Mutualisms and Fungus-Growing Ants

Mutualisms

  • A relationship between 2 or more species in which all benefit (+/+)

  • Question: Rare or ubiquitous?

    • Examples of mutualisms:

    • Endosymbionts in eukaryotic cells

    • Mycorrhizae (90% of land plants)

    • Microbial digestion

Study of Mutualisms

  • Historically ignored in relation to other ecological interactions:

    • Predation

    • Parasitism

    • Competition

  • Approaches to study:

    • Usually unilateral approach (one species benefit)

    • Bilateral approach (two species benefit)

    • Multilateral approach (multiple species benefit)

Fungus-Growing Ants

  • Ant utilization of plants

  • Unique association with Attini (a tribe of ants)

  • Characteristics of this relationship:

    • Ancient and highly evolved

    • Obligate mutualism (ants are dependent on the fungus)

    • Ants carefully tend fungus as their main food source

    • This association originated 45-65 million years ago

    • Reflects amazing complexity of the mutualism

Distribution and Evolution of Fungus-Growing Ants

  • Found only in the New World

  • Originated and underwent subsequent radiation following the separation of South America and Africa

  • Ability to cultivate fungi is unique (believed to have a single origin)

    • Classification: Attini encompasses 12 genera and 212 species

Attini Genera Classification

  • Worker size and their garden substrate:

    • Lower Attines:

    • Myrmicocrypta

      • Worker size: Small

      • Colony size: Small

      • Garden substrate: Insect corpses

    • Mycocepurus

      • Worker size: Small

      • Colony size: Small

      • Garden substrate: Insect feces

    • Apterostigma

      • Worker size: Small/Medium

      • Colony size: Small

      • Garden substrate: Insect feces, woody matter

    • Cyphomyrmex

      • Worker size: Small

      • Colony size: Medium

      • Garden substrate: Insect feces, corpses

    • Mycetophylax

      • Worker size: Small

      • Colony size: Small

      • Garden substrate: Dead grass

    • Higher Attines:

    • Sericomyrmex

      • Worker size: Medium

      • Colony size: Medium

      • Garden substrate: Dead vegetation

    • Trachymyrmex

      • Worker size: Medium

      • Colony size: Medium

      • Garden substrate: Dead vegetation

    • Leaf-cutters:

      • Acromyrmex

      • Worker size: Medium/Large

      • Colony size: Large

      • Garden substrate: Fresh leaves/flowers

      • Atta

      • Worker size: Large

      • Colony size: Very Large

      • Garden substrate: Fresh leaves/flowers

The Ant Colony Structure

  • The fungus serves as the sole food source for larvae and the queen

  • Workers supplement with plant sap

  • Fungi produce gongylidia (nutrient-rich structures)

  • Queen (one individual) tends the garden in early stages and reproduces workers

  • Workers participate in the colony's overall function

  • Males only function in reproduction

Reproductive Strategies of Ant Colonies

  • During the nuptial flight, the queen transfers fungus to new colonies

  • Prior to the presence of workers, the growth of the fungus is supported solely by the queen

  • As the colony develops, it continues to accumulate biomass of both ants and fungi

  • Colonies are generally perennial, reproducing until the queen dies

  • There is an evolutionary trend towards increasing colony and worker sizes

Fungi Cultivated by Fungus-Growing Ants

  • Understanding the mutualism is complex and hindered by a lack of information on fungal species

  • Traditional methods for fungal taxonomy typically rely on identification of basidiocarps (fruiting bodies)

  • Definitive identification aided by molecular phylogenetic techniques

    • Common fungal families associated with ants: Lepiotaceae, Leucocoprinus, Leucoagaricus

Coevolution between Ants and Fungi

  • Ants and their fungal cultivars are descendants of symbionts from previous generations (vertical transmission)

  • Fungal cultivars typically act as ancient clones

  • A strong congruence exists in the evolutionary history of ants and their fungal cultivars

    • Each ant species cultivates a distinct species of fungus

Research Findings by Mueller et al. (1998)

  • An elaborate study focused on fungi and lower attines

  • Extensive sampling across 7 genera over a wide geographic region, encompassing 533 fungal isolates

  • Additionally collected and cultured 309 free-living Leucocoprineae

  • Conducted population genetics and sequencing of conserved genes to understand relationships

Conclusions from Mueller et al. Study

  • Within the same population, distantly related ant species may cultivate the same fungal clone

  • The same ant species (species complex) may cultivate distantly related cultivars

    • This finding indicates lateral transfer (switching fungal cultivars between ant colonies)

