculated from Leslie matrices that weights early reproduction heavily in growing populations. The authors estimate a roughly 10 percent increase in lifetime reproductive success, a difference large enough to carry consequences in nature. Notably, these benefits disappeared when the analysis was restricted, as in many conventional experimental designs, to individuals that matured on or after the peak maturation time of controls. This re-analysis revealed how a methodological artefact the authors call selective appearance can mask the advantages of the fastest developers, who begin reproducing before standard assays, typically set up once 50 to 75 percent of a cohort has matured, even begin.

The reproductive benefits, however, proved temperature-dependent, vanishing without being replaced by costs at both 15 and 25 degrees. A full-factorial experiment across two laboratories, one scoring maturation by visual identification of vulva eversion and the other by time-lapse imaging of the final larval moult, also showed that parental daf-2 knockdown during reproduction slowed offspring development and eroded the benefits of the offspring’s own developmental treatment. Adulthood-only knockdown, by contrast, carried no detectable reproductive costs at all, in line with the developmental theory’s prediction that post-developmental interventions should be comparatively safe.

The critical cost emerged in assays of environmental sensing. C. elegans larvae can abandon reproductive development and enter dauer, a non-feeding, non-reproducing and highly stress-resistant diapause, when conditions such as low food or overcrowding threaten survival. This plasticity is ecologically important because wild nematodes survive on ephemeral food sources that run out rapidly as populations grow, and dauer entry allows migration to new resources. Insulin-like peptides normally signal environmental quality through the DAF-2 receptor to regulate this decision, acting alongside the TGF-beta and steroid hormone pathways. The researchers found that offspring of parents exposed to developmental daf-2 RNAi entered dauer far more frequently in crowded conditions, even when food was abundant enough to support normal growth and reproduction. Because entering dauer costs roughly a day to initiate and at least another to recover from, an unnecessary arrest can delay or entirely prevent reproduction, potentially reducing an individual’s fitness to zero relative to non-arrested competitors.

Knockdown during or after recovery from L1 arrest told a complementary story. In high-throughput imaging assays, larvae treated from recovery onwards grew to a larger maximum size than controls but at the same rate, and took significantly longer to begin producing eggs, whether the knockdown was confined to development or continued for life. The authors interpret this as evidence that IIS performs a dual role: during active larval growth, reduced signalling accelerates progression through larval stages, but when conditions improve after an arrest, intact IIS is needed to sense that improvement and trigger the transition back to reproductive development. Silencing the pathway mimics the molecular signal of a hostile environment, biassing larvae toward diapause and slowing the resumption of reproduction even as intrinsic development runs faster.

To test whether these laboratory findings translate to ecologically realistic settings, the team released founder populations into outdoor mesocosms, conical tubes of autoclaved peat-free compost re-inoculated with a mixture of ten bacterial strains associated with C. elegans in nature, suspended above a salted water bath on the University of East Anglia campus in Norwich, sampling populations weekly over 21 days. For populations founded by worms given 24-hour adulthood knockdown, there was no evidence that treatment changed population growth, despite the lifespan and stress-resistance benefits seen in the laboratory. Developmental knockdown produced significant effects on population growth, but the direction varied across experimental blocks conducted under different weather conditions: some populations grew faster than controls, while others grew more slowly or showed no difference. The authors hypothesise that in more stressful blocks, impaired environmental sensing drove a higher proportion of larvae into unnecessary dauer, while control animals continued developing and reproducing.

Taken together, the results suggest that natural selection maintains developmental insulin signalling at levels that optimise environmental sensing, even though modest fitness gains are available in stable, benign conditions. The IIS pathway acts as a life-history nexus linking development, growth, reproduction and longevity, and its complexity imposes a genetic trade-off: selection tunes the network for performance from zygote to first reproduction, and late-life function is carried along as a constraint. Adulthood-only downregulation can extend lifespan without developmental costs, but the study found no evidence that this translates into population-level benefits in semi-natural environments, leaving open the question of whether post-developmental IIS function is under meaningful selection in nature. The authors propose that such trade-offs, in which complex molecular signalling networks optimised for early life impose late-life costs, may be a common cause underlying the evolution of ageing.

Subject of Research: Age-specific insulin/insulin-like signalling downregulation, life history and environmental sensing in Caenorhabditis elegans

Article Title: Live Fast, Die Old: Lowering Insulin Signalling During Development Can Speed Growth, Increase Reproduction and Lifespan, but Impair Environmental Sensing and Harm Offspring in Caenorhabditis elegans

Article References: Irish, S. D., Kimberley, A., Lenart, P., Carlsson, H., Braybrooke, A., Towbin, B., Immler, S., & Maklakov, A. A. (2026). Live Fast, Die Old: Lowering Insulin Signalling During Development Can Speed Growth, Increase Reproduction and Lifespan, but Impair Environmental Sensing and Harm Offspring in Caenorhabditis elegans. Aging Cell, 25(11), Article e70783. https://doi.org/10.1111/acel.70783

Image Credits: AI Generated

DOI: 10.1111/acel.70783

Keywords: Caenorhabditis elegans, insulin signalling, daf-2, ageing, developmental theory of ageing, dauer, life history, RNA interference, lifespan extension, developmental plasticity, fitness, evolutionary biology