Physiologie des sexués femelles de fourmis (Hymenoptera; Formicidae) en relation avec le mode de fondation

A study was undertaken to determine if there was a relation between the mode of colony founding in ants and the physiology of the new queens produced, in which mature gynes of 24 ant species were examined. Gynes of species utilizing independent colony founding had a far higher relative fat content (X±SD; 54±6%)(g fat/g dry weight) than gynes of species employing dependent colony founding(19±8%). Dimorphism between queens and workers was significantly higher in species employing independent colony founding. Thus independent colony founding not only results in production of queens with a relatively higher fat content and therefore with a higher energy content per g, but also results in the production of larger queens (in comparison with worker size). Of species employing independent colony founding, 80% were monogynous, whereas only 11% of the species employing dependent colony founding were monogynous. These results are discussed with regard to the social structure and life-history of ant species.

Evolution of self-organized division of labor in a response threshold model.

Division of labor in social insects is determinant to their ecological success. Recent models emphasize that division of labor is an emergent property of the interactions among nestmates obeying to simple behavioral rules. However, the role of evolution in shaping these rules has been largely neglected. Here, we investigate a model that integrates the perspectives of self-organization and evolution. Our point of departure is the response threshold model, where we allow thresholds to evolve. We ask whether the thresholds will evolve to a state where division of labor emerges in a form that fits the needs of the colony. We find that division of labor can indeed evolve through the evolutionary branching of thresholds, leading to workers that differ in their tendency to take on a given task. However, the conditions under which division of labor evolves depend on the strength of selection on the two fitness components considered: amount of work performed and on worker distribution over tasks. When selection is strongest on the amount of work performed, division of labor evolves if switching tasks is costly. When selection is strongest on worker distribution, division of labor is less likely to evolve. Furthermore, we show that a biased distribution (like 3:1) of workers over tasks is not easily achievable by a threshold mechanism, even under strong selection. Contrary to expectation, multiple matings of colony foundresses impede the evolution of specialization. Overall, our model sheds light on the importance of considering the interaction between specific mechanisms and ecological requirements to better understand the evolutionary scenarios that lead to division of labor in complex systems. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s00265-012-1343-2) contains supplementary material, which is available to authorized users.

Evolution of gene expression in fire ants: the effects of developmental stage, caste, and species.

Ants provide remarkable examples of equivalent genotypes developing into divergent and discrete phenotypes. Diploid eggs can develop either into queens, which specialize in reproduction, or workers, which participate in cooperative tasks such as building the nest, collecting food, and rearing the young. In contrast, the differentiation between males and females generally depends upon whether eggs are fertilized, with fertilized (diploid) eggs giving rise to females and unfertilized (haploid) eggs giving rise to males. To obtain a comprehensive picture of the relative contributions of gender (sex), caste, developmental stage, and species divergence to gene expression evolution, we investigated gene expression patterns in pupal and adult queens, workers, and males of two species of fire ants, Solenopsis invicta and S. richteri. Microarray hybridizations revealed that variation in gene expression profiles is influenced more by developmental stage than by caste membership, sex, or species identity. The second major contributor to variation in gene expression was the combination of sex and caste. Although workers and queens share equivalent diploid nuclear genomes, they have highly distinctive patterns of gene expression in both the pupal and the adult stages, as might be expected given their extraordinary level of phenotypic differentiation. Overall, the difference in the proportion of differentially expressed genes was greater between workers and males than between workers and queens or queens and males, consistent with the fact that workers and males share neither gender nor reproductive capability. Moreover, between-species comparisons revealed that the greatest difference in gene expression patterns occurred in adult workers, a finding consistent with the fact that adult workers most directly experience the distinct external environments characterizing the different habitats occupied by the two species. Thus, much of the evolution of gene expression in ants may occur in the worker caste, despite the fact that these individuals are largely or completely sterile. Analyses of gene expression evolution revealed a combination of positive selection and relaxation of stabilizing selection as important factors driving the evolution of such genes.

