Fungus-growing ants – Department of Biology - University of Copenhagen

Social Evolution > CSE Research > Model Organisms > Fungus-growing ants

The fungus-growing ants belong to the Attini, a monophyletic tribe of >250 species in 14 genera, including the well-known leaf-cutting ants. These ants rear fungi for food in underground nest chambers, and are completely dependent on their fungal mutualist. Fungus-farming had a single evolutionary origin in South America about 50 million years ago, when the ancestors of all extant attine ants made the transition from hunter-gatherers to fungus-farmers. Today the attine ants are distributed throughout the tropical, subtropical and warm-temperate regions of the Americas.

It was not until ca. 20 million years ago that a single lineage of fungi cultivated by the ants became a specialized fungal crop. In this process it evolved special fungal organs (staphylae), clusters of inflated hyphal tips (gongylidia) whose sole purpose is to feed the ants and their brood. As these organs are only useful for fully committed symbionts, the fungus lost its gene flow with free-living relatives and became a co-evolving mutualist of the attine ants. A single lineage of these ants evolved a herbivorous life-style ca. 10 million years ago, which triggered changes in caste differentiation, mating system and orders of magnitude larger colonies. All the so-called leafcutter ants of today appear to cultivate a single genetically variable species of fungus-garden symbiont.

Species belonging to the basal branches of the attine ant phylogeny have very small colonies of a few hundred workers at best, but the leaf-cutting ants have very large (> 1 million workers) colonies and multiple secondary adaptations to ensure the sustainability of their huge fungus farms. Despite challenges from crop-diseases, these large insect societies have managed to remain evolutionarily stable by evolving genetically more diverse colonies, chemical control of crop disease, and sophisticated individual and collective immune systems. However, socially parasitic species also occur, particularly in the genus Acromyrmex.

Over the years, CSE research has clarified that the ancestral leafcutter ants evolved obligate multiple mating of queens, creating genetically more diverse half-sister colonies. We have also documented that this mating system introduced novel sperm competition conflicts, but that Atta queens have the ability to prevent these conflicts when they store ejaculates of multiple males. We have also shown that the fecal fluid of the ants vectors crucial fungal enzymes from the central fungus garden where the ants eat gongylidia to the top of the gardens where new leaf substrate is incorporated in the garden. We published the draft genome of our main model system species Acromyrmex echinatior in 2011, and are currently analysing the genomes of sixother attine ant genomes in collaboration with BHI Shenzhen and the Smithsonian Institution in Washington DC and Panama.

The genomic part of CSE's research on fungus-growing ants is supported by an advanced ERC grant.

Questions asked

  • What evolutionary transitions in the ants and their crop fungi were necessary to keep fungus-farming productive and sustainable?
  • Which specific challenges does fungus-farming pose with respect to social parasites, and infectious diseases?
  • Which molecular adaptations to the symbiosis have occurred in the ants and their fungal symbionts?
  • Which adaptive functions do bacterial gut and cuticular symbionts have for the ants?
  • How can a single queen produce up to five million daughter workers per year without re-mating, and what are the adaptations in sperm viability and sperm use that make this possible?
  • Does competition between different ejaculates stored by the same queen occur and what are the mechanisms involved to manage such reproductive conflicts?
  • How does social parasitism evolve, both at the level of full-sister lineages among the offspring of multiply mated queens, and at the species level in Acromyrmex, but not in Atta?
  • Does genomic imprinting mediate some of the reproductive conflicts?

Research themes

Fungus growing ants; Concept in social evolutionFunctional Genomics

Research tools

Behavioural experiments; Chemical analyses; Genetic analyses; Genomics; Immune assays; Laboratory colonies; Modelling; Microscopy; Fieldwork