Hidden away among the 15,000 fire ant genes, the Gp-9 gene lurks. This simple gene codes for an odor-binding protein — a molecule that helps determine which scents an animal can detect. That might seem like an insignificant job, but Gp-9 seems to really shake up the way that fire ants behave.
There are two versions of Gp-9, B and b, and which version of the gene an ant carries may spell the difference between how that ant reacts to an invading queen: submission (“God save the Queen!”) or mutiny (“Off with her head!”). Worker ants seem to accept or reject an invading queen based on both how the new queen smells and what odor-binding proteins the worker has. Bb workers in multiple queen colonies encourage polygyny by accepting new Bb queens and executing new BB queens. BB workers in single-queen colonies encourage monogyny by executing all invading queens.
|Genotype BB||Genotype Bb||Genotype bb|
|BB queen produces BB workers; BB workers execute all invading queens||Bb queen welcomed in colonies of Bb workers; Bb workers execute invading BB queens||bb queens are weak; usually die before laying eggs|
Weighing the evidence
How did scientists discover that Bb workers sniff out BB queens? They performed a simple experiment. Researchers took workers and rubbed them up against either BB or Bb queens so that the workers would pick up some of the queen scent. Then the researchers released the queen-rubbed workers back into the colony. Workers that had rubbed against a Bb queen were accepted as usual. But workers that had rubbed against a BB queen were in a bad situation — they were often attacked by their own nestmates. The experiment suggested that it was the smell of BB queens (which rubbed off on the unsuspecting workers) — and not appearance or behavior — that provoked attack.
Eau de ant
Scents are an extremely important part of ant communication — and scientists investigate scent communication using some surprising tools. You might cook with vanilla extract (the essence of vanilla beans) or wash your hair with a botanical extract (chemicals extracted from plants) — but ant researchers run experiments using queen extract (extract of queen ant, of course). Where does queen extract come from? You might imagine biologists whipping up “queen smoothies” in a blender — but in fact, queen extract is made by soaking the bodies of queen ants in alcohol, which allows their scents to be dissolved into the liquid. Then researchers can use the liquid to imitate the scent of a queen — minus her actual body and behavior.
The result of all this gene sniffing and royal intrigue seems to be that different versions of Gp-9 are associated with different numbers of queens. BB queens rule alone, Bb queens always seem to share their power with other queens, and the weak bb queens die before they get to lay any eggs.
There are three essential pieces of evidence linking Gp-9 to queen number:
- observational studies show that different gene forms are associated with different numbers of queens (BB queens rule alone, Bb queens usually share power)
- experiments show that ants behave differently towards queens carrying different gene forms (in some colonies, entering BB queens are executed and entering Bb queens are accepted)
- molecular studies show that Gp-9 has a job that would allow it to influence queen number (Gp-9 is an odor-binding protein).
The evolutionary upshot of all the evidence seems to be that Gp-9 influences queen number and that selection has acted on the gene, favoring the B form of the gene when starting a colony alone was advantageous, and favoring the b form when banding together with other queens was advantageous. Thus, as S. invicta spread throughout the South forming tightly-packed colonies, ants and colonies with the b form of the gene may have been successful and reproduced a lot, causing the b version of the gene to be more common.