SOCIAL IMMUNITY IN ANTS: ANTIPATHOGEN DEFENSES IN INCIPIENT COLONIES AND USE OF ANTIMICROBIAL RESIN

Social insects, and particularly ants, are extraordinarily ecologically successful organisms. Social life confers many advantages, such as co-operation for brood- rearing or foraging. However, social insects may be particularly susceptible to disease transmission, because their colonies consist of often closely related individuals living in densely populated nests. To combat pathogens, social insects have access to individual immune defenses, and have also evolved collective defenses known as "social immunity". In the first part of this thesis, I studied how ants defend themselves against pathogens during particularly sensitive life stages: colony founding and brood rearing. I first tested the ability of queens to detect and avoid pathogenic fungi when establishing incipient colonies. Unexpectedly, Formica selysi queens were attracted rather than repelled by pathogenic fungi. I then tested the hypothesis that under pathogen threat, queens associate during colony founding in order to benefit from social immunity. In F. selysi, the presence of pathogens in a nesting site did not induce queen associations. Moreover, in incipient colonies of Lasius niger, the queens and their workers did not benefit from social immunity, and rather invested in individual immunity. Finally, in F. selysi, the ability of young workers to resist a fungal infection did not depend on their mother queen, nor on the number of workers that reared them. In the second part of this thesis, I studied the use of antibiotic resin by the wood ant Formica paralugubris. I found that wood ants collect more resin when brood is present in their nest, and that they place resin near the brood. I also discovered that ants produce a potent antibiotic by depositing self-produced formic acid on the resin. This thesis illustrates some unexpected aspects of the host-pathogen relationship during colony founding and provides important insights into how ants use antibiotic substances such as resin. = Les insectes sociaux, et particulierement les fourmis, sont des organismes au succes ecologique extraordinaire. La vie sociale confere de nombreux avantages, comme la cooperation pour l'elevage du couvain ou la recherche de nourriture. En revanche, les insectes sociaux peuvent etre particulierement susceptibles a la transmission de maladies, car leurs colonies se composent d’individus souvent tres apparentes vivant dans des nids densement peuples. Pour lutter contre les pathogenes, les insectes sociaux possedent des defenses immunitaires individuelles, et ont egalement evolue des defenses collectives ou « immunite sociale ». Dans la premiere partie de cette these, j'ai etudie comment les fourmis se defendent contre les pathogenes pendant des stades de vie particulierement sensibles chez les fourmis: lors de la fondation d’une colonie et pendant l'elevage de couvain. J'ai d'abord teste la capacite des reines a detecter et eviter des champignons pathogenes lors de l’etablissement de nouvelles colonies. De facon inattendue, les reines de Formica selysi etaient attirees plutot que repoussees par des champignons pathogenes. J’ai ensuite teste l’hypothese selon laquelle en presence de pathogenes, les reines s’associent pour fonder une colonie afin de beneficier d’une immunite sociale. Chez F. selysi, la presence de pathogenes dans un site de nidification n’incitait pas les reines a s’associer. De plus, dans les nouvelles colonies de Lasius niger, les reines et leurs ouvrieres ne beneficiaient pas de l'immunite sociale, et investissaient plutot dans l'immunite individuelle. Finalement, chez F. selysi, la capacite de jeunes ouvrieres a resister a un champignon pathogene ne dependait ni de leur mere, ni du nombre d’ouvrieres les ayant elevees. Dans la deuxieme partie de cette these, j'ai etudie l'utilisation de resine antibiotique par les fourmis des bois Formica paralugubris. J'ai constate que les fourmis des bois recueillaient davantage de resine quand le couvain etait present dans leur nid, et qu'elles placaient la resine pres du couvain. J'ai egalement decouvert que les fourmis produisaient un puissant antibiotique en deposant de l'acide formique sur la resine. Cette these illustre certains aspects inattendus de la relation hote-pathogene lors de la fondation de colonies, et apporte des informations importantes sur la facon dont les fourmis utilisent des substances antibiotiques telles que la resine.