Ant-fungus mutualism is a symbiosis

Symbiosis

Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...

 seen in certain ant

Ant

Ants are social insects of the family Formicidae and, along with the related wasps and bees, belong to the order Hymenoptera. Ants evolved from wasp-like ancestors in the mid-Cretaceous period between 110 and 130 million years ago and diversified after the rise of flowering plants. More than...

 and fungal

Fungus

A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...

 species, where ants actively cultivate fungus much like humans farm crop

Agriculture

Agriculture is the cultivation of animals, plants, fungi and other life forms for food, fiber, and other products used to sustain life. Agriculture was the key implement in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the...

s as a food source. In some species, the ants and fungi are dependent on each other for survival. The leafcutter ant

Leafcutter ant

Leafcutter ants, a non-generic name, are any of 47 species of leaf-chewing ants belonging to the two genera Atta and Acromyrmex.These species of tropical, fungus-growing ants are all endemic to South, Central America, Mexico and parts of the southern United States.The Acromyrmex and Atta ants have...

 is a well known example of this symbiosis. A mutualism with fungi is also noted in some species of termite

Termite

Termites are a group of eusocial insects that, until recently, were classified at the taxonomic rank of order Isoptera , but are now accepted as the epifamily Termitoidae, of the cockroach order Blattodea...

s in Africa

Africa

Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...

.

General overview

Given the exclusive New World

New World

The New World is one of the names used for the Western Hemisphere, specifically America and sometimes Oceania . The term originated in the late 15th century, when America had been recently discovered by European explorers, expanding the geographical horizon of the people of the European middle...

 distribution of the >230 attine ant species, this mutualism is thought to have originated in the basin of the Amazon rainforest

Amazon Rainforest

The Amazon Rainforest , also known in English as Amazonia or the Amazon Jungle, is a moist broadleaf forest that covers most of the Amazon Basin of South America...

 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 hypha

Hypha

A hypha is a long, branching filamentous structure of a fungus, and also of unrelated Actinobacteria. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium; yeasts are unicellular fungi that do not grow as hyphae.-Structure:A hypha consists of one or...

l-tips known as "gongylidia". In some advanced genera

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 the queen ant

Queen ant

A queen ant is an adult, reproducing female ant in an ant colony; generally she will be the mother of all the other ants in that colony. Some female ants do not need to mate to produce offspring, reproducing through asexual parthenogenesis or cloning and all of those offspring will be female.Ant...

 may take a pellet of the fungus with her when she leaves to start a new colony. While this vertical transmission of fungal cultivar

Cultivar

A cultivar'Cultivar has two meanings as explained under Formal definition. When used in reference to a taxon, the word does not apply to an individual plant but to all those plants sharing the unique characteristics that define the cultivar. is a plant or group of plants selected for desirable...

s and strong host-symbiont specificity might suggest a tight coevolutionary relationship, recent phylogenetic

Phylogenetics

In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...

 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

Pseudonocardia

Pseudonocardia is the type genus of the bacteria family Pseudonocardiaceae. Members of this genus have been found living mutualistically in the metapleural glands of the leafcutter ants.-Species:...

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 gland

Exocrine gland

Exocrine glands are a type of ductal glands that secrete their products into ducts that lead directly into the external environment...

s 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 antibiotic

Antibiotic

An antibacterial is a compound or substance that kills or slows down the growth of bacteria.The term is often used synonymously with the term antibiotic; today, however, with increased knowledge of the causative agents of various infectious diseases, antibiotic has come to denote a broader range of...

s 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.

External links

• Fungus-growing ants, Social Insect Research Group, Universities of Copenhagen and Aarhus.
• Ulrich G. Mueller: Publications (Includes links several key papers on ant/fungal symbiosis)
• A breakthrough innovation in animal evolution by Ulrich G. Mueller and Christian Rabeling, PNAS 105(14): 5287–5288, April 8, 2008.