In most animals, mothers give birth to their own species. But scientists studying harvester ants in the Mediterranean found a startling exception. The queen of one species of harvester ants, known as Messor ibericus, gives birth to both her own offspring and males belonging to another harvester ant species, Messor structor. She accomplishes this feat as part of her normal reproductive cycle, even though these species split apart about 5 million years ago. The researchers call this strange reproductive trick xenoparity, which literally means “foreign birth.”
Earlier researchers found hints that something unusual was going on in M. ibericus colonies. The sterile female ants responsible for foraging, nest maintenance, and caring for the offspring, referred to as workers, seemed too genetically diverse to come from just one species. The team behind this new study, led by Juvé and colleagues, wanted to know whether the workers were a mixed species, known as hybrids, and, if so, how the queens were producing them.
To test whether these ants were hybrids, the scientists collected almost 400 ants from 5 species across southern Europe and sequenced their DNA. They compared the genetic material of the worker ants with the ants that lay eggs, known as queens, and the male ants whose only role is to mate, to figure out who their parents were. They also grew colonies of Messor ibericus in the lab and monitored 286 eggs from 5 of the laboratory colonies.
The team conducted weekly checks on the colonies, including monitoring the development and emergence of their offspring. Specifically, they observed the eggs transition into adult males and used genetic and physical analyses to identify their species.
From the genetic results, they found that all the workers in M. ibericus colonies carried half their DNA from M. ibericus mothers and half from M. structor fathers, meaning they were hybrids. They performed further experiments to confirm that when the queens lacked access to M. structor males, they only produced offspring of their own species, but no workers. Thus, they inferred that these queens rely on sperm from a completely different species to build their workforce.
They also found that the queens produced 2 different kinds of male ants: one was covered in body hair, while the other was almost hairless. The “hairy” males only had M. ibericus DNA from the queens, while the hairless ones were pure M. structor, meaning they shared none of the queens’ DNA, but only their father’s. These results meant that the queens lay eggs that hatch into clones of another species’ males.
The researchers proposed that this cloning happens through a process whereby the sperm’s DNA takes over completely and the mother’s DNA is erased, called androgenesis. The researchers found that about 10% of the eggs M. ibericus queens laid in the lab developed into pure M. structor males. These cloned males later mated with M. ibericus queens, providing the sperm needed to make the next generation of hybrid workers, and continuing the cycle.
The team’s discovery reveals the first known case of an animal species producing another species as part of its normal life cycle. Biologists have long assumed that individuals from different species cannot produce viable offspring. However, these findings challenge that assumption, revealing that 2 ant species can form a stable, interdependent reproductive system. In particular, they proposed that the relationship between M. ibericus and M. structor demonstrates how a relationship where one species benefits from the other, called parasitism, can evolve such that both become reliant on each other to complete their life cycles, called co-dependence.
The researchers concluded that these results demonstrate the potential for hidden cross-species reproductive systems to exist in nature. They recommended that future researchers investigate the molecular mechanisms behind DNA swapping during androgenesis and further explore the evolutionary pathways that led to this unique reproductive partnership.