  • Some cultivated fungi found to be genetically identical to free-living taxa, suggesting possible recent domestication or escape

  • The acquisition of new fungal cultivars is regarded as continuous

Coevolution in Higher Attines

  • Detailed phylogenetic analyses of the higher attines remain unexamined as of yet

  • Preliminary works (e.g., Chapela et al., 1994; Hinkle et al., 1994) suggest a stricter coevolutionary dynamic

    • Fungi in these interactions appear as ancient clones that have evolved within this mutualism for millions of years

Garden-Tending Behavior of Ants

  • Genera Acromyrmex and Atta engage in behaviors to maintain their gardens, including:

    • Manuring and promoting growth of the garden

    • Breakdown of vegetative material to create a suitable substrate

    • Eliminating microbes through specific hygiene activities

    • Enhancing the surface area for fungal growth by creating more edges

    • Fecal droplet applications introduce proteolytic enzymes for decomposition management

    • Waste management practices can require around 6 weeks for full breakdown

    • Growth promotion through enzymatic action and climatic control strategies

Protection from Alien Microbes

  • Cultivating fungi necessitates complex behavioral and physiological adaptations from the ants

  • Maintaining a healthy fungus garden is crucial, as it is continually inoculated with bacteria and fungi from the substrate, making potent competitors

  • A critical question arises: How do ants protect their gardens from invasive microbial species?

Promoting Competitive Ability of Cultivars

  • Weber's findings (1972) suggest that the primary mechanism employed is ensuring optimal growing conditions for the fungus

  • There is no conclusive evidence for the suppression of alien microbes; however, creating a large biomass in the substrate may play an important role in competition management

Role of Antibiotics in Ant Gardens

  • Ants utilize antibiotics obtained from their metapleural glands to suppress alien microbial threats

  • Evidence indicates that compounds with antimicrobial properties (such as phenylacetic acid and indole-3-acetic acid) exist within the metapleural glands

    • Not unique to fungus-growing ants, highlighting a common basic function, not strictly tied to fungal cultivation

  • Establishing that these compounds are present in sufficiency to inhibit microbial growth is vital

  • Activity against ecologically relevant microbes is also a key aspect to validate

Antibiotics from Fungal Cultivars

  • Researching antibiotics derived from cultivated fungi yields mixed results

  • It is clear that a diverse assemblage of fungi are cultivated by attine ants, with some producing antibiotics while others do not

Ant Behavioral Defenses

  • Ants maintain the health of their fungus gardens through three key methods:

    1. Licking and masticating leaves

    2. Removing infected parts of the garden

    3. Grooming activities to eliminate possible contaminants

Fungus Gardens and Monocultures

  • Despite incomplete understanding, it has been assumed that ants maintain their mutualist fungi in pure monocultures

  • Limited direct evidence to support this notion exists

  • Observations have shown rapid growth of alien fungi in the absence of ant management

Monoculture and the Red Queen Hypothesis

  • The interactions within parasitic associations can yield oppositions between parasite and host

    • This dynamic often leads to coevolutionary progress known as a “coevolutionary arms race”

  • The Red Queen theory suggests that the maintenance of sexual reproduction is a selective force against asexual or genetically homogenous hosts

    • Asexual organisms can propagate their genes more rapidly than sexual conspecifics, leading to competitive advantages that are counterbalanced by the reproductive advantages of sexual reproduction

Advantage of Sexual Reproduction in Coevolution

  • Sexual reproduction is proposed as an advantage for organisms engaged in coevolution with parasitic agents

  • It is based on the belief that parasites can quickly adapt to asexual or genetically uniform hosts

  • Only by acquiring new or novel resistance genotypes through sexual reproduction can hosts maintain their position ahead of their parasites

  • Fungus-growing ants, especially higher attines that cultivate ancient asexual clones, are postulated to face significant parasite pressures, similar to challenges faced in human agricultural systems

Pathogens of Ant Fungus Gardens

  • Observations indicate that overgrowth typically arises only under controlled lab conditions (e.g., due to airborne contaminants)

  • Currie et al. (1999) conducted a comprehensive survey involving 201 colonies across eight attine genera in Panama, with findings detailing:

    • Sampling of 2400 garden pieces (3mm^3)

    • 40% of pieces had at least one alien fungus present

    • Escovopsis represented 26% of all contaminants

Evidence for Escovopsis as a Pathogen

  • Escovopsis is frequently encountered in field fungus gardens

  • It demonstrates the ability to maintain continuous presence for several months, in contrast to the typical garden cycling period of 6 weeks

  • Absence of workers in Escovopsis-infected gardens leads to rapid overgrowth by the fungus within 12-24 hours

