From Parasites to Protectors

Socially parasitic ants can serve as protective symbionts for their fungus-growing hosts in the face of attacks by predatory raiding species.  

By | September 9, 2013

A Megalomyrmex symmetochus ant (top) confronts a Gnamptogenys hartmani raider ant (bottom)ANDERS ILLUMSocially parasitic Megalomyrmex ants establish colonies in the gardens of fungus-growing ants, where they restrict the growth and reproduction of their hosts by feeding on resident broods and clipping the wings of virgin queens. But according to a study out today (September 9) in the Proceedings of the National Academy of Sciences, the presence of these parasitic guests can be beneficial when the host colony comes under attack from another species of ant—the guests use their potent alkaloid venom to fight the raiders and defend the fungal gardens.

“This is really intriguing,” said evolutionary biologist Joan Herbers of Ohio State University, who was not involved in the study. “It’s a novel demonstration that in a certain context, when this nasty predatory ant is around, the parasite can protect the host.” In other words, the parasite-host relationship has shifted to a context-dependent mutualism in which the cost to the host is compensated for by protection against a shared enemy. “That hadn’t been appreciated before for social insects,” Herbers added. 

“The findings also highlight how ‘parasitic’ or ‘mutualistic’ relationships are not so clear-cut, and [instead] belong on a continuum of interaction between the two,” Natasha Mehdiabadi, an entomologist at the Smithsonian Institution in Washington D.C., who was not involved in the study, told The Scientist in an e-mail.

Working in the field in Panama, the Smithsonian Institution’s Rachelle Adams saw that one species of venom-alkaloid-producing ants, Megalomyrmex symmetochus, was infiltrating host Sericomyrmex amabilis colonies in greater numbers than other socially parasitic species typically would. She also noticed that the guest colonies boasted far more worker ants than usual. “It wasn’t just the queen and a few workers, as you typically find,” she said, “there were hundreds of workers patrolling the cavities in the host colonies.”

One explanation might be that the guests were at risk of attack by host workers. But the guests’ chemical weapons easily overpower host workers’ biting defenses, so Adams and colleagues wondered whether the hordes of guest workers might be essential to the survival of their own queen in another way. They hypothesized that M. symmetochus might act as defenders when host colonies are attacked by the destructive agro-predator Gnamptogenys hartmani, which Adams had observed invading the same colonies on occasion. To test the idea, she and her colleagues observed interactions among the three species in the lab.

In a series of encounters staged in petri dishes, M. symmetochus workers—which attack their adversaries with toxic venom—were much more effective than S. amabilis workers—which use their mandibles to cut off foes’ extremities—at killing G. hartmani raiders. The guest ants were also significantly less likely to be killed by the raiders. What’s more, some raiders daubed in venom by guest ants were later attacked by members of their own raiding party, suggesting that M. symmetochus venom is both toxic and somehow confuses the G. hartmani ants.

When the ants interacted on fungus garden fragments in the lab, the guest ants remained more effective at killing raiders than their hosts, who needed much larger numbers to mount any counter-attack against even just one or two raiders. Because G. hartmani usually attack with a force of 100 or more, the researchers concluded that hosting a guest ant colony would likely have substantial fitness payoffs for M. symmetochus if the risk of raids was high.

Adams and colleagues also showed that even when the raiders could not directly interact with guest ants, they preferred to initiate attacks on colonies without M. symmetochus compared to those with the guests. This suggested that the very presence of guest ants, communicated by the volatile chemicals they emit, deters G. hartmani.

“They’re still parasites because they’re extracting resources,” said Adams. “But in the context of this scenario, [the parasites] use the same chemical weaponry that helps them to invade the host colony against this other raiding species.” Adams compared M. symmetochus workers to the Medieval mercenaries that protected cities for pay.

To Herber’s mind, the work provides a neat illustration of how coevolved relationships can be transformed from purely parasitical to partially mutualistic. “Most—if not all—mutualistic interactions evolved from a parasitic relationship,” she said. “So how do we get from parasitic to mutualistic? Well, this [study] shows us one way.”

Mehdiabadi added that the study also emphasizes the importance of considering all the players influencing such coevolutionary arms races. “A mutualism doesn't involve two mutualists in the absence of other organisms, and a host-parasite interaction doesn't involve only the two and no others. All players need to be studied, and their interactions need to be examined.”

Adams said her team now plans to investigate whether the same results can be found in the field. The researchers also want to see if another socially parasitic ant species from the same genus, Megalomyrmex adamsae—named after Adams, who discovered the species—also protects its fungus-farming hosts.

R.M.M. Adams et al., “Chemically armed mercenary ants protect fungus-farming societies,” PNAS, doi: 10.1073/pnas.1311654110, 2013.

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Avatar of: RJR8222


Posts: 4

September 10, 2013

Hmmm. Kind of like the way the gene for sickle-cell anemia is generally harmful to the host, except during an infection by Plasmodium in which case heterozygosity confers an advantage.

Given that natural selection - driven by differential survival - must always net together ALL of the advantages and disadvantages of a given situation, these kinds of complex advantage-disadvantage relationships should be fairly common.

There's a nice lesson in this study: Assuming an "all other things being equal" mindset when considering the selective advantage or disadvantage of some biological phenonenon is tantamount to assuming a falsehood. And, according to formal logic, if "A" is false, then all statements of the form "if A, then B" are vacuously true. 

Avatar of: James V. Kohl

James V. Kohl

Posts: 481

September 10, 2013

No, it's not "Kind of like the way the gene for sickle-cell anemia is generally harmful to the host, except during an infection by Plasmodium in which case heterozygosity confers an advantage." which is what Ed Yong inferred in his National Geographic report on this article.

The real lesson here is that "An environmental drive appears to have evolved from that of food ingestion in unicellular organisms to that of socialization in insects." That lesson has been learned from what is known about the epigenetic effects of olfactory/pheromonal input in the context of nutrient-dependent pheromone-controlled adaptive evolution sans mutations theory.

Differential survival of individuals (including their survival among other species) is so clearly nutrient-dependent as to remove random mutations theory and current thought about the role of mutations in adaptive evolution from any further consideration whatsoever. Pheromone-controlled reproduction in species from microbes to man offers some reassurance that conserved molecular mechanisms of adaptive evolution are found in species from microbes to man. That means the epigenetic landscape becomes the physical landscape of the organized genome via experience-dependent de novo creation of olfactory receptor genes in all species.

Nevertheless, others should feel free to offer an explanation of how infection-induced heterozygosity enables 'conditions of life' in any species that has adaptively evolved via nutrient-dependent pheromone-controlled reproduction. I always get a kick out of the predator-driven evolution scenarios, for example. They're almost as funny as the mutations in disease-driven models when they are extended to adaptive evolution.


Avatar of: Lucas2


Posts: 1

Replied to a comment from James V. Kohl made on September 10, 2013

October 24, 2013

Bold statements, supported only by your own previous writings, with a clear commercial interest stemming from where you peddle human sex pheromones. Your hypothesis may have merit, but it clearly needs more support before you can dismiss mainstream theories.

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