THE ATTINE ANTS are a clade of New World ants that cultivate fungal gardens for use as a food source. The most widely known group are the leaf-cutter ants, which spend most of their time harvesting slices of leaves that are chewed up and placed in the fungal gardens in their underground colonies. There are many other lesser-known species, and these practice different forms of fungal agriculture. A new study of the phylogenetics of attine ants show that the different methods of fungal cultivation emerged in single events.
Ant fungal agriculture comes in five different forms:
- Lower agriculture: The cultivation of any of a variety of different Leucocoprineae fungi. All fungi known to be cultivated are also capable of independent growth outside the ants’ gardens.
- Coral fungus agriculture: The cultivation of Pterulaceae fungi, a clade of coral fungi.
- Yeast fungus agriculture: The cultivation of yeasts. These fungi normally grow ferally as a web of mycelium, but in ant gardens they grow as globules of single-celled yeasts.
- Higher agriculture: The fungi cultured by these ants appear to be so adapted to garden life that they are incapable of living outside their hosts’ nest. These fungi produce gongylidia, swollen hyphae tips that are harvested by the ants.
- Leaf-cutting agriculture: This is a subtype of higher agriculture. The ants’ activity is so centered upon gathering leaves for their fungal garden that a mature colony is “the ecological equivalent of a large mammalian herbivore in terms of collective biomass, lifespan, and quantity of plant material consumed”.
Schultz and Brady studied the phylogenetic tree of the attine ants and uncovered some interesting findings. First, lower agriculture is the ancestral state for attine ants and dates back to over 50 million years ago. The other methods of agriculture were developed much more recently. Approximately 50 million years ago the Attine ants split into Paleoattine and Neoattine sister clades. Among the Paleoattine ants most retained lower agriculture, but one clade developed coral fungus agriculture about 15 million years ago. This could happen due to accidental introduction of this fungus and loss of the original cultivar. In the Neoattine ants yeast agriculture evolved about 20 million years ago, and a few million years later in the sister clade of the yeast cultivators higher agriculture evolved, although lower agriculture persists among other members of this clade. The leaf-cutting ants themselves evolved from within the higher agriculture ants between 8 and 10 million years ago. This short history helps explain why all leaf-cutting ants known cultivate the same type of fungus.
The authors note that attine ants are not well-studied. Their phylogenetic study demonstrates several relationships not previously known and contradicts some previous theories about attine ant evolution (such as the idea that yeast cultivation is the ancestral condition). Some groups thought to be monophyletic have been shown to be paraphyletic, and these will have to be reorganized. Even with this improvement some of these relationships will probably change in future as we learn more about the attine ants, and the authors even propose that we may discover new fungal agriculture systems in addition to the five already known.
While it was not the focus of this study, the authors mention the distribution of the parasitic fungus Escovopsis among the different types of fungal cultivars. Escovopsis is related to a family of fungi that parasitize free-living fungi. The ancestors of this parasite were probably brought in accidentally with the first fungal cultivars and Escovopsis has evolved as a specialized parasite of ant cultivars ever since. Escovopsis commonly infects fungus gardens, but does not infect yeast gardens. The reversion to yeast growth from mycelial growth may somehow prevent Escovopsis infection, and yeast gardens may have been under selection for yeast growth instead of mycelial growth for this reason. Interestingly, the clade of parasitic Escovopsis infecting higher agriculture ants is evolved from a clade that coevolved with coral fungus gardens (and descended previously from a strain found in lower agriculture gardens) and then was transferred across to the higher agriculture ants. To add a twist to this story, the attine ants have developed a symbiotic relationship with bacteria that live on the ants’ integument and release antibiotic compounds which suppress Escovopsis growth.
This amazing mutualistic network has evolved over the past fifty million years. This study has shed light upon the evolution of the relationship between the attine ants and their cultivars, showing a progression from cultivation of a wide variety of fungi capable of independant growth to cultivation of specialized fungi absolutely dependent upon the ants. Along the way two clades branched out into novel types of agriculture, with one clade cultivating yeasts and another switching to a type of fungus distantly related to its original cultivar. Although the relationship between ants and cultivated fungi started 50 million years ago, major changes have occurred in just the past 20 million years.
Alex Wild at Myrmecos also covered this article today, and includes photographs of various attine ants and their gardens.
Schultz, T.R., Brady, S.G. (2008). Major evolutionary transitions in ant agriculture. Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0711024105