Contribution to the conservation biology of the wood ant Formica lugubris (Hymenoptera, Formicidae) in Switzerland

Summary Among ants, wood ants are probably the most fascinating and studied species in temperate European forests. Unfortunately, due to several threats they are nowadays registered in red lists. Recent studies made in the Swiss Jura Mountains ended up in the description of a new sympatric sibling species of Formica lugubris (i.e. Formica paralugubris Seifert 1996). Because of this confusion the biology of F. lugubris is incomplete. Due to the extreme difficulties to distinguish morphologically F. lugubris from F. paralugubris we studied their cuticular hydrocarbons profiles. Irrespective of their geographic origin, we observed quantitative discrimination between species within each caste (workers, males and gynes =young alate female). Moreover, using a behavioural taxonomic approach (i.e. the pupa-carrying test) we showed that ants preferred conspecific worker pupae to those of the sibling species. These first results allowed us to consider the two species as two separate taxonomic units. To understand their coexistence, habitat distribution models were fitted with GIS predictors and factors known to influence wood ant distribution. In the Jura Mountains, although the two species share very similar habitats, they are spatially segregated. F. lugubris occurs more frequently at woodland borders than in forest interiors. We demonstrated with genetic and field data that Formica lugubris displays two different social forms in close proximity in alpine zone (e.g. unmanaged forests of the Swiss National Park). We discovered populations mostly monogynous to weakly polygynous (i.e. one to a few egg laying queens per colony) and monodomous (i.e. one nest per colony), and polygynous/polydomous populations (new nests being founded by colony budding). It is generally admitted that monogyne species disperse well in order to find suitable habitat to found new colonies whereas polygyne species have restricted dispersal and local mating within the nest. In order to compare reproductive strategies of F. lugubris and F. paralugubris (i.e. matings and dealation process) we conducted experiments with sexuals. F, lugubris gynes from monogynous/monodomous populations do not show a local strategy like the obligately polygynous F. paralugubris (i.e. early dealation even without mating, insemination without flight activity and low fat reserve). They always keep their wings, do not mate when not able to fly and have high amount of fat content revealing high survival capacities. On the other side, F, lugubris gynes from polygynous/polydomous populations have lower lipid reserves and displayed a reproductive behaviour close to the F. para lugubris one. After dispersal, wood ant gynes can either start new societies by temporary social parasitism of another species (i.e. subgenus Serviformica) or be adopted intraspecifically in an existing nest. In F. lugubris, we demonstrated that gynes from monogynous/monodomous colonies showed a high success for temporary social parasitism compare to the lower success of gynes from polygynous/polydomous colonies. However, physiological analyses suggested that only gynes from monogynous/ monodomous populations can efficiently disperse and found new nest by temporary social parasitism. Intraspecifically, gynes were accepted to a high degree in polygynous nest and in monogynous nests as long as these nests contained sexuals. In conclusion, Formica lugubris displays a social and dispersal polymorphism (mixed mating and founding system) representing a behavioural plasticity in relation to environmental and ecological conditions. Therefore, conservation measures directed toward this species should try to maintain a maximum of diversity at the habitat level. Résumé Les fourmis des bois sont probablement parmi les espèces de fourmis les plus fascinantes et les plus étudiées des forêts tempérées Européennes. Actuellement, du fait de différentes menaces, elles figurent malheureusement sur listes rouges. Plusieurs études menées au sein du Jura Suisse ont abouti à la description d'une nouvelle