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Worker ants perform important tasks like gathering food, taking care of eggs and larvae and nest building. However, some species of ants cannot produce workers through reproductive means with their own species. Eggs fertilized by male ants of the same species result only in more queens, while unfertilized eggs result in winged males—those which leave the nest to mate with queens. So, where do the workers come from in these ant species?

Typically, ants incapable of producing workers are known to mate with other species in order to produce these crucial members of their colony. This results in hybrid offspring as workers. Of course, this reliance on reproduction with another species is strange compared to most animals. Yet, the Iberian harvester ant (Messor ibericus) does something even stranger.

A new study, in Nature, documents the Iberian harvester ant queen's bizarre ability to clone another species of ant (Messor structor) to mate with to produce workers.

Researchers involved in the study conducted population genomic analyses of 390 ants from five Messor species across Europe. The analysis revealed that M. ibericus colonies consisted of only first-generation hybrid workers consisting of both M. ibericus and M. structor DNA, with the exception of one Messor ponticus worker. The hybrid workers were found to have an M. structor paternal ancestry and M. ibericus maternal ancestry.

However, this seemed odd to the team, not because the ants were hybrids, but because the hybrids were found in many areas where no M. structor colonies existed. Were queens flying long distances to mate with M. structor in places where their colonies existed, or was something else going on?

"As even more compelling evidence, first-generation hybrid workers from the Italian island of Sicily are found more than a thousand kilometers away from the closest known occurrence of their paternal species. This raises the question of how queens can hybridize in such an isolated area," the authors write.

To shed some light on this mystery, the team isolated M. ibericus colonies in a lab to observe egg laying, reproduction and development. To their surprise, first-generation hybrids still popped up under these conditions.

They found that M. ibericus queens were cloning M. structor males using stored sperm to maintain a clonal lineage of M. structor within their colonies. This allowed them to then produce hybrid workers with the cloned males, allowing M. ibericus to expand beyond the natural range of M. structor.

Genetic analysis of the clones revealed nuclear DNA from M. structor and mitochondrial DNA from M. ibericus. The researchers note that this nuclear–mitochondrial genome mismatch is unique to males in M. ibericus colonies, and hasn't been found in any other M. structor ants found in their own species colonies.

This new frontier in reproduction is referred to by the authors as "xenoparous," meaning the ant queens have a need to produce individuals of another species as an inherent part of their life cycle. The exact developmental mechanism of cross-species male cloning is still unknown, but the authors have some theories.

They say, "Transition towards xenoparity seems to result from sexual evolution along a parasitism–mutualism continuum. Similar to several other harvester ant species, M. ibericus first transitioned into obligate sperm parasitism, a situation in which they lost the ability to produce workers by themselves due to epistatic incompatibilities or selfish caste-biasing genotypes."

The study has left some scientists scratching their heads and wondering what it means for a species to be a species. The authors say on the subject, "Such a 'two-species superorganism' challenges the usual boundaries of individuality."

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