  • This pathogen can significantly impact the health and survivorship of fungus gardens

  • Fulfillment of Koch’s postulates indicates its pathogenicity

Impact of Escovopsis on Fungus Gardens

  • Analysis showed that 9 out of 16 colonies treated with high doses of Escovopsis conidia lost their gardens within 3 weeks

  • Additionally, chronic infections can lead to a substantial reduction in growth rates (biomass) of both fungi and ants

    • Large biomass of fungi is required to support the production of reproductive alates (virgin queens and males)

  • Some colonies may experience a net loss in biomass, preventing them from reaching sufficient size to produce new reproductives

Specialist or Generalist Nature of Escovopsis

  • Escovopsis is solely isolated from habitats linked with fungus-growing ants

  • Recognized for its virulence and apparent ancient association with the mutualism

  • Its presence seems widespread throughout the geographical range of the mutualism and correlates with the phylogenetic diversity of various ant species

Biology of Escovopsis

  • The basic life history of Escovopsis remains inadequately understood

  • It is isolated strictly from gardens and refuse heaps

  • Dispersal biology insights suggest that:

    • Wet conidia may not be wind-dispersed

    • It is absent from the infrabuccal pocket of queens and from incipient colonies

    • Seems to be vectored between colonies via invertebrates or potentially possess a yet-to-be-discovered life history stage

  • Potential mechanisms of pathogenicity:

    • Highly evolved weed or mycoparasite characteristics, or both

    • Teleomorph (sexual reproduction stage) remains unidentified

Summary of Garden Pathogens

  • Escovopsis is highly evolved and acts as a specialist pathogen

  • It is very common within gardens and possesses a capacity to overwhelm these environments

  • Not known from any other habitat, it grows specifically within the garden matrix

  • Colonies infected by the parasite show considerably reduced growth rates in their fungus-gardens compared to uninfected colonies

  • An open question remains regarding how ants and their fungal mutualists defend against such a virulent pathogen

Antibiotic-Producing Bacteria Associated with Ants

  • Some ant species exhibit whitish granular deposits, often crystalline wax, from the presence of filamentous bacteria

  • This association is found among all 22 species of fungus-growing ants, linked with Actinomycete bacteria

  • These bacteria are vertically transmitted through queens

Do Actinomycetes Produce Antibiotics?

  • Extensive bioassays indicate that actinomycetes exhibit potent inhibitory properties specifically against Escovopsis, though no general antifungal properties precluded against a wide range of fungi were found

  • The actinomycete functions as a highly evolved mutualist used by ants to suppress Escovopsis, leading to a quadripartite symbiosis involving ants, fungi, actinomycetes, and the pathogens

Additional Benefits of Actinomycetes

  • Research by Currie et al. (1999) on a specific ant genus showed a substantial increase in the growth of fungal cultivars when grown in broth cultures with filtrate from the bacterium, suggesting the presence of growth-promoting compounds

  • The bacterium may provide protective benefits to ants from other pathogens as well

Benefit to Bacteria

  • Actinomycetes are dispersed by virgin queen ants

  • They occupy a unique habitat associated with the diverse assemblage of attine ants, expanding their ecological niche as it remains largely unoccupied

  • Possible nourishment mechanisms could include secretion of nutrients by ants promoting the growth of the bacteria

Future Research Considerations

  • Potential antibiotic resistance emergence among associated microbes

  • Exploring whether the evolutionary history of the actinomycetes matches with that of:

    • Ants that transmit them

    • Fungal cultivars that it protects

    • Escovopsis, the pathogen it defends against

Recent Updates in Research

  • Actinomycetes have been identified specifically as Pseudonocardia

  • Newly described symbionts include black yeasts (e.g., Phialophora), which could challenge the efficiency of bacteria-derived antibiotic defense in fungus-growing ants

  • Escovopsis is now reclassified into three different genera

  • New findings also indicate the presence of Burkholderia (another bacterial genus) in the gardens of insect symbionts

References

  • Chapela et al., 1994. Science 266:1691-94

  • Currie et al., 1999. PNAS 96:7998-8002

  • Currie et al., 1999. Nature 398:701-4

  • Hinkle et al., 1994. Science 266:1695-97

  • Mueller et al., 1998. Science 281:2034-38

Additional Review Papers for Further Reading

  • Currie et al. (1999). The agricultural pathology of ant fungus gardens. PNAS 96:7998-8002

  • Currie (2001). A community of ants, fungi and bacteria: a multilateral approach to studying symbiosis. Ann. Rev. Microbiol. 55:357-380