espèce jumelle et sympatrique de Formica lugubris (F. para- lugubris Seifert 1996). A cause de cette confusion la biologie de F lugubris est lacunaire. La distinction morphologique de F. lugubris et de F. para lugubris est si difficile que nous avons étudié leurs hydrocarbures cuticulaires. Indépendamment de l'origine géographique, nous avons observé une discrimination quantitative entre les espèces au sein de chaque caste (ouvrières, mâles et jeunes femelles ailées). De plus, à l'aide d'une approche taxonomique comportementale (le test de transport de cocons) nous avons montré que les fourmis préfèrent des cocons d'ouvrières conspécifiques à ceux de l'espèce jumelle. Ces premiers résultats nous permettent de considérer ces deux espèces comme deux unités taxonomiques distinctes et valables. Afin de comprendre leur coexistence, des modèles mathématiques ont été développés avec des données SIG et des facteurs écologiques influençant la répartition des fournis des bois. Dans le Jura, même si elles partagent des habitats fortement similaires, les deux espèces n'occupent pas les mêmes secteurs. F. lugubris est plus fréquente en lisière forestière plutôt qu'en pleine forêt. Nous avons démontré grâce à des données génétiques et de terrain que F. lugubris présente deux formes sociales au sein de la zone alpine (forêts protégées du Parc National Suisse). D'autre part, nous avons découvert des populations monogynes à faiblement polygynes (une à quelques reines pondeuses par colonie) et monodomes (colonies composées d'une seule fourmilière), et des populations polygynes/polydomes (les nouveaux nids étant produit par bourgeonnement). Généralement, les espèces monogynes dispersent sur de grandes distances et peuvent coloniser des habitats favorables à la fondation de nouvelles colonies alors que les espèces polygynes possèdent une dispersion limitée avec des accouplements à l'intérieur des nids. Afin de comparer les stratégies de reproduction de F. lugubris et de F. paralugubris (accouplements et perte des ailes) nous avons mené des expériences avec les sexués. Les jeunes femelles ailées de F. lugubris issues de populations monogynes/monodomes ne présentent pas de stratégie locale comparée à l'espèce obligatoirement polygyne F paralugubris (perte des ailes précoce même si il n'y a pas eu accouplement, insémination possible sans avoir volé activement et faibles réserves de graisse). Elles conservent toujours leurs ailes, ne s'accouplent pas lorsqu'elles sont empêchées de voler et possèdent de grandes quantités de graisse révélant de fortes capacités de survie. D'autre part, les jeunes femelles ailées de F. lugubris provenant de populations polygynes/polydomes ont peu de réserves lipidiques et ont un comportement de reproduction proche de celles de F. paralugubris. Après leur dispersion, les jeunes sexués femelles de fourmis des bois peuvent soit fonder une nouvelle société par parasitisme social temporaire d'un nid d'une autre espèce (sous-genre Serviformica) soit être adoptées dans un nid déjà existant de leur propre espèce. Chez F. lugubris, nous avons pu démontrer que les jeunes sexués femelles de colonies monogynes/monodomes présentent un succès élevé au parasitisme sociale temporaire en comparaison au plus faible succès obtenu avec des sexués provenant de colonies polygynes/polydomes. Cependant, les données physiologiques suggèrent que seules les jeunes sexués femelles de populations mono-gynes/monodomes peuvent disperser efficacement et fonder un nouveau nid par parasitisme social temporaire. Au niveau intraspécifique, les jeunes femelles sont acceptées à un taux élevé dans les nids polygynes mais aussi dans les nids monogynes tant que ces nids possèdent encore de jeunes sexués. En conclusion, F. lugubris est caractérisée par un polymorphisme dans ses structures sociales et ses stratégies de dispersion (système mixte d'accouplement et de fondation) ce qui représente une forte plasticité comportementale en relation avec les conditions environnementales et écologiques. Par conséquent, les mesures de conservation de cette espèce devraient s'attacher à maintenir un maximum de diversité au niveau des habitats.

