Natural and Experimental Coevolution between the Invasive Garden Ant and its Parasites CLUSTER: "Experimental evolution and natural variation of Bacillus-invertebrate interactions"
Final Report Abstract
Aim of the project was to understand the evolutionary dynamics between invasive ants and their pathogens. Species introductions lead to novel combinations of host and parasite species and set the starting point of a co-evolutionary arms race. We analysed the ability of the entomopathogenic fungus Metarhizium (in a mix of six strains, belonging to two species) to adapt to the Argentine ant, Linepithema humile under laboratory conditions. We performed ten passages of an experimental evolution, in which the ancestral fungal mix could adapt to its ant host (in the absence of host evolution) under two distinct evolution regimes: 1) fungus-exposed ants were reared in isolation, leading to pathogen adaptation to the individual immune system of the ants; 2) the exposed ants were reared in a group with healthy nestmates that performed collective hygienic care of the diseased individual ("social immunity"). Our evolution experiment therfore allows to disentangle the effects of individual and social immunity of ant societies on the evolution of their parasites. We found that – as predicted – killing rate of the fungal pathogen increased over the course of the evolution experiment, yet this was less so in the social evolution regime, indicating an effective interference of social evolution with fungal adaptation. We also found that the strain diversity of the ancestral mix was quickly reduced to a single dominant strain in most replicates (with different strains being the winner in differnent replicates, but 2-3 strains performing much better than the others). This reduction of strain variability occurred faster in the solitary treatment, whereas group living led to a longer persistence of multiple strains in the fungal mix. The 'winning strain' in the evolution experiment could be predicted quite precisely by a 'hierarchy of competitive ability' determined for the six ancestral fungal strains in a pairwise competition experiment. One strain was doing particularly well, being the winner in approximately 70% of the replicates. This may be because the strain copes well with both individual and social immune defences of the host, whereas other strains do better or worse under solitary versus group rearing of the host, indicating a trade-off between breaking individual versus social defences in most fungal strains. That is, some strains may be highly efficient penetrating the host cuticle to infect the host's body, but may be easier detected or removed by the nestmates during allogrooming, or vice versa. Moreover, we observed a rare sanitary behaviour by the ants, in which ants dying from the fungus are bitten into pieces. We found that such corpse biting efficiently reduces fungal outgrowth and production of the infectious transmission stages of the fungus (conidiospores). To our knowledge, this is the first study that allows to disentangle the effects of individual versus social immune defences of social insects on the evolution of their multiple co-existing pathogens.
Publications
- (2012) Effects of social immunity and unicoloniality on host-parasite interactions in invasive insect societies. Functional Ecology 26: 1300-1312
Ugelvig, L.V. & Cremer, S.
- (2012) Social transfer of pathogenic fungus promotes active immunisation in ant colonies. PLoS Biology 10(4): e1001300
Konrad, M., Vyleta, M.L., Theis, F.J., Stock, M., Tragust, S., Klatt, M., Drescher,V., Marr, C., Ugelvig, L.V & Cremer, S.
(See online at https://doi.org/10.1371/journal.pbio.1001300) - (2013) Ants disinfect fungus-exposed brood by oral uptake and spread of their poison. Current Biology 23(1): 76-82
Tragust, S., Mitteregger, B., Barone, V., Konrad, M., Ugelvig, L.V. & Cremer, S.
(See online at https://doi.org/10.1016/j.cub.2012.11.034)
