The well-studied, ancient and highly evolved mutualism between fungus-growi
ng ants and their fungi has become a model system in the study of symbiosis
(1-5). Although it is thought at present to involve only two symbionts, ass
ociated with each other in near isolation from other organisms(1-5), the fu
ngal gardens of attine ants are in fact host to a specialized and virulent
parasitic fungus of the genus Escovopsis (Ascomycotina)(6). Because the ant
s and their fungi are mutually dependent, the maintenance of stable fungal
monocultures in the presence of weeds or parasites is critical to the survi
val of both organisms. Here we describe a new, third mutualist in this symb
iosis, a filamentous bacterium (actinomycete) of the genus Streptomyces tha
t produces antibiotics specifically targeted to suppress the growth of the
specialized garden-parasite Escovopsis. This third mutualist is associated
with all species of fungus-growing ants studied, is carried upon regions of
the ants' cuticle that are genus specific, is transmitted vertically (from
parent to offspring colonies), and has the capacity to promote the growth
of the fungal mutualist, indicating that the association of Streptomyces wi
th attine ants is both highly evolved and of ancient origin.