12 mai 2004

Maladies des mâles et évolution de la socialité

Référence
Proceedings of the Royal Society of London - B - Biological Sciences, 2004, 271, 1542, 979-984
"The role of male disease susceptibility in the evolution of haplodiploid insect societies",
Sean O'Donnell et Samuel N. Beshers

Résumé :
Certaines espèces d’hyménoptères, ordre incluant les abeilles, les fourmis et les guêpes, présentent une double particularité : des sociétés complexes, avec division du travail très avancée, et une répartition génétique haplodiploïde, les femelles ayant deux jeux de chromosomes (diploïde), les mâles n’en ayant qu’un (haploïde). Pour Sean O’Donnell et Samuel Beshers, ces deux facteurs ont co-évolué. L’haploïdie des mâles implique en effet une moindre résistance à la maladie : la présence de deux gènes sur un même locus (allèles) augmente la résistance aux infections parasitaires, virales ou bactériennes. La répartition des tâches en porte la conséquence : les femelles des espèces haplodiploïdes, plus nombreuses, sont aussi celles qui font l’essentiel du travail. Les mâles sortent peu du nid, ce qui s’explique s’ils présentent un risque supérieur d’y ramener une maladie. Cette “ hypothèse du mâle malade ” doit désormais être testée. Elle prédit notamment que les femelles ont des procédures d’évitement, d’exclusion ou de meurtre des mâles présentant des symptômes de maladie.

Abstract :
Heterozygosity at loci affecting resistance against parasites can benefit host fitness. We predict that, in haplodiploid species, haploid males will suffer decreased parasite resistance relative to diploid females. We suggest that elevated susceptibility in haploid males has shaped the evolution of social behaviour in haplodiploid species. Male susceptibility will select for behavioural adaptations that limit males' exposure to pathogens and that limit male transmission of pathogens within and between colonies. The relatedness-asymmetry hypothesis that has been advanced to explain female-only workers does not make these predictions. We review the relevant evidence for genetic effects on parasite resistance in insects and summarize empirical evidence that relates to the haploid-susceptibility hypothesis.

© Proceedings of the Royal Society

Commentaire Eurekalert (www.eurekalert.org)

Male susceptibility to disease may play role in evolution of insect societies

A pair of scientists has proposed a new model for behavioral development among social insects, suggesting that a higher male susceptibility to disease has helped shape the evolution of the insects' behavior.
What might be called the "sick-male" theory has been proposed by animal behaviorists Sean O'Donnell of the University of Washington and Samuel Beshers of the University of Illinois at Urbana-Champaign, and appears in the current issue of Proceedings Biological Sciences, published by the Royal Society of London.
Among behaviors possibly affected are the division of labor between males and females and the relative social isolation experienced by males in many social insect colonies.
The researchers looked at Hymenoptera, an order of insects, including bees, ants and wasps, some of which have highly complicated societies and an unusual genetic makeup. These insects are called haplodiploids because males and females have a different number of sets of chromosomes. The females, like most animals, including humans, are diploid and have two sets of chromosomes, one from each parent. Hymenopteran males, however, hatch from unfertilized eggs and are haploids with just one set of chromosomes.
"Disease and infections are a very powerful and ongoing force in natural selection, and natural selection should favor individuals that possess forms of genes, or alleles, that make them more resistant to infection," said O'Donnell, a UW associate professor of psychology. "In some cases, an individual that has more than one form of a gene can ward off more parasites. In humans, for example, there are different forms of a blood gene that can help ward off malaria parasites. People with two different alleles for this gene are more resistant to malaria."
Because they are haploid, Hymenopteran males can't have alternate forms of any genes, or in other words, individual males have no genetic variability. This, O'Donnell and Beshers contend, puts males at a higher susceptibility to disease.