The diverse vegetation of neotropical rainforests is full of a wide array of toxic and repellant chemicals. In this forest of poisons, one group of herbivores reigns supreme: leafcutter ants, which account for 25 percent of all herbivory in neotropical rainforests, including plants that contain pesticides. They accomplish this remarkable feat by outsourcing plant digestion to "gardens" of subterranean fungus. The fungus grows on the leaves, the ants eat the fungus, and everyone is happy.
But researchers at UW Madison and Universidad de Costa Rica recently found that the compounds in harvested plants are toxic to the fungus, too, which lacks the necessary genes to break down many common plant toxins. How can leafcutter colonies thrive with a constant influx of poisonous leaves?
The answer may lie in a third player in this cooperative system: a diverse bacterial community associated with leafcutter fungus. Scientists had known for years that microbes in fungus gardens benefit the colony by synthesizing otherwise-rare nutrients. But these researchers suspected that they might also help break down plant toxins that neither the ants nor the fungus could handle.
In the genomes of dozens of fungus garden bacterial strains, the researchers found many of the genes required to break down plant toxins – and an analysis of “metagenomes”, containing all bacterial DNA from fungus gardens, uncovered even more degradation pathway genes. This suggests that toxin breakdown may be a task shared between multiple strains of bacteria.
To directly test garden residents’ abilities to degrade plant toxins, the researchers exposed bacteria, fungus, or intact garden communities to two plant compounds that reduce fungal growth and repel insects. They then measured the levels of each compound over time. Intact mini-gardens could degrade both compounds, and so could some varieties of bacteria, but fungus alone couldn’t.
While many plant compounds remain incompletely explored, this research suggests that the incredibly broad foraging ability of the ants is enabled by cooperation with an equally diverse set of fungi and bacteria. Together, the community can deal with a set of poisons no single organism could handle alone.