Exposure to bioaerosols in poultry houses at different stages of fattening; use of real-time PCR for airborne bacterial quantification

Previous studies have demonstrated that poultry house workers are exposed to very high levels of organic dust and consequently have an increased prevalence of adverse respiratory symptoms. However, the influence of the age of broilers on bioaerosol concentrations has not been investigated. To evaluate the evolution of bioaerosol concentration during the fattening period, bioaerosol parameters (inhalable dust, endotoxin and bacteria) were measured in 12 poultry confinement buildings in Switzerland, at three different stages of the birds' growth; samples of air taken from within the breathing zones of individual poultry house employees as they caught the chickens ready to be transported for slaughter were also analysed. Quantitative polymerase chain reaction (Q-PCR) was used to assess the quantity of total airborne bacteria and total airborne Staphylococcus species. Bioaerosol levels increased significantly during the fattening period of the chickens. During the task of catching mature birds, the mean inhalable dust concentration for a worker was 26 +/- 1.9 mg m(-3) and endotoxin concentration was 6198 +/- 2.3 EU m(-3) air, >6-fold higher than the Swiss occupational recommended value (1000 EU m(-3)). The mean exposure level of bird catchers to total bacteria and Staphylococcus species measured by Q-PCR is also very high, respectively, reaching values of 53 (+/-2.6) x 10(7) cells m(-3) air and 62 (+/-1.9) x 10(6) m(-3) air. It was concluded that in the absence of wearing protective breathing apparatus, chicken catchers in Switzerland risk exposure beyond recommended limits for all measured bioaerosol parameters. Moreover, the use of Q-PCR to estimate total and specific numbers of airborne bacteria is a promising tool for evaluating any modifications intended to improve the safety of current working practices

Multiple mating and offspring quality in Lasius ants

Genetic diversity benefits for social insect colonies headed by polyandrous queens have received intense attention, whereas sexual selection remains little explored. Yet mates of the same queen may engage in sperm competition over the siring of offspring, and this could confer benefits on queens if the most successful sire in each colony (the majority sire) produces gynes (daughter queens) of higher quality. These benefits could be increased if high-quality sires make queens increase the percentage of eggs that they fertilize (unfertilized eggs develop into sons in social hymenopterans), or if daughters of better genetic quality are over-represented in the gyne versus worker class. Such effects would lead to female-biased sex ratios in colonies with high-quality majority gynes. I tested these ideas in field colonies of Lasius niger black garden ants, using body mass of gynes as a fitness trait as it is known to correlate with future fecundity. Also, I established the paternity of gynes through microsatellite DNA offspring analyses. Majority sires did not always produce heavier gynes in L. niger, but whenever they did do so colonies produced more females, numerically and in terms of the energetic investment in female versus male production. Better quality sires may be able to induce queens to fertilize more eggs or so-called caste shunting may occur wherever the daughters of better males are preferentially shunted to into the gyne caste. My study supports that integrating sexual selection and social evolutionary studies may bring a deeper understanding of mating system evolution in social insects.

Gene by environment interaction: effects of a single-gene and social-environment on reproductive phenotypes of fire ant queens

1. The gene Pgm-3 (or a closely linked gene) influences the phenotype and reproductive success of queens in multiple-queen (polygynous) colonies but not single-queen (monogynous) colonies of the Fire Ant Solenopsis invicta. 2. We investigated the mechanisms of differential phenotypic expression of Pgm-3 in these alternate social forms. Mature winged queens with the homozygous genotype Pgm-3(a/a) averaged 26% heavier than queens with the genotypes Pgm-3(a/b) and Pgm 3(b/b) in the polygynous form. Heterozygotes were slightly heavier (2%) than Pgm-3(b/b) queens in this form, demonstrating that the allele Pgm-3(a) is not completely recessive in its effects on weight. 3. There was no significant difference in weight among queens of the three Pgm-3 genotypes in the monogynous form, with the mean weight of monogynous queens slightly greater than that of polygynous Pgm-3(a/a) queens. Differences in weight between queens of the two social forms and among queens of the three genotypes in the polygynous form are not evident at the pupal stage and thus appear to develop during sexual maturation of the adults. This suggests that some component of the social environment of polygynous colonies inhibits weight gains during queen maturation and that Pgm-(3a/a) queens are relatively less sensitive to this factor. 4. To test whether the high cumulative queen pheromone level characteristic of polygynous colonies is the factor responsible for the differential queen maturation, we compared phenotypes of winged queens reared in split colonies in which pheromone levels were manipulated by adjusting queen number. Queens produced in colony fragments made monogynous were heavier than those produced in polygynous fragments, a finding consistent with the hypothesis that pheromone level affects the reproductive development of queens. However, genotype-specific differences in weights of queens were similar between the two treatments, suggesting that pheromone level was not the key factor of the social environment responsible for the gene-environment interaction. 5. To test whether limited food availability to winged queens associated with the high brood/worker ratios in polygynous colonies is the factor responsible for this interaction, similar split-colony experiments were performed. Elevated brood/worker ratios decreased the weight of winged queens but there was no evidence that this treatment intensified differential weight gains among queens with different Pgm-3 genotypes. Manipulation of the amount of food provided to colonies had no effect on queen weight. 6. The combined data indicate that cumulative pheromone level and brood/worker ratio are two of the factors responsible for the differences in reproductive phenotypes between monogynous and polygynous winged queens but that these factors are not directly responsible for inducing the phenotypic effects of Pgm-3 in polygynous colonies.

