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TERMITE SOLDIERS: MUSCLE AND MEDIC

Researchers from Berlin have discovered that termite soldiers perform what could be a vital role in protecting the colony from disease.

The contribution soldier termites make in protecting the colony from animal predators is well recognised, but their role in other societal functions is less understood. There is new evidence to suggest termite soldiers are more than just the physical ‘muscle’, also helping in the protection of the colony from infection and disease.

In a study published in Insect Molecular Biology in April, researchers from the Division of Zoology at Freie Universität Berlin in Germany examined the role termite soldiers play in colony immunity. The study, which looked at Mastotermes darwiniensis, showed that the oral secretions produced by termite soldiers, which are transferred to worker termites through trophallaxis, offered their nestmates a level of defence against infection, both in preventing disease and curing infection.

The role of the soldier

In social insects, the evolution of a sterile caste (i.e. of non-reproductives) allowed for the creation of sophisticated societal structures. Over time, each caste developed its own physiological and behavioural adaptations that allowed it to perform a specific function for the benefit of the entire colony. Whereas bee and ant societies evolved through the development of sterile workers, termite societies developed through the initial development of sterile soldiers.

Soldiers make up 5-20% of a typical termite colony and display defensive adaptations such as a ­­­specialised head, mandibles and/or chemical secretions. However, soldiers cannot feed themselves and depend on trophallaxis with workers – the most abundant individuals in the colony, which look after the brood and forage for food – to get their nutrition.

What is ‘social immunity’?

Social insects are a promising target for parasitic organisms as a colony offers high numbers of hosts in frequent contact with each other, allowing for the easy transfer of the parasite. ‘Social immunity’ is the term used to describe anti-parasite defences of co-operating individuals to protect the colony against infection.

Most investigations into social immunity have been conducted on workers. Thanks to a number of behaviours workers exhibit, fellow insects in social structures are shown to be protected against pathogens. These worker behaviours include barring, burying or even cannibalising infected individuals or communicating the presence of pathogens to other nestmates. It also includes mutual grooming and the transfer of molecular effectors that trigger a type of virus defence mechanism in nestmates to protect them against infection.

While there is evidence for workers exhibiting this behaviour, to date there has been little evidence to indicated that soldiers also contribute to colony-level immunity.

The study

The researchers examined the role Mastotermes darwiniensis soldiers played in group-level immunity. They conducted several experiments to measure the impact the presence of soldiers made on nestmates’ survival, after worker termites had received lethal exposure to the parasitic fungus Metarhizium anisopliae. The fungus grows naturally in soils throughout the world and is not an uncommon threat to termites.

Mastotermes soldier with workers
Mastotermes soldier with workers

Infected termites were separated into groups, with each group being joined by one unexposed termite soldier or one unexposed worker. Researchers allowed the termites to interact and observed survival rates, whilst also making particular note of the behaviour between soldiers and workers. This included allogrooming, stomodeal (mouth-to-mouth) trophallaxis and proctodeal (anus-to-mouth) trophallaxis.

After 48 hours of interaction there was no evidence of proctodeal trophallaxis, so researchers used a puff of air to encourage soldiers to produce oral secretions, which were then removed from the soldiers’ mouths. These secretions were then applied directly to additional infected workers to assess survival rates.

Soldiers saving lives

Survival rates of infected workers that interacted with one uninfected soldier were significantly higher than the survival rate in groups consisting of infected workers only. The oral secretions extracted from these soldiers also improved the survival rate of infected workers when applied topically to the workers’ abdomens, indicating these oral secretions provided the source of the anti-microbial activity.

This anti-microbial performance was confirmed in petri dish trials, not only fungicidal activity against M.anisopliae but anti-bacterial activity against a range of bacteria.

Protein power

Analysis of the oral secretions identified 139 protein components, some of have been linked to antimicrobial activity in carpenter ants, other termite species and worms. Others, notably the immunity-related proteins, were entirely unique to M. darwiniensis. The identification of these immunity-related proteins in soldier secretions further strengthens the view that secreted molecules derived from soldiers play a role in colony-level immunity.

Valued members of society

The fact that termite soldiers cannot feed by themselves or engage in allogrooming could potentially make them a burden to the colony during pathogen exposure, as their usefulness would be minimal. However, findings from this study show that termite soldiers can improve the chance of survival in their nestmates – and therefore the overall survival of the colony – better than termite workers. This makes them a valuable asset to the colony.

Journal reference: Shulin He, Paul R. Johnston, Benno Kuropka, Sophie Lokatis, Christoph Weise, Rudy Plarre, Hans‐Jörg Kunte, Dino P. McMahon. ‘Termite soldiers contribute to social immunity by synthesizing potent oral secretions’. Insect Molecular Biology, April 2018. 10.1111/imb.12499.

More research on termite nests and termite reproduction