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SYMBionts in Insect Societies: reproductive manipulation and the fitness effects of Wolbachia in leaf-cutting ants

Final Report Summary - SYMBIS (SYMBionts in Insect Societies: reproductive manipulation and the fitness effects of Wolbachia in leaf-cutting ants)

Project context and objectives
%Parasites have significant effects on host biology. Most research has focused on infections by single, virulent parasites that cause obvious disease. However, infections frequently involve more than one parasite, many of which may appear relatively avirulent and thus hard to detect. Although their effects may be more subtle, these cryptic parasites can nevertheless have important effects on host fitness. This project investigated the bacterium Wolbachia in leaf-cutting ants. Wolbachia is a cryptic parasite in a wide variety of insects, but very little is known about its fitness effects or infection dynamics in ants, in spite of their ecological importance. If Wolbachia has negative fitness effects on its leaf-cutting ant host, as is seen in other insects, then genetic variability in resistance would be predicted to evolve. Leaf-cutting ants are a particularly useful group for investigating genetic variation in resistance because queens mate with multiple males (polyandry). The workers in a colony thus consist of multiple patrilines, each the offspring of a different father and differences between patrilines have to be due to genetic effects because patrilines share maternal and environmental cues, and differ only in their paternal genotype. Genetic variation in the resistance of leaf-cutting ants to a virulent parasite has been found previously and host phenotypes (castes) have also been found to differ in resistance.
%Project results
% In this project we found no evidence of genetic variation in resistance to Wolbachia, nor differences in infection between the small and large worker castes. This suggests that Wolbachia has little fitness effect on workers. Work during the project was expanded to look also at the effect of genotype on behaviour and demonstrated significant genotypic variation. Genotype therefore influence behaviour but does not appear to influence resistance to Wolbachia.

We also discovered remarkably high infection prevalence in workers of our study species. Previous studies found approximately 50 % infection rates of workers, but in this project we used a more rigorous approach and found infection rates to be at least 80 % and to reach 100 % of workers in some colonies. This further suggests that Wolbachia infection does not negatively impact host worker fitness. This is surprising given that the sterile workers represent a reproductive dead-end for vertical transmission of Wolbachia and raises the possibility that horizontal transmission may be important.
In accord with this, we incorporated the project's results in a phylogenetic analysis and confirmed that horizontal transmission of Wolbachia is commonplace, with the same strain of Wolbachia being found in a diversity of leaf-cutting ants.

The results of the project therefore suggest that horizontal transmission of Wolbachia through workers may be much more important to the biology of the symbiont than has previously been thought, and raise the possibility that this may be true for social insects in general.