Séverine Devers

Hydrocarbons in the ant Lasius niger: from the cuticle to the nest and home range marking.

The cuticular hydrocarbons (CHCs) of the ant Lasius niger are described. We observe a high local colony specificity of the body cuticular profile as predicted for a monogynous and multicolonial species. The CHCs show a low geographical variation among different locations in France. The CHCs on the legs also are colony specific, but their relative quantities are slightly different from those on the main body. For the first time, we demonstrate that the inner walls of the ant nest are coated with the same hydrocarbons as those found on the cuticle but in different proportions. The high amount of inner-nest marking and its lack of colony-specificity may explain why alien ants are not rejected once they succeed in entering the nest. The cuticular hydrocarbons also are deposited in front of the nest entrance and on the foraging arena, with a progressive increase in n-alkanes relative amounts. Chemical marks laid over the substrate are colony specific only when we consider methyl-branched alkanes. Our data confirm that these “footprint hydrocarbons” are probably deposited passively by the contact of ant tarsae with the substrate. These results suggest that the CHCs chemical profiles used by ants in colony recognition are much more complex than a single template: ants have to learn and memorize odors that vary depending on their context of perception.

Production of sexuals in a fission-performing ant: dual effects of queen pheromones and colony size

Models based on the kin selection theory predict that in social hymenopterans, queens may favor a lower investment in the production of sexuals than workers. However, in perennial colonies, this conflict may be tuned down by colony-level selection because of the trade off between colony survival and reproductive allocation. In this study, we present a survey of sexual production in colonies of Aphaenogaster senilis, a common species of ant in the Iberian Peninsula. Similar to most species that reproduce by fission, males were found in large excess compared to gynes (172:1). Sexuals were more likely to be found in queenless than in queenright (QR) field colonies. However, we also found a few gynes and numerous males in very large QR colonies. We compared these data with those available in the literature for A. rudis, a congeneric species from North America that has independent colony founding. The sex ratio in this species was only five males for each female, and sexuals were mostly found in QR nests, irrespective of colony size. We confirmed queen inhibition of sexual production in A. senilis in laboratory experiments and provide evidence that this inhibition is mediated by a nonvolatile pheromone. To seek the potential source of such a queen pheromone, we analyzed the secretions of two conspicuous exocrine glands, the Dufour’s and postpharyngeal glands (DG and PPG, respectively) in both queens and workers. Both secretions were composed of hydrocarbons, but that of DG also contained small quantities of tetradecanal and hexadecanal. The hydrocarbon profile of the DG and PPG showed notable caste specificity suggesting a role in caste-related behavior. The PPG secretions also differed between colonies suggesting its role in colony-level recognition. We suggest that in A. senilis, there are two modes of colony fission: First, in very large colonies, gynes are produced, probably because of the dilution of the queen pheromone, and consequently one or more gynes leave the mother colony with workers and brood to found a new nest. This is beneficial at the colony level because it avoids the production of costly sexuals in small colonies. However, because the queen and workers have different optima for sexual production, we hypothesize that queens tend to overproduce the pheromone to delay their production. This in turn may drive workers to leave the mother colony during nest relocation and to produce sexuals once they are away from the queen’s influence, creating a second mode of colony fission.

Ant cuticles: A trap for atmospheric phthalate contaminants

Phthalates are universal contaminants. We show that they are trapped by the ant cuticles and maintained permanently at a low level, generally less than 1% of cuticular components. They are found throughout the interior of the insect, predominately in the fat body, which suggests that they are adsorbed by the cuticle. In open plastic boxes free of phthalates the ants became more contaminated with phthalates over a period of time, whereas in closed glass jars they did not. This finding suggests that the main source of pollutants is the atmosphere. Different ant species collected from multiple places showed similar levels of contamination. It appeared that in some pristine places the contamination was lower, but this needs to be confirmed. Ants can be considered as bio-indicators of phthalate pollution.

Ectosymbionts and immunity in the leaf-cutting ant Acromyrmex subterraneus subterraneus

Associations with symbiotic organisms can serve as a strategy for social insects to resist pathogens. Antibiotics produced by attine ectosymbionts (Actinobacteria) suppress the growth of Escovopsis spp., the specialized parasite of attine fungus gardens. Our objective was to evaluate whether the presence or absence of symbiotic actinobacteria covering the whole ant cuticle is related to differential immunocompetence, respiratory rate and cuticular hydrocarbons (CHs). We evaluated these parameters in three worker groups of Acromyrmex subterraneus subterraneus: External workers (EXT), internal workers with actinobacteria covering the whole body (INB) and internal workers without actinobacteria covering the whole body (INØ). We also eliminated the actinobacteria by antibiotic treatment and examined worker encapsulation response. INB ants showed lower rates of encapsulation and respiration than did the EXT and INØ ants. The lower encapsulation rate did not seem to be a cost imposed by actinomycetes because the elimination of the actinomycetes did not increase the encapsulation rate. Instead, we propose that actinobacteria confer protection to young workers until the maturation of their immune system. Actinobacteria do not seem to change nestmate recognition in these colonies. Although it is known that actinobacteria have a specific action against Escovopsis spp., our studies, along with other independent studies, indicate that actinomycetes may also be important for the individual health of the workers.

