Défenses immunitaires et division du travail chez les fourmis

Chez les insectes sociaux, les défenses immunitaires sont importantes à la fois pour la survie individuelle et pour la protection de la colonie contre les infections parasitaires. Nous avons étudié comment les défenses immunitaires, qui sont un trait coûteux, étaient modulées par la division du travail concernant la reproduction ou les autres tâches. En effet, les différentes fonctions assurées par les individus au sein de la colonie peuvent être associées à des contraintes sélectives distinctes et donc faire varier l investissement immunitaire. Réciproquement, considérer les défenses immunitaires comme un trait d histoire de vie peut aider à comprendre certains aspects de l évolution sociale. Les fourmis offrent des modèles intéressants pour ces problématiques. Dans un premier temps, nous avons étudié l impact des conflits reproducteurs dans une espèce de fourmi sans reine, Diacamma sp. Nilgiri. Un taux plus faible de travail et une immunosuppression ont été observés dans les groupes affectés par les conflits, ce qui suggère un coût des conflits reproducteurs. Dans un deuxième temps, nous avons examiné les niveaux de phénoloxydase (PO) et prophénoloxydase (PPO), deux enzymes importantes du système immunitaire des insectes, chez les ouvrières de la fourmi Cataglyphis velox. Nous avons trouvé que les fourrageuses présentaient des niveaux plus élevés de PO que les ouvrières internes. Ceci semble indiquer une élévation du niveau de PO chez les fourrageuses en lien avec le risque élevé de blessures et d infection à l extérieur du nid. Enfin, nous avons fait l hypothèse que les ouvrières inactives, fréquemment observées dans les colonies, pourraient investir plus de ressources dans les défenses immunitaires. Nous avons conduit une étude comportementale chez Cataglyphis velox associée à des mesures de défenses immunitaires. Nous n avons pas mis en évidence de lien entre l inactivité des ouvrières et leurs défenses immunitaires mais d autres paramètres immunitaires pourraient être analysés. L ensemble de ces travaux suggère que la division du travail pourrait influencer la variation intra-coloniale dans l immunité et souligne l importance de prendre en compte les défenses immunitaires dans l étude des insectes sociaux.Immune defence is a fundamental trait determining fitness in social insects, both by increasing individual survival and by limiting contamination at colony level. We investigated how individual variation in immune defence, which is a costly trait, is related to division of labour concerning reproduction or sterile tasks. Indeed, the different worker functions within the colony may be associated to distinct selection pressures and thus induce a variation in immune defence. Reciprocally, including immune defence as a life-history trait can help to understand some aspects of social evolution. Ants offer interesting models to address these questions. Firstly, we studied the effects of reproductive conflicts on labour and immune defence in a queenless ant, Diacamma sp. from Nilgiri. The groups affected by conflicts showed a lower rate of labour and an immunosuppression, suggesting a cost of reproductive conflicts. Secondly, we investigated the levels of phenoloxidase (PO) and prophenoloxidase (PPO), two major enzymes of the insect immune system, in workers of the ant Cataglyphis velox. We found a higher PO activity in foragers than in intra-nidal workers, which could result from an adaptive up-regulation of PO in foragers in relation to the high risk of infection and wounding outside the nest. Finally, we hypothesized that inactive workers, often observed in social insect colonies, may invest more resources in immune defence. We conducted a behavioural and immunological study in Cataglyphis velox to analyse the relation between inactivity and immune defence. Our hypothesis was not supported but could be further studied by measuring other immune parameters. This work suggests that division of labour may influence the intra-colonial immune variation and highlights the importance of considering immune defence for the study of social insects.PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

To have and not to have sex: when multiple evolutions of conditional use of sex elegantly solve the question in the ant genus Cataglyphis

