Background: The obligate mutualism between fungus-growing ants and microbial symbionts offers excellent opportunities to study the specificity and stability of multi-species interactions. In addition to cultivating fungus gardens, these ants have domesticated actinomycete bacteria to defend gardens against the fungal parasite Escovopsis and possibly other pathogens. Panamanian Acromyrmex echinatior leaf-cutting ants primarily associate with actinomycetes of the genus Pseudonocardia. Colonies are inoculated with one of two vertically transmitted phylotypes (Ps1 or Ps2), and maintain the same phylotype over their lifetime. We performed a cross-fostering experiment to test whether co-adaptations between ants and bacterial phylotypes have evolved, and how this affects bacterial growth and ant prophylactic behavior after infection with Escovopsis. Results: We show that Pseudonocardia readily colonized ants irrespective of their colony of origin, but that the Ps2 phylotype, which was previously shown to be better able to maintain its monocultural integrity after workers became foragers than Ps1, reached a higher final cover when grown on its native host than on alternative hosts. The frequencies of major grooming and weeding behaviors co-varied with symbiont/host combinations, showing that ant behavior also was affected when cuticular actinomycete phylotypes were swapped. Conclusion: These results show that the interactions between leaf-cutting ants and Pseudonocardia bear signatures of mutual co-adaptation within a single ant population.

Bacterial cover dataBacterial cover of A. echinatior ants scored from 0-12. Each ant is identified by an ID and the host and donor colony noted. Each treatment was replicated and each sub colony harboured 4 ants. Bacterial cover was scored on day 1, 4, 8, 12 and 16 after eclosion, at the time of experimental infection with Escovopsis and 2 weeks post infection. From these scores the logistic growth rate was estimated and subsequently this rate was Box-Cox transformed. Some subcolonies were excluded from the analysis due to garden loss.Behaviour dataBehaviours performed by cross-fostered A. echinatior ants following experimental Escovopsis infection. For each sub-colony the frequency of behaviours performed by all individuals is noted, together with the proportion of time spent on each and the first four principal components following PCA. Some sub-colonies were excluded from the analysis due to garden loss.