Ant-fungus Mutualism - General Overview

General Overview

Given the exclusive New World distribution of the >230 attine ant species, this mutualism is thought to have originated in the basin of the Amazon rainforest some 50–65 million years ago. There are five main types of agriculture that the attine ants practice include: lower, coral-fungus, yeast, generalized higher, and leafcutter agricultural systems. Lower agriculture was the first type of attine agricultural system and is currently practiced by 80 atttini species in 10 genera. Coral-fungus agriculture is practiced by 34 species by a single derived clade within the attine genus Apterostigma. The coral fungus farmers underwent a switch of cultivars between 10 and 20 million years ago to a nonleucocoprineacoeous fungus, which makes its choice of cultivar different from all other attines. Yeast agriculture is practiced by 18 species of Cyphomyrmex rimosus. The C. rimosus group is hypothesized to have evolved growing fungus in a yeast form between 5 and 25 million years ago. Generalized higher agriculture is practiced by 63 species in two genera and refers to the condition of highly domesticated fungus. The fungi used in higher agriculture cannot survive without its agriculturalists to tend it and has phenotypic changes that allow for increased ease of ant harvesting. Leafcutter agriculture, which is a more highly derived form of higher agriculture, is practiced by 40 species in two genera and has the most recent evolution, originating between 8 and 12 million years ago. Leaf cutters use living biomass as the substrate to feed their fungi, whereas in all other types of agriculture, the fungus requires dead biomass.

In all of these types of agriculture,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. While this vertical transmission of fungal cultivars and strong host-symbiont specificity might suggest a tight coevolutionary relationship, recent phylogenetic analyses suggest this is not the case. Multiple domestications of the same fungus, fungal escape from domestication, or cultivar switching could lead to the observed diffuse coevolutionary pattern.

This mutualism is further complicated by the introduction of two other organisms, a fungal parasite Escovopsis and Pseudonocardia bacterial species residing on the ants' integuments that assist in defending the fungus from this parasite through the production of secondary metabolites. In fact, some species of ants have evolved exocrine glands that apparently nourish the antibiotic-producing bacteria inside them. A black yeast has been recorded as a partner in the mutualism. The yeast has a negative effect on the bacteria that normally produce antibiotics to kill the parasitic fungus and so may affect the ants' health by allowing the parasite to spread.

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.