Geneticists studying fire ants have published a paper detailing the discovery of what they are calling the first known social chromosome -- an inherited group of genes, which together dictate behaviour.
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.
Ordinarily, genetic information from two parents is muddled to make new and unique genetic encoding. This process is referred to as recombination. However, when it comes to sex chromosomes, one chunk of genetic information located in the Y-chromosome is unable to recombine. The Y-chromosome has evolved to retain the genetic information that ensures important male characteristics are never muddled in a gene exchange. Keller and his team found that a similar suppression of the recombination practice is happening in fire ant DNA, around where the Gp-9 gene is located. It's located within a pair of heteromorphic chromosomes (two chromosomes that share some characteristics but are not identical), with the two possible versions referred to as social B (SB) and social b (Sb). They found that 55 percent of the chromosome is unable to recombine, with a total of 616 genes retained and inherited. SB chromosomes can exchange information between each other, but genetic information cannot be crossed between SB and Sb, as in sex chromosomes. Therefore, as the paper states, "locally limiting recombination" is responsible for the inherited traits.
Further bolstering this argument, the authors concluded: "importantly, most of the genes with demonstrated expression differences between individuals of the two social forms reside in the non-recombining region."
The group suggests this suppression is down to "at least one large inversion of around 9 megabases" -- meaning a strand of DNA has been inverted and thus could be preventing chunks of genes from detaching and reattaching as they normally would during recombination.
Speaking about the find to ScienceNow, Kenneth Ross -- who identified the disparate Gp-9 gene in monogyne and polygyne fire ants back in the 90s -- called it "a spectacular piece of work".
"They've unlocked a whole new mechanism for how a supergene can determine something as complex as behaviour."
