Ant-Fungal Symbiosis: The World's First True Agriculturists






Introduction
While human agriculture may have started within the last few thousand years, ants are truly the planet’s pioneers of agriculture, becoming the world’s first farmers over 50 million years ago.[1]Attine ants have formed a symbiotic obligate mutualism with fungi that has become more complex throughout evolutionary time. Attine ants, whose more derived genera are also known as “leaf-cutter ants”, cannot digest leaves, but instead cut them off of trees and feed them to their symbiotic fungus. The fungus then digests the plant matter, creating nutrient-rich threads that the ants then eat.[2] Lower, more primitive fungus-gardening ants forage for decaying plant and insect matter instead of fresh leaves.
While the ant-fungal smbiosis has long been considered a model one to one symbiotic relationship, recent findings have unveiled up to three more members of this symbiotic network, keeping each other in check through antagonistic interactions. The fungal parasite Escovopsis attacks the ants’ symbiotic fungus, and can be detrimental to the colony’s food source and survival. To counteract this, the ants have actionmycete bacteria living on their outer surface that produce antibiotics to kill Escovopsis parasites. The symbiosis was thought to contain these four members until a fifth was discovered: a black yeast that feeds on the actinomycete bacteria. All parties involved have stayed “in check” for the last 50 million years, and it is thought that their interaction prevent cheating from occurring.[3]

To put the strength of a leaf cutter ant in perspective, an ant lifting that leaf is analagous to a human carrying an elephant on its shoulders.  Source: http://topnews.in/usa/files/Leaf-cutter-ants.jpg
To put the strength of a leaf cutter ant in perspective, an ant lifting that leaf is analagous to a human carrying an elephant on its shoulders. Source: http://topnews.in/usa/files/Leaf-cutter-ants.jpg

This graphic outlines the relationship between attine ants, their fungal cultivars, and the escovopsis parasite.  The Coevolution tree (part 4) is based on the categories developed by Chapela et. al. Source: http://www.biosci.utexas.edu/IB/faculty/mueller/positi1.jpg
This graphic outlines the relationship between attine ants, their fungal cultivars, and the escovopsis parasite. The Coevolution tree (part 4) is based on the categories developed by Chapela et. al. Source: http://www.biosci.utexas.edu/IB/faculty/mueller/positi1.jpg

Ant-fungal specificity
Attine ants make up a monophyletic group consisting of 13 genera, ranging from the primitive Apterostigma to the more derived Atta and Acromymrmex, those ants traditionally thought of as “leaf cutter” ants. Ants, Escovopsis, and the fungal symbiont have been growing together for over 50 million years, as seen in the way their phylogenetic trees parallel each other. Typically, the relationship between ant, fungus, and fungal parasite has been considered to be one of specificity. Currie categorized 4 sets of ant, fungal, and parasite symbioses ranging from the most primitive to the most derived: the Apterostigma symbiosis, the Lower Attine symbiosis, the Trachymyrmex symbiosis, and the leaf-cutter symbiosis.[4]The symbiosis between ant, fungus, and parasite began with the most primitive group, and over evolutionary time became more and more complex and derived, with each group having specifically adapted ant, fungus, and parasite combinations. While this study by Currie did recognize the existence of a fourth symbiont, the actinomycete bacteria, its phylogeny was not considered.
A different study by Chapela sought to show that rather than there being one initial fungus cultivated by primitive ants in the original symbiosis that became more and more derived as ant queens faithfully took a fungal pellet from their nest and used it to seed the fungus for a new colony for millions of years, the fungal symbionts are actually polyphylectic, having been acquired multiple times throughout evolutionary history. Chapela divides the fungal symbiont into three groups: G1, G2, and G3. G1 fungi release nutrient-rich hyphae swells to feed their highly derived Atta and Acromyrmex cultivators. G2 fungi are cultivated by less derived Apterostigma ants, while G3 fungi, the most diverse group, are cultivated by the most primitive ants. The high genetic variability of the polyphyletic G3 group, according to Chapela, shows that initially primitive ants took up multiple free-living fungal symbionts. As the ant-fungal relationship progressed through evolutionary time, more successful ant-fungus pairs began to persist, and gave rise to the monophyletic G2 and G1 groups of higher, more derived attines.[5]
In contrast to these theories, studies by Mikheyev propose that all the ant genera cultivate the same fungus. Since the fungi of derived ants had no free-living counterparts and where thought to reproduce clonally (asexually), they were assumed to be unique and specific to those ant genera and carried on through vertical transmission. By looking at the conservation of DMC1 and RAD51, two proteins involved in meiosis, it was shown that, “the strong conservation of DMC1 is not consistent with the long-term arrest of the meiotic chromosome metabolism in leaf-cutter ants.”[6] This study ultimately showed that all the ants were cultivating the same species of fungus, Leucaogaricus gongylophorus. Mikheyev concludes that, “Rather than one to one correspondence between ant and cultivar lineages, a many to one relationship exists, in which a single general-purpose fungal mutualist acts as a nexus for indirect interaction between divergent lineages.”[7]
In addition to vertical transmission of fungi to new gardens by queens, frequent horizontal cultivar exchange occurs as well.[8] Some, such as Mikheyev, believe that ant cultivars could belong to one species, leading to the conclusion that cultivar lineages may not be as specific to ant genera as previously thought. Frequent cultivar exchange between ant colonies can make it difficult for co-adaptations to persist. According to Mikheyev, “fungal molecular variance was largely unstructured with respect to ant phylogeny”, meaning that ants of completely different genera could end up having fungal cultivars of identical genotypes.[9]

