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.