Solid-phase microextraction as short-term sampling technique for BTEX occupational exposure

Solid phase microextraction (SPME) has been widely used for many years in various applications, such as environmental and water samples, food and fragrance analysis, or biological fluids. The aim of this study was to suggest the SPME method as an alternative to conventional techniques used in the evaluation of worker exposure to benzene, toluene, ethylbenzene, and xylene (BTEX). Polymethylsiloxane-carboxen (PDMS/CAR) showed as the most effective stationary phase material for sorbing BTEX among other materials (polyacrylate, PDMS, PDMS/divinylbenzene, Carbowax/divinylbenzene). Various experimental conditions were studied to apply SPME to BTEX quantitation in field situations. The uptake rate of the selected fiber (75 μm PDMS/CAR) was determined for each analyte at various concentrations, relative humidities, and airflow velocities from static (calm air) to dynamic (>200 cm/s) conditions. The SPME method also was compared with the National Institute of Occupational Safety and Health method 1501. Unlike the latter, the SPME approach fulfills the new requirement for the threshold limit value-short term exposure limit (TLV-STEL) of 2.5 ppm for benzene (8 mg/m3).

The presence of conifer resin decreases the use of the immune system in wood ants

1. Wood ants (Formica paralugubris) incorporate large amounts of solidified conifer resin into their nest, which reduces the density of many bacteria and fungi and protects the ants against some detrimental micro-organisms. By inducing an environment unfavourable to pathogens, the presence of resin may allow workers to reduce the use of their immune system. 2. The present study tested the hypothesis that the presence of resin decreases the immune activity of wood ants. Specifically, three components of the humoral immune defences of workers kept in resin-rich and resin-free experimental nests (antibacterial, lytic, and prophenoloxidase activities) were compared. 3. The presence of resin was associated with reduced bacterial and fungal densities in nest material and with a small decrease in worker antibacterial and lytic activities. The prophenoloxidase activity was very low in all workers and was not affected by the presence of resin. 4. These results suggest that collective medication with resin reduces pathogen pressure, which in turn decreases the use of the inducible part of the immune system. More generally, the use of plant secondary compounds might be an efficient and economical way to fight pathogens.

Caste determination through mating in primitively eusocial societies.

Eusocial animal societies are typified by the presence of a helper (worker) caste which predominantly cares for young offspring in a social group while investing little in their own direct reproduction. A key question is what determines whether an individual becomes a worker or leaves to initiate her own reproduction. In some insects, caste is determined nutritionally during development. In others, and in vertebrate societies, adults are totipotent and the cues that determine caste are less well known. The mate limitation hypothesis (MLH) states that a female's mating status acts as a cue for caste determination: females that mate become reproductives, while those that fail to mate become workers. The MLH is consistent with empirical observations in sweat bees showing that over the course of the nesting season, there are increases in both the proportion of females that become reproductives and the frequency of males in the mating pool. We modelled a foundress's offspring sex-ratio strategy to investigate whether an increasingly male-biased operational sex-ratio over time is evolutionarily stable under the MLH. Our results indicate that such a pattern could occur if early workers were more valuable than late workers. This pattern was then more likely if male mortality was high, if worker mortality was low, if the value of a worker was high and if the period over which workers can help was short. Our results suggest that the MLH can be evolutionarily stable, but only under restrictive conditions. Manipulative experiments are now required to investigate whether mating determines caste in nature.

