Symbiosis Between Leaf-cutting Ants, a Fungal Cultivar, a Parasitic Fungus, and a Bacterium that Produces an Antibiotic that Inhibits the Parasite

Instructor:

Elizabeth Shank
UNC – Chapel Hill

Focus:

This module would be most appropriate for an upper-level class focusing on microbiology or microbial ecology.
I selected these papers (5 papers in sequence from a single lab) for a number of reasons. They are focused on a well-studied symbiosis between four different organisms: leaf-cutting ants, a fungal cultivar (their food), a parasitic fungus, and a bacterium that produces an antibiotic that inhibits the parasite. I thought they might be appealing to the students because it is an interesting, complex, and interconnected symbiosis. From a teaching perspective, there are interesting co-evolutionary relationships between these organisms, and the papers cover a broad range of topics and experimental approaches.

Overview:

Ant and Its Fungus Are Ancient Cohabitants (Natalie Angier, December 13, 1994)
For Leaf-Cutter Ants, Farms Life Isn’t So Simple (Nicholas Wade, August 03, 1999)
These are two New York Times articles that I might use (either in their entirety or excerpts of them in conjunction with Paper 1 and 2) to cover some of the background of this symbiosis.

Applicable for Courses:

microbiology or microbial ecology.

Educational Level:

Upper-level

Roadmap Objectives:

- Article: Cameron R. Currie, James A. Scott, Richard C. Summerbell & David Malloch Nature (1999) 398:701-704
- Content area/major concepts: This paper identifies a new bacterial member of the fungal-farming ant symbiosis that wards off a parasitic invader
  
  Mutualisms/parasitisms/symbioses/host-microbe interactions
  Distribution of microbes in the environment
  Microscopy
  Growth and inhibition assays
  Differences between microbes and fungi?
  How observation leads to hypothesis leads to experiments to test
  Morphological phenotying vs. genetic identification (they incorrectly identify the genus of the bacterium)
- Methods or technology used to obtain data: Scanning Electron Microscopy Morphological characterization Bioassay (inhibition plate assay)
- How the CREATE strategy was used:
- Biggest teaching challenge: Complicated multiple-partner interaction system Jumping right into system without discussion of what bacteria even are
- Article: Currie CR, Wong B, Stuart AE, Schultz TR, Rehner SA, Mueller UG, Sung GH, Spatafora JW, Straus NA. Science (2003) 299(5605):386-8.
- Content area/major concepts: This paper presents evidence that the ant, farmed fungus, and parasites in this community have co-evolved.
  
  Phylogenetic relationships
  Microbial diversity
  Evolution/co-evolution
- Methods or technology used to obtain data: Phylogenetic trees
- How the CREATE strategy was used:
- Biggest teaching challenge: Difficult to understand how to build phylogenetic trees; hard concept of co-evolution
- Article: Nicole M. Gerado, Ulrich G. Mueller, Shauna L. Price, and Cameron R. Currie Proc. R. Soc. Lond. B (2004) 271:1791-1798
- Content area/major concepts: This paper examines the host specificity of the parasitic fungus to determine whether it is phylogenetically more congruent with the ant or food fungus of different ant nests, as well as doing cross-infection experiments to address potential mechanisms constraining the parasite range.
  
  Genotypes vs. phenotypes
  Mechanisms of host-parasite interactions
  Phylogenetic trees
  Evolution
  Genotyping using multiple loci
  Analysis of molecular variance (AMOVA)
- Methods or technology used to obtain data: Phylogenetic trees Genotyping Cross-infection experiments
- How the CREATE strategy was used:
- Biggest teaching challenge: Difficult concepts New representations of phylogenetic data
- Article: Adrián A. Pinto-Tomás, Mark A. Anderson, Garret Suen, David M. Stevenson, Fiona S. T. Chu, W. Wallace Cleland, Paul J. Weimer, Cameron R. Currie Science (2009) 326:1120-1123
- Content area/major concepts: This paper shows that nitrogen fixation occurs in the fungal gardens of leaf-cutter ants, and that this nitrogen is incorporated into biomass.
  
  Nitrogen fixation
  Nitrogen cycle
  Using proxies to measure metabolic activities
  Measuring microbial activity
  Microbial activity in the environment
  Biological functioning of ecosystems
  How to interpret histograms
- Methods or technology used to obtain data: Quantification of nitrogen incorporation Quantification of nitrogen fixation Nitrogen incorporation Bacterial isolation
- How the CREATE strategy was used:
- Biggest teaching challenge: Technical assays

Advice for Using Module/Activity:

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