Ants change the rules of an evolutionary arms race

When I was told that I shared 50% genetic similarity with my siblings, I was aghast. But we were so different! It turns out, these genetic family ties are even closer when it comes to ants.

Insect colonies generally include three types of ants: a larger mating queen, mating males, and hordes of sterile, female workers. The evolution of so many sterile workers may sound like an evolutionary disadvantage - sterile organisms can't pass their genes on to the next generation!

But check this out--the peculiarities of insect colonies and hives make more sense if we focus on key features of their reproduction. If the ant egg is fertilized, it will develop into a diploid female. If the ant egg is not fertilized, it will develop into a haploid male. So an ant has 50% genetic similarity with any potential offspring. Remember that females have two sets of each chromosome, while males only have one copy of each chromosome. This means that every daughter receives the same exact paternal chromosome, but has only a 50/50 chance of inheriting the same maternal chromosome. So sister ants share 75% genetic similarity. For females, "farming" or nurturing their own mother to produce sisters is actually a better way of furthering their own genes. If they reproduced, their offspring would only have 50% genetic commonality with them. (1)

Slave rebellion

The plot thickens when relationships between colonies are taken into account. Slave-making ants such as Protomagnathus americanus engage in violent raids on the nests of other species. During these excursions, the marauding ants kill all the adults, and steal the larvae. Yikes. When these captive larvae mature, they begin to broadcast the same chemical signature as their kidnappers, and become servants of their enslaving queen.  Eventually, they are so well assimilated that they will actually participate in raids on other colonies!

A different ant called Temnothorax has managed to change the rules of this evolutionary arms race. Once kidnapped, Temnothorax larvae mature in their captive environment. Then they turn and kill the pupae of their captors.  How they manage to do this has baffled scientists, because these ants are sterile and removed from their original home.

Here is what they do. These rebel Temnothorax attackers use their jaws to rip their captors' progeny apart, or drag pupae outside of the colony where the pupae die of neglect. In a study published in Evolution, Achenbach and Foitzik found that up to 2/3 of captor pupae are murdered.(2) We can't explain this as just generally aggressive Temnothorax behavior, because the ants do not behave like this in their own colonies. Amazingly, the slaughter appears to be selective: captives kill 83% of pupae containing queens, but only 3% of males, and selectively kill healthy pupae.(3)

Since these sterile ants cannot pass genes on to the next generation, it is unclear how this is an adaptive trait. Also, these ants cannot protect the survival of the queen ant back home who carries their own genes. So why are they doing it?

From an evolutionary perspective, this is particularly puzzling.  Researchers wonder how this behavior evolved, because these enslaved workers seem to be caught in an evolutionary trap. How can they increase the frequency of any gene in the population?

The answer may be that slave rebellion protects their relatives back home. Ants that prevent future raids could protect cousins in nearby colonies, since multiple colonies over a given area are often sister colonies sharing the same gene pool. How they know to do this after being stolen and enslaved as teeny pupae is still a mystery.  Stories like these make us wonder-is loyalty genetic?

1 Dawkins, Richard. The Selfish Gene. Oxford University Press, 1976.
2 Achenbach & Foitzik. First evidence for slave rebellion: enslaved ant workers systematically kill the brood of their social parasite protomognathus americanus. Evolution, April 2009.
3 Yong, Ed. The rebellion of the ant slaves. ScienceBlogs, April 1, 2009.