Leafcutter ants recognize and fight pathogen with social immunity

A study conducted by researchers from São Paulo State University (UNESP), in Brazil, and collaborators shows that lemon leafcutter ants (Atta sexdens) exhibit behaviors that go beyond so-called social immunity, i.e. the ability to detect pathogens and try to get rid of them for the benefit of the colony.

In an article published in the journal Proceedings of the Royal Society B, scientists report that these insects are able to recognize a pathogenic fungus they have already been exposed to – even 30 days after the initial contamination – and fight it even more intensely. To do this, they increase their cleaning behavior and recruit more workers for the activity, indicating what could be called “social immune memory”.
 
“If we think of the colony as a superorganism, these behaviors would be like the immune system. In the same way that our defense cells attack invaders, such as viruses and bacteria, the workers fight infections that could harm the colony,” explains Aryel Goes, first author of the study, carried out as part of his FAPESP master’s degree at the Institute of Biosciences (IB) of UNESP, in Rio Claro, with the support of FAPESP.
 
“What we’re showing is that the social immune system of ants can store information about previous infections with the same pathogen, a phenomenon seen in our immune cells, but which occurs at a behavioral level in ant colonies,” he adds.

Social immune memory duration

In one of the experiments, the colonies were exposed to the same pathogen for the first time, then again after seven days, and finally after a month. This last exposure had the most intense and rapid response, with more ants involved in both cleaning themselves and the fungus they cultivate to eat.
 
Atta sexdens is a leafcutter ant, a group that has a mutualistic relationship with fungi, which they cultivate inside their colonies. The fungi, in turn, provide the ants with nutrients.

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Of the four pathogens tested, two were harmful to the ants and two to the cultivated fungi. The study is part of a project supported by FAPESP and coordinated by André Rodrigues, professor at IB-UNESP.
 
In a second experiment, the researchers exposed other colonies to the pathogen, repeated the contamination after seven days, and then a third time only 60 days after the initial contact. The responses were much less intense than in the colonies re-exposed after 30 days.
 
“Unlike our immune system, which in some cases responds expressively to a specific pathogen even many years after initial exposure, ants’ social immune memory has a shorter time frame, so they need to be constantly exposed to the pathogen for the information to persist,” adds Goes, who is currently a PhD student at Ohio State University in the United States.

Specific response

In the third experiment, the researchers tested the specificity of the ants’ response. Seven days after contamination with one pathogen, they exposed the ants to another. Cleaning behavior and recruitment of other workers were less intense when this second pathogen was introduced.

The following week, the first pathogen was reapplied to the colony. The return elicited a moderate response, similar to that of the first infection, but much less intense than that of the ants that had a third contact 30 days after the first.

“One hypothesis is that the lemon leafcutter reduced its hygienic response to avoid spreading the new pathogen to the rest of the colony. This is because cleaning altruism, in which ants lick each other to get rid of pathogens, can also spread new contaminants. That’s why they would have been more careful with a second infection that was completely new to the colony,” says Rodrigues, a researcher associated with the Center for Research on Biodiversity Dynamics and Climate Change (CBioClima), one of the Research, Innovation and Dissemination Centers (RIDCs) supported by FAPESP.

Recognition of pathogens

One question that the experiments may raise is whether the hygienic behavior is simply the ants’ response to an unknown substance that is not necessarily pathogenic.

To test this possibility, the researchers applied a solution that does not harm ants or fungi to five colonies as a control. The hygienic responses were the least intense of all the experiments in the 80 colonies analyzed throughout the study, showing that the ants do indeed recognize pathogens and different exposures.

One hypothesis put forward by the researchers is that the social immunity observed in the study may indicate an improvement in the efficiency of contaminant removal, with a positive effect on group survival.

More research needed

Because there is no information on how these variable responses affect the ants’ survival, it is not yet possible to determine whether they actually constitute a social immune memory, but further experiments could test this possibility.

“Lemon leafcutters are adapted to open areas, so they benefit from monocultures and are a pest. Their cleaning behavior is one of the obstacles to effective biological control without the use of chemical insecticides. Understanding the social immunity of leafcutters can help to develop more effective and ecologically correct controls,” concludes Rodrigues.