A piece of fungal garden extracted from a colony.  Source: http://www.blueboard.com/leafcutters/pics/images/fiero_2.jpg
A piece of fungal garden extracted from a colony. Source: http://www.blueboard.com/leafcutters/pics/images/fiero_2.jpg


Higher Attine Caste System
Although science has not shed any light on whether ants believe in reincarnation or karma, observational studies of higher attine colonies have shown that these ant societies are divided into castes based on function and morphology. Three main factors evident in studies of Atta sexdens colonies are polyethism, allometry, and alloethism. Polyethism is the specialization of colony members leading to division of labor. Related to this is allometry, the differential growth of the size of one body part compared to the whole organism, which leads to alloethism, the differing growth patterns of different ant caste members based on function.[10]
There are currently 29 specified tasks carried out by fungus-growing ants, of which higher leaf cutting attines peform 24. Rather than forage for substrates that do not require extra processing before feeding to their fungal symbiont, as seen in lower attines that feed decaying plant and insect remains or excrement to their fungal symbionts, higher attines forage for leaves and petals, which require a long and complex series of steps to gather, break down, and treat before they can be fed to the fungus.[11]
There is a strong correlation between the tasks ants perform and allometric differentiation of body parts based on caste, which is based on function. The four main categories of tasks engaged in by Atta are foraging, processing, gardening, and defending, each generally performed by members of the corresponding caste. The main trait used to differentiate between various castes is head width, ranging from <1mm to 6mm. Workers who cut leaves have heads adapted to this task, ranging from 1.6mm-3mm. Defender ants who fight off other insects and do not cut leaves have larger heads that can reach up to 6mm wide. The smallest ants with heads <1.2mm wide remain inside the colony. Their small heads allow them to perform more precise, fine movements such as tending to larvae or pruning and maintaining the fungal garden.[12] Besides head width, adaptation to task roles can take the form of positive organ allometry. Large defender ants have oversized mandibular glands and adductor muscles, while forager ants that lay down scent trails have the largest post-pharyngeal glands compared to other ant castes. Ants that spend the most time outside of the colony, such as foragers and leaf cutters, have the longer pronotal spines to protect themselves from predators than the ants whose function is largely within the colony itself. [13]
All this shows that ants within leaf-cutter colonies have become specialized and adapted to their respective tasks throughout evolutionary time. As one of the most complex and derived genera of ants, leaf cutter ants show a higher degree of body type variety than do other, less derived species of ants. This variety is due to the division of labor that has arisen in leaf cutter ant societies, which has led to the physical adaptation of specialized ants to their specific tasks. Rather than one type of ant performing multiple functions, smaller ants better suited to navigating the colony’s narrow pathways perform in-colony tasks, while larger ants with large jaws better suited for attacking outsiders perform defense functions, and so on.
It is truly humbling and fascinating to witness the complexity of ant colonies, or better yet ant “societies”. Every member knows its role and performs it unquestioningly, and has been doing so for millions of years, giving rise to a tiny creature capable of amazing feats of strength, teamwork, and architecture.

