by Judy Purdy
The queen is dead. Long live the other queens.
Assuming, that is, they don't possess the same gene that provoked their fellow fire ants to rip the first queen to shreds.
Certain fire ant queens may signal their own executions according to two entomologists, Dr. Kenneth Ross of the University of Georgia and Dr. Laurent Keller, of the universities of Bern and Lausanne (Switzerland). The two have shown for the first time that such fire ant behavior may be caused by a single gene carried by some queens. And that such behavior only occurs under certain environmental influences.
"These executions are anything but random events," Ross said. "Worker ants are acting on basic instincts when they attack certain queens and tear them apart limb by limb."
What appears even more perplexing is that the murdered queens are young and highly fertile.
"The queens with the `execution' gene are the ones with the greatest reproductive potential," Ross said. "Most species are organized to maintain these individuals. This goes against everything we know about social insects, but it could be a way to help maintain the genetic diversity of the colony."
Such executions only occur in multi-queen colonies, and they may preserve the multi-queen system by preventing the most fertile queens from dominating reproduction. On the other hand, the entomologists found that the highly fertile queens account for 70 percent of the queens in the single-queen ant hills, where high fertility has a much greater survival value.
Their finding shows that fire ant workers play a far greater role in the colony's reproductive options than ever before suspected.
"They are not without a voice even though they may be without ovaries," Ross said.
"In fact, they choose who reproduces, and they regulate the number of queens in the colony," Keller said.
What causes worker ants in multi-queen nests to attack and kill only certain queens?
The pair said they suspect that a queen with the execution gene may release a chemical signal, called a pheromone, that induces worker ants in multi-queen colonies to attack and kill her.
While conducting studies of population genetics in red imported fire ants, Solenopsis invicta, Keller and Ross noticed that workers seemed to be selectively executing queens in multi-queen colonies. After studying hundreds of colonies and thousands of individual queens, they determined what triggered the selective execution pattern: Only queens of a specific genotype, those with the Pgm-3a/-3a gene, signaled their own demise.
In lab experiments the entomologists mimicked the two ways that fire ant queens are recruited in multi-queen nests: A queen joins a nest by recruitment from within the nest or after she takes a mating flight. Newly recruited queens begin to lay eggs when certain queen-produced pheromones, which suppress reproductive development of daughter queens, fall below a critical level.
In experiments where new queens were recruited from within the nest, those with the execution gene were one-sixth as likely to survive as others. Even more dramatic was the execution within hours of every Pgm-3a/-3a queen reintroduced to the nest after a mimicked mating flight.
"When we mimicked the process of the mating flight, not a single one of the incipient reproductive queens of this genotype was alive after introducing them into the nest," Keller said. "The workers apparently don't sting them, but instead simply start taking off the antennae and legs until all we find is the head and thorax."
Ross and Keller can't say whether such behavior also exists in other social insects, "but we can say that people should be looking for it," Ross said.
Although Ross and Keller have yet to figure out the exact identity and location of the execution gene, they have learned that Pgm-3a/-3a queens weigh more and have larger, heavier abdomens than other winged queens in single or multi-queen nests; in single-queen nests they do not exhibit these traits.
Ross and Keller want to learn just what chemical the execution gene produces and also whether it could be synthesized and used to control the ants' spread.
"Entomology is more and more about the study of the naturally occurring biochemicals that insect species use to survive," Ross said. "Because they're a very complex species, science is still at a pretty crude state of understanding them."