Most male yellow crazy ants carry two separate sets of DNA, resulting from sperm and egg cells that don’t mix their genetic material after fertilisation.
This means the males are chimeras – individuals with some parts of the body carrying one gene set, and other parts carrying another. The males’ female offspring develop into either workers or queens, depending on the DNA in the sperm cell that fuses with the egg, while male descendants become chimeras themselves.
The findings, which resolve a 15-year mystery about the insect’s genetics, reveal a mode of reproduction that, until now, was “unknown to science”, says Hugo Darras at Johannes Gutenberg University Mainz in Germany.
“There were lots of crazy hypotheses to explain what was happening in these ants, but none was as crazy as the one we discovered,” he says.
Ants usually reproduce either by sexual reproduction – a male’s sperm fertilising a female’s egg – or by clonal reproduction, meaning the queen produces offspring without the added genes of a second parent. Generally, males result from unfertilised eggs and females from fertilised eggs. In most cases, queens are genetically similar to workers in the same colony, but they develop into queens because they have been given special care and nourishment.
In 2007, scientists studied the genomes of yellow crazy ants (Anoplolepis gracilipes) and found, to their surprise, that males had mixed genetics as if they had two parents. Six years later, another team discovered that all the queens in a colony descended from the same genetic line, and their worker sisters all descended from a very distinct, second line.
Intrigued by this puzzle, Darras and his colleagues sequenced the DNA of 53 yellow crazy ant queens and 91 workers collected from 14 locations in South-East Asia. The queens were inbred, but the workers had much higher genetic diversity, he says. In other words, it seemed as though the queens were born to parents of the same lineage, whereas workers were born to parents of different lineages – even though the males that fathered them all came from the same colony.
Assuming this meant the colonies included two lineages of males, the researchers then collected 574 males from the same colonies as the queens and workers and sequenced the DNA in their legs. They found that some of the males seemed to share a genetic line with the queens, whereas the others seemed more closely related to the workers.
That might have made sense if the males had resulted from fertilised eggs, says Darras, because it would mean they had two copies of each chromosome – one from each parent. But his team’s analysis revealed that, like most ant, bee and wasp males, they only had one copy of each chromosome and thus appeared to result from unfertilised eggs.
Perplexed, the team then tested DNA from individual cells in 20 of the males. They discovered that within the same individual insect roughly half the cells had genes of one lineage, and the other half had genes of the other lineage. Looking specifically at sperm cells, the team found that one lineage – the one that led to workers – was much more abundant than the other.
Further analysis revealed that the two lineages in these males did come from two parents, meaning the males actually resulted from fertilised eggs, but unlike in females, the nucleus of the egg didn’t fuse with the nucleus of the sperm. The males thus ended up with two different sets of chromosomes, carried into different parts of the body.
In retrospect, the researchers realised that the vast majority of yellow crazy ant males are probably chimeras – and that the DNA samples from the insects’ legs only showed the genetic material of that particular leg, says Darras.
The ants may have evolved such a bizarre reproductive system because of an old conflict between lineages, he says. In particular, if the worker-producing lineage always creates sterile females when the egg and sperm fuse, then it can ensure its survival across generations if it can slip into an egg without fusing. It then becomes a “selfish” lineage by becoming the primary DNA line in the sperm of fertile males.
“This is only speculation though,” says Darras. “We’re just at the beginning of our understanding.”