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The role of somatic mutation in the lifespan difference of queen and worker ants

Final Report Summary - ANTAGING (The role of somatic mutation in the lifespan difference of queen and worker ants)

1. Introduction

The social insects provide a unique system for the study of aging and senescence. The striking differences in longevity that exist between queens and workers can be used to examine how different life histories can lead to very different aging patterns despite shared genetic heritage. For this reason, insect societies have been used to test models such as the free radical theory of aging. This model predicts that longer lifespan can be achieved by investing more resources into the removal from the organism of damaging oxidising agents such as Reactive Oxygen Species (ROS). Intriguingly, studies on social insect systems have consistently found results opposite to expectations: shorter-lived workers show greater expression of ROS removal genes than longer-lived queens. It is necessary now to take a step back and ask whether the different levels of damage of the type predicted by the free radical theory, such as DNA mutations, do indeed exist. Furthermore, enquiries are needed into whether the counter-intuitive pattern shown by ROS-removal genes are consistent across other maintenance processes.

The project objectives were to find out:

1. Whether queens and workers show an increase in somatic DNA mutations with age.
2. Whether this increase occurs more rapidly in workers than queens.
3. Whether queens invest more than workers in expression of DNA repair genes.

2. Progress so far

All three of the above objectives requires obtaining samples of age-matched queens and workers.

For objectives 1 and 2, we obtained both 1-day-old and 1-year-old individuals, in order to compare the extent of damage that accumulates over the course of one year. This is a biologically relevant timeframe as workers live around 1-2 years in the lab. A 1-year-old worker therefore should represent an old individual, whereas a 1-year-old queen should still be relatively young. If accumulation of DNA mutations underlie a difference in aging between these two castes, we can therefore expect little to no difference in mutation abundance between 1-day-old and 1-year-old queens, but a substantial difference between 1-day-old and 1-year-old workers.

For objective 3, we obtained both 1-day-old and 2-month-old individuals, in order to compare the expression of DNA repair genes both in very young and in more mature individuals. Using very young individuals is useful in order to compare the two castes before differences due to age start affecting gene expression. 2-month-old individuals are useful because at this stage the queen has already founded a colony and is thus in the condition that typifies the majority of queen life (whereas at 1-day-old, queens are in a unique period of preparing for a mating flight).

Progress towards objectives 1 and 2:

The initial hurdle towards these objectives is to find a method that can be used to detect damage. We have broadened our scope in this regard, to include damage to proteins as well as DNA, as both types of damage are relevant to physiological decline and can be dealt with by various maintenance pathways. We have tested two methods for detecting protein damage. The first we found to be insufficiently sensitive for the amount of tissue we have available. The second method, involving Western Blotting to detect ubiquitinated proteins shows more promise and is nearly optimised. For detection of damage to DNA, we have experimented with an approach of sequencing and looking for variants. We will soon be in a position to decide whether to proceed with this approach. We will also soon be testing a method for detecting the abundance of oxidised bases in DNA extractions.

Progress towards objective 3:

Objective 3 has been addressed by using high throughput RNA sequencing. We have extracted RNA from the legs and from the brains of our age-matched samples and obtained sequencing data for all of these samples. The data from legs has been analysed. The data from brains still requires analysis, but as it can be put into the same pipeline as has been developed for the leg data, results will be available very soon.

3. Results and Conclusions

Results from the quantification of molecular damage (objectives 1 and 2) are currently preliminary, but we have early evidence that there is little accumulation of detectable damage over one year in either queens or workers.

Results from the study of gene expression point towards an age-specific difference in the expression of DNA repair genes in leg tissue, with 2-month-old queens expressing these genes more than 2-month-old workers. Results concerning brain tissue will be available soon.

4. Expected final results

The final results will allow us to answer the three questions outlined above. We will be in a position to state whether an accumulation of molecular damage is linked to age in Lasius niger, and whether it shows an accelerated pattern in workers relative to queens. We will also know whether caste-specific patterns of gene expression can help explain the different aging and damage accumulation profiles of queens and workers.

Previous studies did not find evidence for the predicted differences between queens and workers in the expression of antioxidant genes. One potential explanation for this may be that damage caused by ROS is in fact not the primary cause of senescence in social insects. Verifying whether the types of damage caused by ROS, such as DNA mutations and protein damage, really do accumulate more rapidly in longer-lived castes is therefore an important next step in the investigation of aging in these species. Confirming that such a difference exists would support the ROS theory of aging and encourage research into the processes that allow queens to be more resistant to damage despite investing less into the removal of free radicals. Finding no such difference would on the other hand suggest that an increased ability to resist mutations and protein oxidation is not a method by which queens ensure their greater longevity.

The fact that expression of antioxidant genes is not linked to slowed aging in queens does not in itself invalidate the idea that queens investing more into somatic maintenance. It may simply be that the increased investment on the part of the queens comes at a different point in the maintenance process, such as macromolecule repair. The results from our gene expression analysis so far suggest that queens do indeed show higher expression of somatic maintenance genes, but not at all ages. Whether such age-specific differences are sufficient to explain the difference in lifespan is a question that can be addressed by future studies.

This study furthers our understanding of the causes of senescence and of the evolution of life history strategies that can prolong lifespan.

Research fellow: Eric Lucas, University of Lausanne, Lausanne, Switzerland.
Scientist in charge: Laurent Keller, University of Lausanne, Lausanne, Switzerland.