Ant–fungus mutualism
The fungus’ ability to convert the plant material into a food source accessible to their host makes them the ideal partner.
A similar mutualism with fungi is also noted in termites within the subfamily Macrotermitinae which are widely distributed throughout the Old World tropics with the highest diversity in Africa.
[7] There are three castes of female worker ants in Attini colonies which all participate in foraging plant matter to feed the fungal cultivar.
The lowest caste, minor, is smallest in size but largest in number and is primarily responsible for maintaining the fungal cultivar[8] for the rest of the colony.
The symbiosis between basidiomycete fungi and attine ants involves the fungal pathogen, Escovopsis, and an actinomycetes bacterial symbiont, Pseudonocardia.
Lower agriculture commonly involves smaller nests and they use techniques besides cutting leaves to obtain plant material.
[9] 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.
[10] Within this system of higher agriculture, leafcutter ants craft the most optimal environment for their fungus by excluding them from the competition.
Lower attines have less specialized cultivars that more closely resemble Leucocoprineae found in the wild and use "ancestral substrates" composed of plant, wood, arthropod, and flower detritus.
[16] The cultivars of higher attines often have growths called gongylidia -—nutrient-rich structures that have evolved for easy harvesting, ingesting, and feeding to larvae, while simultaneously serving as propagules for the fungi.
It is clear that evolutionary pressure has been exerted on these ants to develop an organized system in which they feed the fungus and continue its reproduction.
[22][23] Targeted microbial isolations revealed that Pseudonocardia bacteria are located in specialized structures along the exoskeleton of the ant.
[23] In fact, some species of ants have evolved exocrine glands that apparently nourish the antibiotic-producing bacteria inside them.
[25] It has been suggested that the black yeast impedes the growth of Pseudonocardia by using up nutrients produced by the bacteria which indirectly helps the survival and spread of the Escovopsis infection.
[8] To optimize the growth of their fungal cultivar, attine ants open and close tunnels to maintain the ideal temperature and humidity in their below-ground structures for fungus farming.
Delayed rejection has been observed in leaf-cutting ants in response to plant substrates which were detrimental to the fungal colony.
[27] Given the exclusive New World distribution of the over 200 fungus-growing ant species,[1] this mutualism is thought to have originated in the basin of the Amazon rainforest some 50–66 million years ago.
During the fallout of the K-Pg event, the ancestor of the attine ants speciated as the resources it depended on as a generalized hunter-gatherer grew scarce.
At the same time, the sister group of the attine ants Dacetina developed predatory behavior during the same drive for new resources.
This created an immediate dependency on their cultivars for providing the needed amino acid and is supported by the lack of reversal to hunter-gatherer lifestyles.
This nest building behaviour is more similar to that of lower attine ants which engage in cultivation of Lepiotaceous fungi belonging to the G3 group.
A third species, Myrmecopterula moniliformis was placed in this genus in 2020 and has been observed to produce both fertile and infertile fruiting bodies meaning it is capable of surviving without the ants.
[36] Studies now show that fungi belonging to lower attine ants are not obligate mutualists and are capable of free-living.
[13] At the same time they appear to have committed fully to propagation by the vertical transmission practiced by attine ants and at the end of their allopatry were no longer able to sexually reproduce with their free-living cousins or lower-attine counterparts.
