Scientists have found the final missing link answering a long-standing question about how social behaviour evolves. New research shows that a social supergene in fire ants coordinates a host of biological processes and behaviours, that ultimately determine how sociable their colonies are, and who they choose to be their queen.
The social b gene is amazing because it makes ants that carry it selectively offer help to other ants who also carry it. The ultimate discriminatory gene, it basically says “be nice to others “, ensuring that the helpfulness of those carrying the gene isn’t wasted on unhelpful ants. But how do the ants know who carries the gene and who doesn’t? New research shows this gene is the full package – it also controls the perceptual system that enables the ants to make the distinction.
Fire ants and social evolution
The remarkable cooperative behavior of ants is complex and controlled by many genes. For decades, entomologists have been working to understand how different genes produce the collective intelligence we observe of social insect colonies as a whole. These efforts have been reinvigorated by massive improvements to the speed, quality and cost of next generation sequencing methods over the past decade. Particular attention has been given to the Fire ant, (Solenopsis invicta), not only because of their status as an invasive pest in North America, but because their genetics are quite unusual.
Fire ants offer biologists a rare window into the evolution of cooperation, because a key facet of their social behaviour is controlled by a single genetic region. Known as the “social b” supergene, this region determines whether the colony will accept a single, or multiple queens. The social b gene has attracted attention as a rare example of a phenomenon known as the Green Beard Effect, where a gene for cooperation can thrive by directing that cooperation only towards individuals that also carry the gene.
The social B supergene proves a long-debated thought experiment, first proposed in the 1960s by W.D. Hamilton, and developed in the 1970s by Richard Dawkins, known as the “Green Beard Effect”. In the thought experiment, the green beard gene produces two traits in individuals that carry it – it gives them a green beard, and it makes them behave cooperatively only towards other individuals with a green beard.
Why does evolution need green beards?
A green beard gene offers a convenient way for cooperation to evolve. It’s actually surprisingly hard to persuade evolution to favour cooperative behaviour, because cooperative groups are highly susceptible to cheaters – there’s always one that has to ruin it for everyone – unless you can find a way to make sure that cheaters (who don’t cooperate) can’t benefit from the good will of cooperators. The green beard offers the perfect visual identifier enabling cooperative individuals to identify fellow cooperators to direct their helpful behaviour towards
A key part of the green beard concept is that the two aspects of the trick – cooperative behaviour, and discrimination against non-cooperators – must be controlled by the same gene. Otherwise, the two traits could slowly be uncoupled during evolutionary time (because of something called ), rendering the green beard useless. Only when the two traits are inextricably linked, can the green beard work as a route towards stable cooperation.
The Green Beard effect, although a neat solution to the problems of evolving cooperation, was thought to be highly unlikely in nature. It seemed implausible that a single gene could code for multiple, complex and disparate behaviours that compose the action of selective cooperation, the signal for possessing the gene and the molecular apparatus required to detect the signal. But the social b gene has a neat way around this. It isn’t actually a gene. It’s a supergene – a large region of DNA, containing many separate genes and non-coding regions, but which behaves as though it’s a gene when it comes to recombination. Because the supergene doesn’t recombine at each generation, the genes aren’t scrambled up, and the traits can remain tightly linked.
We’ve known for a while that the social b gene fulfills the first part of the criteria for a green beard gene – it causes workers to accept multiple queens only if they also carry the social b gene. Worker fire ants give a lot to their queens – rearing their young, building and defending the nest – but the social b gene ensures that sociable fire ant workers are rearing the young of sociable queens, and continuing the social gene on to the next generation.
New research published in the Journal of Evolutionary Biology completes the story by showing that the social b gene also controls workers’ ability to perceive unique chemical cues produced only by social b queens. This is the second half of the equation – the detection system. The green beard might actually be a chemical odour, but this discovery shows that the “Green Beard Effect” really does occur in nature.
Want to Know More?
- Trible and Ross (2015) Chemical communication of queen supergene status in an ant Journal of Evolutionary Biology.
- Unenumerated (2006) Are green beards ubiquitous?
- Keller and Ross (1998) Selfish genes: a green beard in the red fire ant
- Dawkins (1976) The Selfish Gene