International audienceOrganisms use an amazingly large diversity of mechanisms to pass on their genes to the next generation. Sex is ancestral in eukaryotes, where it remains the most widespread way of reproduction. By combining one's genes with those of a partner, sex entails a dilution of one's genes at each generation. Evolution has been particularly creative in devising mechanisms allowing females to avoid this dilution, from classical parthenogenesis to the elimination of male genes after fertilization (Bell, 1982). Moreover, the term parthenogenesis include various forms. Parthenogenesis can be used for female (thelytoky) or male (arrhenotoky) production and it can be associated with different cytological mechanisms, from strict clonality to meiotic division with the fusion of two of the four products of meiosis to restore diploidy (Suomalainen, Saura, & Lokki, 1987). Understanding the evolution of these diverse reproductive systems remains one of the most exciting and longstanding questions in evolutionary biology. By characterizing the reproductive systems of 11 species from the thermophilic ant genus Cataglyphis, in this issue of Molecular Ecology, Kuhn, Darras, Paknia, & Aron (2020) show the high lability of parthenogenesis, with multiple independent evolution of facultative thelytoky from sexual ancestors. The diversity of life history traits and social characteristics of this genus (e.g. mode of colony foundation, female polyandry) provides a unique and exciting opportunity to investigate the social and environmental factors driving the evolution of reproductive systems in social Hymenoptera

Worker Size Diversity Has No Effect on Overwintering Success under Natural Conditions in the Ant Temnothorax nylanderi

International audienceWinter is a difficult period for animals that live in temperate zones. It can inflict high mortality or induce weight loss with potential consequences on performance during the growing season. Social groups include individuals of various ages and sizes. This diversity may improve the ability of groups to buffer winter disturbances such as starvation or cold temperature. Studies focusing on the buffering role of social traits such as mean size and diversity of group members under winter conditions are mainly performed in the laboratory and investigate the effect of starvation or cold separately. Here, we experimentally decreased worker size diversity and manipulated worker mean size within colonies in order to study the effect on overwintering survival in the ant Temnothorax nylanderi. Colonies were placed under natural conditions during winter. Colony survival was high during winter and similar in all treatments with no effect of worker size diversity and mean worker size. Higher brood survival was positively correlated with colony size (i.e., the number of workers). Our results show that the higher resistance of larger individuals against cold or starvation stresses observed in the laboratory does not directly translate into higher colony survival in the field. We discuss our results in the light of mechanisms that could explain the possible non-adaptive size diversity in social species

Structure socio-génétique des sociétés et des populations chez la fourmi Cataglyphis cursor (Formicidae)

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fichier données pour Mol Ecol driad

First line : ID = individual identity; nest = colony number; caste: R = queen, W = workers, G = gynes (young queens); the number refers to the microsatellite locus number. For each microsatellite, the two alleles are given except when they are missing data. The file is formatedd for the software Relatedness. The individuals and loci surlined in blue refered to the individuals removed from the analysis in the paper (see the first paragraph of the result section)

Evolution des stratégies de dispersion et de reproduction chez la fourmi Cataglyphis cursor

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Manipulation of worker size diversity does not affect colony fitness under natural conditions in the ant Temnothorax nylanderi

International audienceIn social insects, within-colony worker diversity has long been thought to improve colony fitness. Recent studies start to question this assertion in species with moderate diversity. However, demonstrating an absence of effect is difficult as this absence could be due to the measurement of only few life history traits or to artificial conditions. In order to circumvent these limitations, we experimentally decreased worker size diversity within colonies of the ant Temnothorax nylanderi, with or without affecting mean worker size and we reintroduced them in the field for four months (spring and early summer). We then thoroughly measured their fitness based on survival, growth and reproductive success. Our results show that our manipulation did not affect colony fitness. In addition, colonies did not restore diversity to its initial level, further suggesting that worker size diversity is not a key parameter to them. We found the classically observed positive relationship between colony size, colony growth and reproductive success. Overall, our results confirm that worker size diversity within colony is not necessarily adaptive in species where it is moderate. We discuss the alternative mechanisms that could explain the evolutionary persistence of moderate worker size diversity. Significance statement Organisms that live in groups can greatly benefit from the emergence of novel group-level traits. For instance, social insects show significant variability in worker size within colonies. This size diversity increases the division of labour among workers and improves colony fitness. However, in species where size diversity is moderate, this relationship may not always be verified. Here, we manipulated both worker size diversity and mean worker size within colonies of the ant Temnothorax nylanderi, we reintroduced them in the field, and we extensively measured colony fitness after four months. We found no impact on colony survival, growth and reproduction. We discuss how social life and its associated cooperation and conflicts could cause variation in worker size, without any positive effect on colony fitness

Queen replacement in the monogynous ant Aphaenogaster senilis: supernumerary queens as life insurance

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