In questa tesi, varie specie di Imenotteri sociali sono usate come modello per lo studio degli adattamenti attuati nel fronteggiare una delle principali problematiche conseguenti alla vita di gruppo, l’aumento del rischio di infezione. L’alta densità di individui tra loro imparentati e l’alto tasso di interazione tra essi nello spazio ristretto del nido, facilitano la diffusione dei parassiti tra i membri della colonia. D’altra parte, il successo evolutivo degli insetti sociali indica l’evoluzione di risposte efficaci alla pressione biotica imposta dai parassiti. Tali risposte risultano dalla combinazione di adattamenti individuali e collettivi basati su meccanismi fisiologici, comportamentali e organizzativi che, tramite dinamiche collettive, generano strategie coloniali integrate chiamate complessivamente immunità sociale. Questa tesi esamina vari aspetti dell’immunità sociale con un approccio sperimentale. Infatti, nonostante essa abbia un ruolo fondamentale nella sopravvivenza coloniale, le evidenze empiriche della sua espressione e dell’interazione con lo stato fisiologico dell’individuo sono ancora frammentarie. La tendenza a focalizzarsi su aspetti singoli e su specifiche componenti coloniali complica la comprensione del fenomeno nonché una visione definita dei suoi costi e benefici. La formica Crematogaster scutellaris è stata qui largamente studiata per descrivere l’espressione dei fenomeni di immunizzazione nelle regine e della sua trasmissione alla prole. Tramite un primo studio metodologico sono state quantificate le conseguenze a lungo termine di stress microbiologici e manipolativi sulla sopravvivenza delle regine e sullo sviluppo coloniale. In un secondo studio l’esistenza di immunizzazione trans-generazionale contro il fungo patogeno Metarhizium anisopliae è stata dimostrata per la prima volta nelle formiche in questo organismo modello. In un terzo studio, si è osservato che le regine di C. scutellaris non si immunizzano contro il medesimo patogeno e che, invece, una prima esposizione ne aumenta la mortalità a seguito della seconda. Questi risultati indicano che l’immunizzazione e l’immunizzazione trans-generazionale sono fenomeni disaccoppiati in questo sistema parassita-ospite in quanto le fondatrici esposte al patogeno trasmettono una maggiore resistenza alla propria prole senza però beneficiare esse stesse di una migliorata risposta immunitaria. È stata poi studiata l’integrazione tra componenti fisiologiche e comportamentali dell’immunità sociale nella formica Temnothorax unifasciatus, che abitualmente nidifica in piccole e promiscue cavità. In assenza di contatto diretto le colonie inibiscono la crescita micotica all’interno dei nidi, presumibilmente tramite composti volatili, e le dimensioni coloniali sono risultate positivamente correlate con l’attività fungistatica. In un secondo esperimento, le colonie piccole a cui venivano presentate fibre contaminate hanno mostrato una maggiore rimozione di fibre rispetto a colonie piccole a cui venivano presentate solo fibre non contaminate. Colonie di grandi dimensioni rimuovevano invece le fibre a prescindere dalla presenza di contaminazione. Ciò dimostra che le dimensioni coloniali influenzano qualitativamente il comportamento di rimozione. Inotre il comportamento igienico di allogrooming è stato studiato in Apis mellifera, tramite un approccio multidisciplinare abbiamo caratterizzato il comportamento e gli individui che lo esprimono, focalizzandoci sul bilancio tra specializzazione comportamentale e plasticità fenotipica. La possibilità di studiare un’associazione di nidificazione recentemente scoperta tra l’ape senza pungiglione Partamona testacea e la formica pallottola Paraponera clavata ha generato uno studio sulle interazioni che avvengono tra le colonie associate di queste due specie.

