Males pit their genes against females by chucking DNA out of eggs.
In a bizarre war of the sexes, little fire ants have evolved a novel way to fight for their gender's genes, according to new research.
The sperm of the male ant appears to be able to destroy the female DNA within a fertilized egg, giving birth to a male that is a clone of its father. Meanwhile the female queens make clones of themselves to carry on the royal female line.
The result is that both the males and females have their own, independent gene pools, leading some to speculate whether each gender ought to be technically classified as its own species. "We could think of the males as a separate, parasitic species that uses host eggs for its own reproduction," says Denis Fournier of the Université Libre in Brussels, Belgium, who led the work.
Many insects, including most bees, wasps and ants, sexually reproduce in order to create both queens and sterile female workers. Males are created when a female egg goes unfertilized. Unlike humans, whose males require genetic input from a father, these male insects simply have less genetic material than the females.
But when Fournier and his team were studying little fire ants (Wasmannia auropunctata) in French Guiana, they found something quite different.
There's no other genetic system that's quite like this. Benjamin Normark , evolutionary biologist at the University of Massachusetts in Amherst
"It is by chance that we discovered this extraordinary genetic system," says Fournier. The team had set out to investigate how colonies in human-disturbed areas, such as plantations or quarries, differ from those in undisturbed rain forests. But after collecting 34 nests and analyzing the genomes of the queens and workers, and the sperm of the males, an unusual pattern emerged. Although the sterile workers carried one maternal and one paternal set of chromosomes as expected, the queens carried only maternal genes and the males carried only paternal genes.
"There's no other genetic system that's quite like this," says Benjamin Normark, an evolutionary biologist at the University of Massachusetts in Amherst.
Little fire ant queens produce two types of eggs: one that carries the full complement of maternal genes and develops without fertilization into future clones of the queen, and a second group that carries only one set of chromosomes and is fertilized with sperm from a male. Of this latter group of eggs, most develop into sterile workers. In some of the fertilized eggs, however, the maternal genes are somehow destroyed, leaving the eggs to develop into male ant clones.
Something similar is known to happen in some fish, amphibians and insects, in which the paternal genes can be eliminated from a developing egg. But it is unusual for maternal genes to be wiped out. The researchers don't know why this happens, but they speculate that it is the outcome of an extreme case of conflict between the sexes.
"It's a selfish strategy, initiated by females," explains Fournier. "Under this strategy, queens transmit 100% of their genome to future queens. Males must adapt or disappear, so they choose to thwart queens by eliminating the female genome in fertilized eggs."
"This illustrates the extraordinary imagination of nature - or of males - to counteract the female strategy," he adds.
The system may also help the worker ants to maintain as high a genetic diversity as possible, speculates Normark, since their genes come from two pools that do not intermix from one generation to the next. This could help to counteract the negative genetic effects of inbreeding.
FournierD., et al. Nature, doi: 10.1038/nature03705 (2005).
evolutionary biologist at the University of Massachusetts in Amherst
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Ebert, J. War of the fire ants. Nature (2005). https://doi.org/10.1038/news050627-7