Periodic Reporting for period 4 - GermlineAgeingSoma (Getting to the root of ageing: somatic decay as a cost of germline maintenance)
Berichtszeitraum: 2022-03-01 bis 2023-02-28
Ageing is an evolutionary paradox, but it is also a debilitating condition in humans, which explains why it commands such high level of interest among scientists and general public alike. Recent progress in the study of ageing has challenged the current paradigm that ageing results exclusively from the resource allocation trade-offs between survival and reproduction. In recent years, there has been a surge in empiricial studies that challenged the ubiquity of the resource allocation trade-off between reproduction and longevity, and in particular, its importance for the evolution of ageing. The ‘expensive germline’ hypothesis argues that expensive germline maintenance is a missing link in the puzzle of cost-free lifespan extension. Germ cells enjoy elevated levels of costly protection and repair of their genomes and proteomes diverting resources away from somatic maintenance even in the absence of actual reproduction. This hypothesis predicts that when germline signalling is manipulated to increase investment into somatic cells, the germline maintenance will suffer resulting in increased mutation rate and reduced offspring fitness. However, a different theory, called developmental theory of ageing (also known as ‘early-life inertia’) suggests instead that ageing is caused by suboptimal gene expression in adulthood because the force of natural selection on gene expression is maximal during pre-adult development and weakens with increasing adult age. This theory predicts that experimental optimisation of gene expression in adulthood can simultaneously increase lifespan, healthspan and reproductive performance and may even have beneficial effects on the resulting offspring. Our primary goal was to test the two competing explanations for the evolution of ageing: ‘expensive germline’ hypothesis and the developmental theory of ageing (‘early-life inertia’).
At first the project focused on how increased investment into parental somatic maintenance affects offspring fitness. We discovered that increased investment into parental soma by downregulating insulin signalling via RNA interference (RNAi) against daf-2 gene in C. elegans, increases the quality and Darwinian fitness of the resulting offspring. This was an exciting finding that set the course for the rest of the project and ultimately led to the development of a new theoretical framework. In line with this we also showed additionally that evolution under dietary restriction decouples survival from fecundity further challenging the resource allocation theory. The key insight was that because selection optimises gene expression during development and early adulthood, until the age at first reproduction, reduced selection on gene expression in late adulthood can lead to the evolution of ageing. In line with this idea, we knocked-down the expression of several different genes that play important roles in different physiological processes at different time points across the life cycle and showed that it is possible to optimise expression of different physiological processes in adulthood without any cost. We further demonstrated that these results apply not only to benign laboratory conditions, but also in variable and fluctuating environments simulating natural variation in abiotic factors. We established 800 mutation accumulation (MA) lines where we downregulated insulin signalling using daf-2 RNA interference (RNAi) approach. We found that downregulation of insulin signalling reduces extinction and increases fitness under mutation accumulation when mutations accumulate spontaneously or induced by low-level UV radiation. We directly estimated the effect of reduced insulin signalling on germline mutation rates and these results are being written up for publication. Nevertheless, germline maintenance can be costly, and we have two exciting pieces of evidence in this regard. First, we quantified germline mutation rate in parents of different ages in C. remanei but creating MA lines from young, peak-reproduction and old parental animals. We run whole-genome resequencing and variant calling after three generations of MA. We found that reproductive-peak lines had an overall reduced mutation rate compared to young and old lines. These results suggest that animals at the peak of reproduction have better DNA maintenance and repair compared to young and old animals. We propose that C. remanei start to reproduce before they optimize their DNA maintenance and repair, trading the benefits of earlier onset of reproduction against offspring mutation load. The costs of germline maintenance are predicted to be higher in males because of continuous production high number of gametes over a lifetime. To directly examine this, we employed germline ablation through glp-1 RNAi in the dioecious nematode Caenorhabditis remanei. The elimination of the germline significantly heightened heat-shock resistance in both sexes, affirming the germline's role in regulating somatic maintenance. Interestingly, while germline removal extended the lifespan of males, it had no such effect on females. Moreover, germline removal led to a reduction in male growth before maturation, but this effect was not observed in adulthood. In contrast, the growth rate of females was diminished both before and notably after maturation. Consequently, germline removal enhanced male lifespan without imposing significant growth costs (most of the growth in nematodes is after reproductive maturity), whereas germline-less females exhibited slower growth and no increase in longevity compared to their reproductively functional counterparts in the absence of environmental stress. In summary, these findings imply that maintaining the germline incurs higher costs for males than for females, as predicted. We are currently finalising data analyses (the bioinformatics part) from germline manipulation experiment in zebrafish. We also established an experimental set-up to evaluate the effect intermittent fasting on the soma and the germline in zebrafish, Dario rerio.
The most important achievements of the project so far are:
1. The discovery that downregulation on insulin signalling pathway in parents can increase offspring fitness. This is a novel and very significant finding that has important implications for our understanding of the biology of ageing and for translational research. This work has been published.
2. We revealed how multigenerational downregulation of insulin signalling pathway in adulthood affects the soma and the germline under mutation accumulation. Part of this work has been just completed and the key bioinformatics part is currently being prepared for publication.
3. We investigated, for the first time, how downregulation of insulin signalling pathway in adulthood affects lifespan, healthspan, reproduction and Darwinian fitness in complex and variable environments.
4. The most unexpected part was the discovery that optimising gene expression in late adulthood can simultaneously slow down ageing and increase fitness. The project ultimately contributed to the development of a new integrated framework for the evolutionary theories of ageing (Lemaître et al. 2024 A unified framework for evolutionary genetic and physiological theories of aging. PLoS Biology, accepted).
1. The discovery that downregulation on insulin signalling pathway in parents can increase offspring fitness. This is a novel and very significant finding that has important implications for our understanding of the biology of ageing and for translational research. This work has been published.
2. We revealed how multigenerational downregulation of insulin signalling pathway in adulthood affects the soma and the germline under mutation accumulation. Part of this work has been just completed and the key bioinformatics part is currently being prepared for publication.
3. We investigated, for the first time, how downregulation of insulin signalling pathway in adulthood affects lifespan, healthspan, reproduction and Darwinian fitness in complex and variable environments.
4. The most unexpected part was the discovery that optimising gene expression in late adulthood can simultaneously slow down ageing and increase fitness. The project ultimately contributed to the development of a new integrated framework for the evolutionary theories of ageing (Lemaître et al. 2024 A unified framework for evolutionary genetic and physiological theories of aging. PLoS Biology, accepted).