Ant-fungus mutualism is one of the many interesting interactions in the animal kingdom. It is a highly evolved symbiosis observed in certain species where the ants cultivate or farm fungus for food -- fungiculture and/or fungivory, if you will. The ants grow, fertilize, clean, and weed patches or gardens of fungi in their underground tunnels and caverns. This mutualism is thought to have originated approximately 45-65 million years ago in the ancestor of fungus-growing ants (Formicidae, tribe Attini). In the course of evolution, a switch was made from the hunter-gatherer lifestyle to a farming strategy. This switch has evolved only twice in ants: (1) in the attine ants and (2) in some species of the solenopsidine genus Megalomyrmex. In this second case the ants either coexist as trophic parasites in the fungal gardens cultivated by attine ants or they take over the gardens from the attine ants. (Read more here) It is estimated that there are more than 200 species of New World ant fungiculturists.
A new study in The American Naturalist reports that these ants update the crops they grow over time. The authors compared the multigene phylogeny to the fossil phylogenies of attine ants as well as the phylogenies of associated fungal clades. Basically, they looked at the accumulations of mutations in short stretches of DNA to see when certain species or strains of ant and fungi emerged. The researchers discovered that the fungi were significantly younger than the ants. For example, leaf cutter ants (one of the best known farmers) diverged from their anty ancestors 12 million years ago, they are significantly younger than the corresponding ant genera. It was thought that the fungus that they cultivate would have to be older, evolutionarily speaking, so that the ants could aquire and domesticate it. However, the fungus these ants farm only arose only 2-3 million years ago. This time difference suggests that the ants are cultivating a new fungal strain, a strain which has been spread to other colonies throughout the ants' range.
Mikheyev Alexander S., Ulrich G. Mueller and Patrick Abbot (2010) Comparative Dating of Attine Ant and Lepiotaceous Cultivar Phylogenies Reveals Coevolutionary Synchrony and Discord. The American Naturalist: 175, E126–E133. (DOI: 10.1086/652472)