Parasites and sociality in ants

SUMMARY : Parasites and sociality in ants This thesis investigates the complex relationships between sociality, defences against parasites and the regulation of social structures. We studied how fungal parasites influenced colony organization, collective defences and social immunity in the ant Formica selysi. We first describe the diversity and prevalence of fungal pathogens associated with ant nests. The richness of fungal parasites community may increase the risk of multiple infections and select for a diversification of anti-parasitic defences in ants. Collective defences are powerful means to combat parasites, but can also increase the risk of disease transmission. Here, we showed that allo-grooming (mutual cleaning) was directed towards every returning individuals, be they contaminated or not. This collective behaviour removed conidia more efficiently than self-grooming but did not improve the survival of contaminated individuals. This suggests that allo-grooming may rather protect the group than cure contaminated individuals. It may also permit "social vaccination" if a contact with contaminated ants protects groomers frorn a second fungal exposure. Social transfer of immunity is an emerging theme in insect immunology. Here, we showed that ants in contact with an ant from a different genetic lineage had a higher disease resistance. We also found that naïve ants had a higher resistance after a contact with an immunized ant. This suggests that a transfer of resistance is possible and that "social vaccination" may improve the resistance of the group. However, it remains unclear whether repeated exposure to parasites may also increase the resistance of infected individuals themselves. lmmune memory in invertebrates is still debated. We tested whether immune priming against fungal parasite arose in ants and whether it was strain-specific. We found no evidence of immune priming. Naïve and immunized ants had a similar survival when infected. Together with our previous results, this suggests that ants have evolved efficient collective anti-fungal defences but that these defences aim at protecting the group rather than the contaminated individuals. ln colonies of our study population, there is a strong variation in the number of breeders. This is associated with important changes in life-history traits like demography or queen and worker body size. In the second part of the thesis, we investigated how social structures evolved and were maintained. We showed that queens from monogyne and polygyne colonies were able to found new colonies both alone or in association. We also found that there was no difference between monogyne and polygyne colonies in the acceptance of additional queens. These results suggest that a high plasticity has been maintained in this population, which may permit to adapt rapidly to changing environmental conditions. RESUME : Parasites et socialité chez les fourmis Durant cette thèse, nous avons étudié comment la socialité apporte de nouvelles réponses a des problèmes complexes telle que la défense contre les parasites ou l'organisation de la vie en groupe. Nous avons choisi comme modèle la fourmi Formica selysi et ses champignons pathogènes. Nous avons d'abord montré que la diversité et la prévalence de champignons pathogènes associés aux nids de fourmis étaient très élevées. Cela a pu pousser les fourmis à diversifier le champ de leur défenses anti-parasitaires afin d'éviter les infections multiples, La socialité a en particulier permis l'évolution de défenses collectives qui pourraient être plus efficaces que les défenses individuelles. Nous nous sommes donc intéressés de plus près aux défenses collectives et avons étudié quels en étaient les coûts et les bénéfices pour le groupe et pour ses membres. Nous avons trouvé que les fourmis nettoyaient tous les individus entrant dans la colonie, qu'ils soient contaminés ou non. Cela permettait d'ôter plus de spores que le nettoyage individuel et n'augmentait pas la transmission de maladie. Cependant, le nettoyage mutuel n'augmentait pas non plus la survie des individus contaminés. ll se pourrait donc que ce comportement serve plutôt a éviter une dissémination de la maladie qu'à soigner les individus contaminés. Le nettoyage mutuel pourrait aussi permettre aux individus sains d'avoir un premier contact non-létal avec un parasite et d'être vaccinés contre une future exposition. Cette hypothèse a été soutenue par une expérience dans laquelle nous avons montré que le contact avec une fourmi immunisée permettait d'augmenter la résistance d'individus naïfs. Les fourmis avaient aussi une meilleure résistance lorsqu'elles étaient en contact avec une fourmi provenant d'une autre lignée génétique. Cette "vaccination sociale" pourrait permettre d'une part d'augmenter le nombre d'espèce de parasites contre lesquelles le groupe serait protégé et d'autre part de faire l'économie d'autres défenses individuelles telles que la réponse immunitaire. Nous avons testé si les fourmis étaient elles-mêmes "vaccinées", c'est-à-dire, si elles exprimaient une mémoire immunitaire après un premier contact avec un champignon parasite. Nous n'avons trouvé aucune différence de survie entre les individus naïfs et immunisés ce qui suggère les fourmis favorisent d'autres défenses que la mémoire immunitaire contre les champignons entomopathogènes. Cela suggère également que les comportements coopératifs anti-parasitaires pourraient compléter, voire remplacer les défenses individuelles. La socialité telle qu'elle est pratiquée par les fourmis pose un autre problème de poids qui est celui de savoir combien d'individus se reproduisent. En effet, si les ouvrières sont stériles, le nombre de reines assurant la reproduction peut varier considérablement. Dans la population de E sebrsi étudiée, les colonies monogynes (une reine) co-existent avec des colonies polygynes (plusieurs reines) dans le même habitat. Nous nous sommes demandés si ces structures sociales étaient fixes ou si un changement de l'une à l'autre était possible. Pour cela nous avons comparé la fondation de nouvelles colonies par les jeunes reines issues de colonies monogynes et polygynes. Nous avons également observé si l'acceptation de nouvelles reines était possible dans les deux types de colonies. Nous n'avons trouvé aucune différence entre les deux types de colonies. Cela suggère qu'un changement est possible et que l'évolution des structures sociales est un processus dynamique. Cela pourrait être dû à l'habitat particulièrement changeant dans lequel se trouve notre population qui exigerait d'être capable de s'adapter très rapidement a de nouvelles conditions.