Microgynous queens in the Paleartic ant, Manica rubida: Dispersal morphs or social parasites?

Abstract In many ant species, queen size is dimorphic, with small microgynes and large macrogynes, which differ, for example, in size, insemination rate, ovary development, and dispersal tactics. These polymorphic queens often correspond with alternative reproductive strategies. The Palearctic ant, Manica rubida (Latreille) (Hymenoptera: Formicidae), lives mostly in mountainous regions in either monogynous colonies, containing one macrogynous queen or polygynous colonies, containing a few large macrogynous queens. In 1998, a colony of M. rubida was discovered containing macrogynes and many small alate microgynes that did not engage in a nuptial flight but, instead, stayed in the home nest the following winter. These microgynes were studied more closely by investigating their size, behavior, and spermatheca in relation to M. rubida macrogynes and workers. Mitochondrial DNA of macrogynes, microgynes and workers from four nests was sequenced to detect possible genetic differences between them. The microgynes were significantly smaller than the macrogynes, and the head width of the gynes was completely bimodal. The microgynes behaved like workers of the macrogynes in every experiment tested. Furthermore, the microgynes had a normal spermatheca and could be fecundated, but rarely (only one in several years). Finally, all the individuals were genetically identical, except three workers that differed by only one codon position. Because these microgynes have features of both queens and workers, their functional significance in the colony is not yet clear.

Impact of ecological doses of the most widespread phthalate on a terrestrial species, the ant Lasius niger

Phthalates are synthetic contaminants released into the environment notably by plastic waste. Semi-volatile, they adsorb to atmospheric particles and get distributed in all ecosystems. Effects of this major anthropogenic pollution in economical species in aquatic habitats have attracted large interest. On the contrary, very few studies have focused on wild terrestrial species. Yet, these lipophilic molecules are easily trapped by insect cuticle; ants and other insects have been shown to permanently bear among their cuticular components a non-negligible proportion of phthalates, meaning that they suffer from chronic exposure to these pollutants. Oral route could also be an additional way of contamination, as phthalates tend to stick to any organic particle. We show here via a food choice experiment that Lasius niger workers can detect, and avoid feeding on, food contaminated with DEHP (DiEthyl Hexyl Phthalate), the most widespread phthalate found in nature. This suggests that the main source of contamination for ants is atmosphere and that doses measured on the cuticle correspond to the chronic exposure levels for these animals. Such an ecologically relevant dose of DEHP was used to contaminate ants in lab and to investigate their physiological impact. Over a chronic exposure (1 dose per week for 5 weeks), the egg-laying rate of queens was significantly reduced lending credence to endocrine disruptive properties of such a pollutant, as also described for aquatic invertebrates. On the contrary, short term exposure (24h) to a single dose of DEHP does not induce oxidative stress in ant workers as expected, but leads to activation of the immune system. Because of their very large distribution, their presence in virtually all terrestrial ecosystems and their representation at all trophic levels, ants could be useful indicators of contamination by phthalates, especially via monitoring the level of activation of their immune state.

Blochmannia endosymbionts and their host, the ant Camponotus fellah: Cuticular hydrocarbons and melanization

Carpenter ants (genus Camponotus) have mutualistic, endosymbiotic bacteria of the genus Blochmannia whose main contribution to their hosts is alimentary. It was also recently demonstrated that they play a role in improving immune function as well. In this study, we show that treatment with an antibiotic produces a physiological response inducing an increase in both the quantity of cuticular hydrocarbons and in the melanization of the cuticle probably due to a nutritive and immunological deficit. We suggest that this is because it enhances the protection the cuticle provides from desiccation and also from invasions by pathogens and parasites. Nevertheless, the cuticular hydrocarbon profile is not modified by the antibiotic treatment, which indicates that nestmate recognition is not modified.

The Guianese population of the fire ant Solenopsis saevissima is unicolonial

In this study, conducted in French Guiana, a part of the native range of the fire ant Solenopsis saevissima, we compared the cuticular hydrocarbon profiles of media workers with previous results based on intraspecific aggressiveness tests. We noted a strong congruence between the two studies permitting us to delimit 2 supercolonies extending over large distances (up to 54 km), a phenomenon known as unicoloniality. Solenopsis geminata workers, taken as an out‐group for cluster analyses, have a very different cuticular hydrocarbon profile. Because S. saevissima has been reported outside its native range, our conclusion is that this species has the potential to become invasive because unicoloniality (i.e., the main attribute for ants to become invasive) was shown at least for the Guianese population.