Eusocial insects represent the highest level of social organisation in Arthropods and a highly valuable model to study group living dynamics. In this thesis, different species of social Hymenopteran are used as a model to study the adaptations to cope with one of the main issues of the group living lifestyle, the potential increase of disease risk. The high density of related and interacting individuals coexisting in the reduced area occupied by a social insect nest facilitates the spread of parasites among colony members. Anyway, the evolutionary success of social insects indicates that they evolved effective responses to the biotic pressure imposed by parasites. These responses result from the combination of individual and collective immune adaptations, and rely on physiological, behavioural and organizational mechanisms composing, through collective dynamics, the integrated colonial strategies referred as social immunity. This thesis examines some aspects of social immunity by facing questions experimentally. Social immunity has a fundamental role in colony survival but empirical evidence about its expression and its interactions with the physiological status of individuals is still scattered. The tendency to focus on single aspects of social immunity and on specific colony components complicates the understanding of the phenomenon and a balanced overview of its costs and benefits. An ant species, Crematogaster scutellaris, has been investigated with effort to describe the existence of immunization in queens and of trans-generational immunization in their worker offspring. A methodological study quantified the long-term consequences of microbiological and manipulative stress on queen survival and on colonial development. In a second study the occurrence of trans-generational immunization against the entomopathogenic fungus Metarhizium anisopliae has been demonstrated for the first time in ants using this model organism. In a third study we found that C. scutellaris queens do not express immunisation against the same pathogen; conversely, a first exposure increases mortality during the challenge. These results indicate that immunization and trans-generational immunization are uncoupled phenomena in this host-parasite system since foundresses exposed to M. anisopliae can elicit an increased resistance in the offspring without providing themselves with an increased immune response. A study on the integration between physiological and behavioural components of social immunity have been carried out on in the ant Temnothorax unifasciatus, showing the habit of nesting in small and promiscuous cavities. In the absence of direct contact, T. unifasciatus colonies inhibited fungal growth inside their nests, presumably through volatile compounds and colony size was positively correlated with this fungistatic activity. In a second experiment, we found that small colonies presented with contaminated fibres showed an increased removal of both contaminated and non-contaminated items compared to small colonies presented with control fibres only. Conversely, larger colonies removed items regardless of the presence of the spores inside the nest. This demonstrated that colony size qualitatively affects removal behaviour. A behavioural component, the hygienic behaviour of social grooming, has been also studied in Apis mellifera. Using a highly multidisciplinary approach we characterized this behaviour and individuals expressing it, elucidating the relevance of a trade-off between behavioural specialization and phenotypical plasticity. The possibility to study a newly discovered nesting association between the stingless bee Paraponera clavata and the bullet ant Partamona testacea led to the production of a study about the peculiar interactions occurring between associated colonies of these two species.

Social immunity in social insects. New empirical evidence

BORDONI, Adele
2019-03-13T00:00:00+01:00

Abstract

Eusocial insects represent the highest level of social organisation in Arthropods and a highly valuable model to study group living dynamics. In this thesis, different species of social Hymenopteran are used as a model to study the adaptations to cope with one of the main issues of the group living lifestyle, the potential increase of disease risk. The high density of related and interacting individuals coexisting in the reduced area occupied by a social insect nest facilitates the spread of parasites among colony members. Anyway, the evolutionary success of social insects indicates that they evolved effective responses to the biotic pressure imposed by parasites. These responses result from the combination of individual and collective immune adaptations, and rely on physiological, behavioural and organizational mechanisms composing, through collective dynamics, the integrated colonial strategies referred as social immunity. This thesis examines some aspects of social immunity by facing questions experimentally. Social immunity has a fundamental role in colony survival but empirical evidence about its expression and its interactions with the physiological status of individuals is still scattered. The tendency to focus on single aspects of social immunity and on specific colony components complicates the understanding of the phenomenon and a balanced overview of its costs and benefits. An ant species, Crematogaster scutellaris, has been investigated with effort to describe the existence of immunization in queens and of trans-generational immunization in their worker offspring. A methodological study quantified the long-term consequences of microbiological and manipulative stress on queen survival and on colonial development. In a second study the occurrence of trans-generational immunization against the entomopathogenic fungus Metarhizium anisopliae has been demonstrated for the first time in ants using this model organism. In a third study we found that C. scutellaris queens do not express immunisation against the same pathogen; conversely, a first exposure increases mortality during the challenge. These results indicate that immunization and trans-generational immunization are uncoupled phenomena in this host-parasite system since foundresses exposed to M. anisopliae can elicit an increased resistance in the offspring without providing themselves with an increased immune response. A study on the integration between physiological and behavioural components of social immunity have been carried out on in the ant Temnothorax unifasciatus, showing the habit of nesting in small and promiscuous cavities. In the absence of direct contact, T. unifasciatus colonies inhibited fungal growth inside their nests, presumably through volatile compounds and colony size was positively correlated with this fungistatic activity. In a second experiment, we found that small colonies presented with contaminated fibres showed an increased removal of both contaminated and non-contaminated items compared to small colonies presented with control fibres only. Conversely, larger colonies removed items regardless of the presence of the spores inside the nest. This demonstrated that colony size qualitatively affects removal behaviour. A behavioural component, the hygienic behaviour of social grooming, has been also studied in Apis mellifera. Using a highly multidisciplinary approach we characterized this behaviour and individuals expressing it, elucidating the relevance of a trade-off between behavioural specialization and phenotypical plasticity. The possibility to study a newly discovered nesting association between the stingless bee Paraponera clavata and the bullet ant Partamona testacea led to the production of a study about the peculiar interactions occurring between associated colonies of these two species.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11392/2488323
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