Sex allocation conflict in ants: when the queen rules.

Insect societies are paramount examples of cooperation, yet they also harbor internal conflicts whose resolution depends on the power of the opponents. The male-haploid, female-diploid sex-determining system of ants causes workers to be more related to sisters than to brothers, whereas queens are equally related to daughters and sons. Workers should thus allocate more resources to females than to males, while queens should favor an equal investment in each sex. Female-biased sex allocation and manipulation of the sex ratio during brood development suggest that workers prevail in many ant species. Here, we show that queens of Formica selysi strongly influenced colony sex allocation by biasing the sex ratio of their eggs. Most colonies specialized in the production of a single sex. Queens in female-specialist colonies laid a high proportion of diploid eggs, whereas queens in male-specialist colonies laid almost exclusively haploid eggs, which constrains worker manipulation. However, the change in sex ratio between the egg and pupae stages suggests that workers eliminated some male brood, and the population sex-investment ratio was between the queens' and workers' equilibria. Altogether, these data provide evidence for an ongoing conflict between queens and workers, with a prominent influence of queens as a result of their control of egg sex ratio.

Respiratory effects of an exposure to wheat dust among grain workers and farmers: a longitudinal study

OBJECTIVES: Agriculture is considered one of the occupations most at risk of acute or chronic respiratory problems. The aim of our study was to determine from which level of exposure to organic dust the respiratory function is chronically affected in workers involved in wheat grain or straw manipulation and to test if some of these working populations can recover their respiratory function after an exposure decrease. METHOD: 87 workers exposed to wheat dust: farmers, harvesters, silo workers and livestock farmers and 62 non exposed workers, were included into a longitudinal study comprising two visits at a six months interval with lung function measurements and symptom questionnaires. Cumulative and mean exposure to wheat dust were generated from detailed work history of each worker and a task-exposure matrix based on task-specific exposure measurements. Immunoglobulins (IgG and IgE) specific of the most frequent microorganisms in wheat dust have been determined. RESULTS: FEV1 decreased significantly with the cumulative exposure and mean exposure levels. The estimated decrease was close to 200 mL per year of high exposure, which corresponds roughly to levels of wheat dust higher than 10 mg/m(3). Peak expiratory flow and several acute symptoms correlate with recent exposure level. Recovery of the respiratory function six months after exposure to wheat dust and evolution of exposure indicators in workers blood (IgG and IgE) will be discussed. CONCLUSIONS: These results show a chronic effect of exposure to wheat dust on bronchial obstruction. Short term effects and reversibility will be assessed using the full study results.

Population viscosity can promote the evolution of altruistic sterile helpers and eusociality.

