Queen ants in southern Europe produce male clones of an entirely different species — tearing up the playbook of reproductive biology and suggesting we need to rethink our understanding of species barriers.
The workers in Iberian harvester ant (Messor ibericus) colonies are all hybrids, with queens needing to mate with males from a distantly related species, Messor structor, to keep the colony functioning. But researchers found that some Iberian harvester ant populations have no M. structor colonies nearby.
"We had to face the facts and try to see if there is something special within Messor ibericus colonies," Romiguier said.
In setting out to resolve this paradox, Romiguier and his team found that queen Iberian harvester ants also lay eggs containing male M. structor ants, with these males ultimately fathering the workers. This discovery, published Sept. 3 in the journal Nature, is the first time any animal has been recorded producing offspring from another species as part of their normal life cycle.
"In the early stages, it was kind of a joke in the team," Romiguier said. "But the more we got results, the more it became a hypothesis and not a joke anymore."
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Ants are eusocial insects, meaning their colonies form cooperative super-organisms predominantly made up of infertile females, called workers, and a small number of reproductive females, called queens. Males solely exist to fertilize queens during their mating flight and die soon after.
Queens only mate once in their lives and store the sperm from this meeting in a special organ. She then draws from this sperm stash to lay new eggs containing one of three types of offspring: queens, workers or males.
However, Iberian harvester ants mating with males of their own species can only produce new queens. This is thought to be a result of selfish queen genes, where the DNA from male M. ibericus guarantees its survival across generations by biasing larvae to produce fertile queens rather than infertile workers — known as "royal cheaters."
To avoid this, queens must use sperm from male M. structor ants to produce their workers.
This was why the presence of thriving isolated M. ibericus colonies was such a conundrum.
To find answers, the researchers first sampled 132 males from 26 Iberian harvester ant colonies to figure out whether there were M. structor males present. They found that 58 were covered in hair and 74 were hairless. A closer inspection of the nuclear genomes of a subset of these ants revealed that all hairy ones were M. ibericus and all bald ones were M. structor.
But this was not proof that the queens were laying male eggs of two different species — there could have been some hidden M. structor queens producing the odd male. So the team sequenced the mitochondrial DNA, which is passed down by the mother, of 24 of the M. structor males, and found it came from the same mother as the M. ibericus male nestmates.
"This was the detail that made me realize that 'maybe we are on to something very, very, very big,'" Romiguier said.
The team then separated 16 queens from laboratory colonies and looked at the genetic sequences of their freshly laid eggs. They found that 9% of their eggs contained M. structor ants. They then directly observed a single queen producing males of both species by monitoring its broods weekly over an 18-month period.
Together, all these findings show that Iberian harvester ant queens are cloning M. structor males and not passing on any of their own nuclear DNA. Researchers now need to pinpoint the exact mechanism underlying this cloning, Romiguier said, and find out at what point the maternal DNA is removed.
Denis Fournier, an evolutionary biologist and ecologist at the Free University of Brussels, Belgium, who was not involved in the research, said that it was "almost like science fiction" when he first learned of this discovery. "It's jaw-dropping! Most of us learn that species boundaries are firm, yet here is a system where ants regularly cross them as part of normal life," he told Live Science in an email.
The team have called this new reproductive system "xenoparity," meaning the birth of a different species. Romiguier said the team aren't exactly sure when this system first emerged in the Iberian harvester ants, but it's somewhere between when M. ibericus and M. structor split along different evolutionary paths 5 million years ago and a few thousand years ago.
"This discovery is a great reminder to stay open to the unexpected," Fournier said, noting that the finding opens up new questions about cooperation, conflict and dependency in nature. "Now that we know such a system is possible, it’s exciting to think that old, puzzling data might suddenly make sense in light of this discovery," he added.