These workerless social parasites, sometimes called inquilines (from the Latin word for \u201ctenants\u201d), have a distinctive appearance that to human eyes easily sets them apart from their hosts. But their parasitic scheme succeeds because they have evolved ways to steal chemical odors from the host nest to camouflage themselves.<\/p>\n

Genomic analyses have shown that ant inquiline species have independently evolved dozens of times, and nearly all of them parasitize a closely related species that looks and behaves as ants normally do. For evolutionary biologists, that posed a mystery: How could a new species of obligate social parasites evolve from its host species? If their ancestors had lived together in the same nest, they would have interbred too easily.<\/p>\n

For many years, researchers hypothesized that the initial step had been reproductive isolation: that the early ancestors of the inquilines were normal ants that were reproductively isolated from their kin long enough to diverge genetically from them and become a new species. They could live on their own, but some of them eventually discovered the benefits of sneaking back into the nests of their ancestors for help. Their dependence on their hosts gradually increased, and they evolved from a state of optional or \u201cfacultative\u201d parasitism to obligate parasitism.<\/p>\n

The problem with that idea, Kronauer explained, is that nobody has ever observed in the wild what should be an essential, early step of the process: free-living, facultative social parasites living in isolation from their close relatives.<\/p>\n

Trible and Kronauer\u2019s new findings turn the previous assumptions on their head. Their alternative scenario focused on the pair of mismatched supergenes in the clonal raider ants. Sometime in history, one of those ants had experienced a mutation that replaced the supergene on one chromosome with a copy of the supergene from the other chromosome. The resulting mutant ant with two copies of the \u201cparasitic\u201d version of the supergene could have suddenly developed into a miniature queen that looked a lot like an inquiline.<\/p>\n

The work showed that a single mutation in a supergene was sufficient to produce the full suite of changes observed in the obligate parasites, even before the ants were split by speciation.<\/p>\n

\u201cYou can go from free-living to obligately parasitic in one step, and you don\u2019t need to take a number of gradual steps involving a reproductively isolated facultative intermediate population,\u201d said Trible, who is now at Harvard University. \u201cWhat we can be sure of is that a free-living parent had a daughter who was immediately an obligate parasite.\u201d<\/p>\n

He continued: \u201cThat is the scenario that had never been entertained by any of the classical evolutionary theorists, because that\u2019s the scenario that was thought to be too big of a jump for you to take.\u201d<\/p>\n

The fact that a single mutation can shift all of these traits in a single step \u201creally changes the way we think about the evolution of these weird, workerless social parasites,\u201d Kronauer said.<\/p>\n