Because it increases relatedness between interacting individuals, population viscosity has been proposed to favour the evolution of altruistic helping. However, because it increases local competition between relatives, population viscosity may also act as a brake for the evolution of helping behaviours. In simple models, the kin selected fecundity benefits of helping are exactly cancelled out by the cost of increased competition between relatives when helping occurs after dispersal. This result has lead to the widespread view, especially among people working with social organisms, that special conditions are required for the evolution of altruism. Here, we re-examine this result by constructing a simple population genetic model where we analyse whether the evolution of a sterile worker caste (i.e. an extreme case of altruism) can be selected for by limited dispersal. We show that a sterile worker caste can be selected for even under the simplest life-cycle assumptions. This has relevant consequences for our understanding of the evolution of altruism in social organisms, as many social insects are characterized by limited dispersal and significant genetic population structure.

Symptoms of chronic stress at work in NGO workers

Working in a NGO often involves providing life saving resources (food, medicine, equipment, water, etc) to needy populations around the globe. Such duty requires highly dedicated employees and humanitarian workers are said to face a hign degree of pressure in their daily work. Despite the evidence of taxing work demands, and a high potential for stress related problems, very few studies on occupational chronic stress have specifically looked at NGO workers. Assuming that "field stress" can relay to workers at headquarters, we carried out an exploratory study about occupational health among employees of a NGO's headquarters. We sent a questionnaire to all employees (N=130) of a NGO headquarters located in Switzerland. We used the TST questionnaire (French version of the Langner's questionnaire on psychiatric symptoms) to identify cases with potential mental health problems. We also included in the questionnaire some items about motivation, acknowledgment, work-life balance, job demand, and autonomy. A total of 75 employees answered our questionnaire (57% response rate). 44% of our sample were men (n=33) and 56% were women (n=42). The mean age was of 40 years (SD=7.6). 56% were working at the headquarters of the NGO in questions as of 2 years or less. Not surprisingly, a majority of respondents reported to be highly motivated (74%) and the meaning of work was important for 80% of them. However, 35% indicated having problems in conciliating their private and professional life. Most frequent reported symptoms included feeling "weak all over" (81%), having "trouble getting asleep often" (35%), "clogging in nose" (35%), feeling "nervous often" (33%), and "memory not all right" (33%). The score for psychiatric symptoms was high in 8 (11%) employees whose health might therefore be at risk. In comparison, other sudies showed that this proportion was 9% for French teachers and 16% for sales personnel1. Results show that symptoms of mental health problems do occur among NGO workers. Some of these symptoms are known to be linked to occupational stress. Chronic stress manifests itself first in non-specific symptoms (e.g. fatigue) and later in specific pathologies. This could explain the relatively low proportion of cases with a high score in Langner's scale than was expected. Therefore, we hypothesize a healthy worker effect. The fact that our sample is 40 years old in average, and that the turnover is quite high can also support this hypothesis. Further research is needed in order to better understand occupational stress in this specific population. An upcoming study will investigate the role of organizational factors associated with health complaints. Therefore, a longitudinal survey including quantitative and qualitative methods is appropriate.