ANT–FUNGUS MUTUALISM is a symbiosis seen in certain ant and fungal
species, in which ants actively cultivate fungus much like humans farm
crops as a food source. 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
* 1 Overview * 2 Types * 3 The attines * 4 Secondary symbiotic relationships * 5 Partner fidelity * 6 Evolution * 7 References * 8 External links
Fungus-growing 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.
There are five main types of agriculture that fungus-growing ants
practice: lower, coral fungi , yeast , generalized higher, and
leafcutter agricultural systems. Lower agriculture is the most
primitive system and is currently practiced by 80 species in 10
genera. Coral-fungus agriculture is practiced by 34 species by a
single derived clade within the genus
Apterostigma . The coral fungus
farmers underwent a switch of cultivars between 10 and 20 million
years ago to a non-leucocoprineacoeous fungus, which makes its choice
of cultivar different from all other attines.
The ants of the
SECONDARY SYMBIOTIC RELATIONSHIPS
There are additional symbiotic relationships that affect fungal agriculture. The fungus Escovopsis is a parasite in ant colonies, and the bacterium Pseudonocardia has a mutualistic relationship with ants. Pseudonocardia resides on the ants' integuments and assists in defending the ants from Escovopsis 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 interferes with this 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.
Partner fidelity can be witnessed through vertical gene transmission
of fungi when a new colony is begun. First, the queen must mate with
several males to inseminate her many eggs before she flies off to a
different location to begin a new colony. As she leaves, she takes
with her a cluster of mycelium (the vegetative portion of the fungus)
and actually begins a new fungal garden at her resting point using
this mycelium. This grows to become the new fungal farm complete with
the genes of the original cultivar preserved for another generation of
ants. The relationship between attine ants and the
Studies done (with the concept of the prisoner\'s dilemma in mind) to test what further drives partner fidelity among species have shown that external factors are an even greater driving force. The effects of cheating ants (ants who did not bring plant biomass for fungal food) had a much smaller effect on the fitness of the relationship than when the fungi cheated by not providing gongylidia. Both effects were exacerbated in the presence of infection by escovopsis, resulting in close to a 50% loss in fungal biomass. It is clear that the risk of infection from parasites is a driving external factor in keeping these two species loyal to one and other. Though external factors play a large role in maintaining fidelity between the mutualists, genetic evidence of vertical transmission of partner fidelity has been found among asexual, fungus cultivating ant species. Factors such as vertical transmission do not play as strong a role as environmental factors in maintaining fidelity, as cultivar switching among ant species is not a highly uncommon practice.
Given the exclusive
Though 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. It is hypothesized that fungi have evolved to make themselves more attractive to ant species through the development of enzymes that allow the ants to access nutrition in the fungal mass.
There is debate in the field on the "tightness" of the coevolution between ants and their fungal cultivars. While the observed 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. The alternative perspective of a "tight" coevolution points to evidence of instability in horizontal transmission events, while also postulating that the observed differences between the phylogenies of attine ants and their fungal cultivars correspond to speciation events.
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* Fungus-growing ants, Social Insect