The find came about during an investigation into the group behaviour of the species Solenopsis invicta. The species has two distinct social colony structures stemming from which type of queen ant heads it up, explains Elizabeth Norton in ScienceNow. On the one hand there is the large, fertile and rather dominant monogyne queen -- a colony bred from this queen will be highly territorial and aggressive, killing any new queen that attempts to attach itself to the colony. A polygyne colony, as the name suggests, will accept multiple queens. It is primarily headed up by a smaller queen who doesn't mind getting a helping hand bolstering the numbers of her brood.
For decades biologists have tried to uncover the genetic reasoning behind such different behaviours being displayed within one species, behaviours which go deep into explaining the colony structure as a whole. While monogyne ants appear inherently aggressive and are threatened by intruders, a polygyne colony is predisposed to accept strangers and only gets its guard up if a monogyne queen infringes on it (in which case, she is slaughtered).
Thanks to genome sequencing, we finally have an answer.
In the hunt to identify the gene or group of genes responsible for dictating social behaviour in fire ants, the team -- headed up by Laurent Keller of Lausanne University -- had already sequenced the fire ant genome. Differences in monogyne and polygyne ants' Gp-9 gene had already been identified in the 90s, so this is where their investigation focused. After studying the DNA of 500 specimens it became clear that the ants' behavioural differences are built into their DNA in a similar way to how gender is.