LAUSANNE - Is it better to be ruled by one queen or several queens? You can encounter both situations within colonies of fire ants, social insects known for their nasty stings. The fact that both systems coexist as a single species is rare, and until now the factor responsible for it remained unknown.

A new inquiry conducted by researchers in Lausanne, Switzerland concludes that there is in fact a set of genes that determines numerous ant characteristics, for instance, whether they tolerate one queen or several.

The fire ants –- in Latin, Solenopsis invicta -- are known for the severe rashes they leave after using the toxic venom injected through a sting on the bottom of their abdomen. Since their unscheduled arrival in the United States in 1930, these insects from South America kept multiplying, reached Australia a few decades ago and more recently arrived in China. Apart from the danger emanating from getting stung, they are a scourge for both crops and other types of ants they do battling with, according to the study, which has been published in the latest issue of Nature magazine,

The S. invicta knows two models of social organization. The first, called monogyna, has only one regent ruling the workers; the other, polygyna, has several queens within the same colony. In the first case, the new queens born in the community fly off to mate with a male before creating a new nest of her own, far from her original colony. They store a large quantity of fat to feed her larvae. In the polygyna regime, young queens don’t store fat before leaving; once impregnated, they join an already settled neighboring colony to lay their eggs, or they simply stay where they are.

But this species has another specificity that may have a broader significance: The polygyna and monogyna “citizens” are able to mutually detect each other thanks to their scent. This ability allows them to know which regime the other queen belongs to when they meet her for the first time. The communities have strict defensive rules: if the newcomer isn’t fit for their society, they go for the kill.

In order to get to the bottom of it, Laurent Keller’s team of biologists from Lausanne University compared the genome of several queen ants with their descendants. A long-term study rendered possible by the progress made in sequential genetics and data analysis thanks to computer tools from the Swiss Bioinformatics Institute.

Follow the scent

This analysis proved that a part of a chromosome from the fire ant was remarkably preserved from mother to child. This piece of chromosome identified by the scientists comprises 600 genes including the famous Gp-9.

According to Keller and his coworkers, this piece of chromosome, this “super-gene” as they call it, is the source of the social models of the fire ant. It is said to contain determinant information on the ants’ scent, but also the queen’s fat level and other traits linked to the social organization, such as the height of the workers.

“All these genes appear to have been selected to end up together into this piece of chromosome,” explains Keller.

Serge Aron, social insects specialist at Brussels University, calls it a "major step forward, for it’s the only evidence of a "social chromosome" in the animal kingdom.”

Joel Meunier from Mainz University, Germany, says the study provides “a better understanding of how the two community models could survive within the same species all these years.”

As for Laurent Keller, he believes that such a “super-gene” also exists within other insects or even birds.

Such a gene conglomerate running through generations isn’t new for geneticists since the same process applies to the mammals’ Y sexual male chromosome. “The problem with this phenomenon is that the chromosome that won’t recombine will degenerate: that’s why most Y genes aren’t functional,” explains Keller. Nevertheless, this system grants its bearer every aspect that goes with masculinity.