Abstracts (first author)
Adaptation of a fungal pathogen to individual versus social immunity in ants
Social insects fight diseases not only by the hygiene behaviour and physiological immune system of the individual group members, but also by their collectively performed, social defences ranging from sanitary behaviours, use of antimicrobials and organisational adaptations. Parasites infecting social insect colonies thus have to overcome both individual and social immune responses, which likely exert very different selection pressures on the parasite. To disentangle these effects and to understand the complexity of host-parasite-coevolution in social insect hosts, we performed a selection experiment on parasite adaptation towards solitary reared ants and ants in a social group, i.e. individual immune defence only or both, individual and social defences. We used the Argentine ant, Linepithema humile, and its fungal pathogen Metarhizium as our study system. Metarhizium is a soil-born entomopathogen and obligate killer of a broad range of insect hosts. It was recently found that the previously named M. anisopliae in fact is a species complex comprising a diversity of different Metarhizium species, which differ in ecological dominance and virulence against insects. To reflect this natural diversity, we used a mix of both M. roberstii and M. brunneum to perform ten passages through both solitary and grouped ants (10 replicates each). We found that fungal species composition and virulence changed over the ten generations, revealing a difference in the adaptation potential of the two sympatric fungal species, as well as an effect of the evolution regime. We can thus conclude that the ecological diversity of the different generalist Metarhizium species may reflect adaptations to a diversity of host species, and that the additional group level defences in insect societies adds new selection pressures for their coevolving pathogens with measurable effects on disease dynamics.