Ant-fungus mutualism

Ant-fungus mutualism is a symbiosis seen in certain ant and fungal species, where ants actively cultivate fungus much like humans farm crops as a food source. Interestingly, in some species, the ants and fungi are dependent on each other for survival. The leafcutter ant is a well known example of this symbiosis. A mutualism with fungi is also noted in some species of termites in Africa.

General overview
This mutualism is thought to have originated in the basin of the Amazon rainforest some 50-65 million years ago. The attine ants actively propagate, nurture and defend the basidiomycete cultivar. In return, the fungus provides nutrients for the ants, which may accumulate in specialized hyphal-tips known as gongylidia. In some advanced genera the queen ant may take a pellet of the fungus with her when she leaves to start a new colony. This mutualism is further complicated by the introduction of two other factors, a fungal parasite Escovopsis and a bacterial species residing on the ants' integuments that assists in defending the fungus from this parasite through the production of secondary metabolites.

Whereas the ants are monophyletic, their symbionts are not. They fall roughly into three major groups, only G1 having evolved gongylidia. Some G2 species grow long hyphae that form a protective cover over the nest. Those in G3 are paraphyletic, the most heteregenous, and form the most loose relationships with their cultivators. Studies now show that the fungi themselves may not be completely dependent on the ants. The fungi were earlier thought to be propagated by ants purely through clonal (vegetative) means. However considerable genetic variation in the fungi suggests that this may not be the case.