Royal Freeloaders

For these imposter ant queens, real work is entirely out of the question.

To thrive, ant colonies need every member to pull its weight. For raider ants, this means diligent scouts track down other nests, then direct hundreds of savage foragers to attack.

They return carrying dead young ants to feed the settlement.

But raider ants are among about 50 ant species plagued by impostors; parasitic ants that resemble queens.The impostors greedily eat the colony's food, but shirk their own foraging duties.

Also, when they reproduce, they can only hatch more parasites instead of workers. Where the fake queens come from has long puzzled scientists.

''It's a real mystery how these things arrive,'' said Ken Ross, an evolutionary geneticist at the University of Georgia.

A new study offers a solution. A ''supergene'' that mutates rapidly, in a single generation of raider ants, is most likely responsible for the royal impostors.

The discovery arose from an observation in a lab at Rockefeller University, where Waring Trible and his graduate adviser, Daniel Kronauer studied raider ants.

'' These weird, mutant queens just showed up,'' said Mr. Trible, now at Harvard, who led the study. Isolated from the rest of the colony in a petri dish, it was plain to see : Several of the ants had wings.

Wings are a typical trait for queens in many species, but raider ants don't normally have them.

''Seeing those winged females are very shocking, very striking, right away,'' Dr. Trible said. ''I immediately thought it was something genetic.''

He set about sorting through the 10,000 ant-colony. His needle-in-a-haystack search found a total of 14 impostor queens, which he then let reproduce. Their progeny were always winged parasites.

Dr. Trible and his colleagues devoted years to studying the mutants and trying to figure out their origin. 

Another geneticist, Sean McKenzie, compiled the regular ants' whole genome, while Dr. Trible analyzed the mutants' genome. Comparing the genomes let Dr. Trible see where the regular and mutant ants differed.

It is the first time that parasitic ants of the same species as their hosts have been observed. [ Rebecca Dzombak ]


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