The native South American Solenopsis invicta comes in both single and multi-queen families. In the U.S., this behavioral division follows a genetic one: workers bearing two copies of one form, or allele, of the Gp-9 gene live in nests governed by a single matriarch, whereas those that have one copy or a pair of a second allele take orders from a female oligarchy. The researchers determined that the Gp-9 protein is most similar to a moth pheromone receptor. Such receptors allow insects to identify members of their own species. S. invicta workers are known to accept a potential egg-laying queen based on chemical cues she provides, so the authors presume that the distribution of the two Gp-9 alleles among the workers factors heavily into the size of the monarchy.
The authors found the same organizing principle at work in a range of Solenopsis species living in Argentina. Comparisons of genetic data from the different ants suggest that single-queen colonies evolved first, they note. They add that other genes in the region of the genome around Gp-9 are probably involved in regulating fire ant social organization as well. "Future studies of the gene content of this gene region, combined with biochemical analyses of the gene products," they write, "promise to yield new insights into the genetic basis of social evolution."