Displayed are seven different leaf cutter ant colony members, ranging from the smallest ones who are involved in larval and fungal care within the colony, to larger ants who are involved in leaf foraging and defense.  The ant with the largest head (third in from the right) is an example of a defender ant.  The two largest ants on the right are queens. Source: http://animals.y2u.co.uk/Photos/ants_leafcutter.jpg
Displayed are seven different leaf cutter ant colony members, ranging from the smallest ones who are involved in larval and fungal care within the colony, to larger ants who are involved in leaf foraging and defense. The ant with the largest head (third in from the right) is an example of a defender ant. The two largest ants on the right are queens. Source: http://animals.y2u.co.uk/Photos/ants_leafcutter.jpg







[1] Mikheyev, A. (2006) "Cryptec Sex and Many One-To-One Coevlolution in the Fungus-Growing Ant Symbiosis". PNAS. vol. 103. no.28. p.10702
[2] Pennisi, E. (2003) "On Ant Farm, a Threesome Coevolves". Science. vol. 299. p. 325
[3] Mueller, U. (2002) "Ant vs. Fungus vs. Mutualism: Ant-Cultivar Conflict and the Deconstruction of the Attine Ant-Fungus Symbiosis". The American Naturalist. vol. 160, p. S68
[4] Currie, C. (2003). "Ancient Tripartite Coevolution in the Attine Ant-Microbe Symbiosis". Science. vol. 299. 17 January. p.386
[5]Chapela, I. (1994) "Evolutionary History of the Symbiosis Between Fungus-Growing Ants and Their Fungi". Science. vol. 266 No. 5191. p. 1692
[6] Mikheyev, A. (2006) "Cryptec Sex and Many One-To-One Coevlolution in the Fungus-Growing Ant Symbiosis". PNAS. vol. 103. no.28. p.10703
[7] Mikheyev, A. (2006) "Cryptec Sex and Many One-To-One Coevlolution in the Fungus-Growing Ant Symbiosis". PNAS. vol. 103. no.28. p.10704
[8]Mikheyev, A. (2007) "Population Genetic Signatures of Diffuse Co-Evolution Between Leaf-Cutter Ants and Their Cultivar Fungi". Molecular Ecology. vol. 16, p. 214
[9] Mikheyev, A. (2007) "Population Genetic Signatures of Diffuse Co-Evolution Between Leaf-Cutter Ants and Their Cultivar Fungi". Molecular Ecology. vol. 16, p. 214
[10] Wilson, E. (1980). "Caste and Division of Labor in Leaf-Cutter Ants (Hymenoptera: Formicidae: Atta): I. The Overall Pattern in A. sexdens". Behavioral Ecology and Sociobiology. vol. 7 no. 2 p.144
[11] Wilson, E. (1980). "Caste and Division of Labor in Leaf-Cutter Ants (Hymenoptera: Formicidae: Atta): I. The Overall Pattern in A. sexdens". Behavioral Ecology and Sociobiology. vol. 7 no. 2 p.153
[12] Wilson, E. (1980). "Caste and Division of Labor in Leaf-Cutter Ants (Hymenoptera: Formicidae: Atta): I. The Overall Pattern in A. sexdens". Behavioral Ecology and Sociobiology. vol. 7 no. 2 p.154
[13] Wilson, E. (1980). "Caste and Division of Labor in Leaf-Cutter Ants (Hymenoptera: Formicidae: Atta): I. The Overall Pattern in A. sexdens". Behavioral Ecology and Sociobiology. vol. 7 no. 2 p.152