Fire ants avoid sex to protect their genes

Brussels, 01 Jul 2005

Sexual reproduction can lead to major conflicts between sexes and within genomes. A new study, published in this month's edition of the journal Nature, reports an extreme case of such conflicts in the little fire ant Wasmannia auropunctata, which is considered an invasive pest in tropical habitats.

A team of researchers from Belgium, France, Switzerland, Japan and New Caledonia has found a system of reproduction particular and unique in the animal kingdom in the ant: the queens and the males both result from clonal reproduction. Only the workers originate from normal sexual reproduction between the queens and the males. But, these workers are sterile.

Sexual reproduction is the predominant force in the propagation of animals and many plants. However, it can lead to conflicts between the sexes. Characteristics that enhance the reproductive success of one sex can reduce the success of the other: Asexual females, for example, do not need to produce males to ensure future reproduction. In most ants, females are typically produced by sexual reproduction, while males develop from unfertilised eggs.

Clonal or asexual reproduction is not unique to little fire ants. Some lizard species, for example, produce female offspring clonally from adult females. However, what is unique about this ant is that not only females are reproduced clonally by females, but that males also reproduce males clonally.

In an apparent response to this conflict between sexes, genetic analyses reveal that males reproduce clonally. The sperm of the male ant appears to be capable of destroying the female DNA within a fertilised egg, giving birth to a male that is a clone of its father.

Little fire ant queens produce therefore two types of eggs: one that carries the full complement of maternal genes and develops without fertilisation into future clones of the queen, and a second group that carries only one set of chromosomes and is fertilised with sperm from a male. Of this latter group of eggs, most develop into sterile workers. In some of the fertilised eggs, however, the maternal genes are somehow destroyed, leaving the eggs to develop into male ant clones.

The clonal production of males and queens from individuals of the same sex effectively results in a complete separation of the male and female gene pools. 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.

'In the evolutionary battle of opposing sexes, queens transfer all their genes to the reproductive females, and males thwart queens by eliminating the female genome during sexual brood development,' said Denis Fournier from the Université Libre de Bruxelles (ULB), Belgium, lead researcher of the study.

The ant's unusual reproductive strategy probably arose because queens sought to protect their own genes by clonal reproduction, using sexual reproduction only to produce workers. A selfish strategy initiated by females in which queens transmit 100 per cent of their genome, and males are not essential for the evolution of the species - they only genetically participate in the generation of sterile workers.

But fire ant males do not apparently resign themselves to the role of mere evolutionary spectators and hit back by reproducing themselves clonally to pursue their own genetic lineage.

The biological system only retained the males so that diverse sexual workers could be produced and this, apparently, gave males both the time and the means to evolve a counterattack-converting some of the workers into males. Scientists hypothesise that genetic diversity in an ant colony is important for defending against parasites, as well as adapting to changes in environmental conditions.

'From an evolutionary point of view, this discovery concretely indicates that genetic variability is the major advantage of sexual reproduction and illustrates the extraordinary imagination of nature - or of the male ants - to counteract this female strategy,' says Mr Fournier.

While males and females remain affiliated by the mutual production of workers, the sexual conflict between the two could ultimately lead to each sex becoming its own species. The system may also help the worker ants to maintain as high a genetic diversity as possible since their genes come from two pools that do not intermix from one generation to the next.

For further information, please contact:
Arnaud ESTOUP
INRA Montpellier
Unité Mixte de Recherche INRA-CIRAD-ENSA-M-IRD 'Centre de biologie et de gestion des populations'
Tel: +33 4 99 62 33 38
E-mail: estoup@ensam.inra.fr

Denis FOURNIER
Université Libre de Bruxelles
Bruxelles, Belgium
Tel: +32 2 650 44 97 -
E-mail: Denis.Fournier@ulb.ac.be
Remarks: Peer reviewed publication and references:
Clonal reproduction by males and females in the little fire ant
Nature, Vol 435, No. 7046, 30 June 2005

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