Insects have farmed monocultures longer than we have

Modern agriculture’s dependence on monoculture crops — vast fields of nothing but corn, wheat or soybeans — has been widely criticised lately for being unsustainable. However, a new study reports on how farming ants and termites have successfully used monoculture techniques far longer than we have.

While humans have developed agriculture only in the past 10,000 years or so, certain insects have had tens of millions of years to evolve their fungus farming techniques, a team of researchers writes this week in the journal Science. By all appearances, those techniques are stable and sustainable, conclude the scientists from the Laboratory of Genetics of Wageningen University and the Centre for Social Evolution at the University of Copenhagen.

Fungus-growing termites in the Old-World tropics build impressive mounds consisting of thousands of workers and soldiers. These societies domesticated African Termitomyces mushrooms more than 30 million years ago and became obligatorily dependent on farming their own fungal food in their often gigantic nest mounds. The termite fungus-farming symbiosis had a single African rain-forest origin but now comprises some 330 species.

The fungal farming is also of major ecological importance for decomposition and mineral cycling.

A colony-founding termite queen and king normally do not acquire their first garden until they have raised the first workers. These helpers collect Termitomyces spores while foraging, together with the plant material that they defecate in the nest to establish a garden substrate. These spores are amply available because the fungus gardens produce large mushrooms once a year on top of the termite mounds.

This farming technique, however, poses a paradox. According to evolutionary theory, symbioses with multiple types of fungus per colony should be unstable, because each fungal genotype can be expected to compete for making mushrooms rather than collaborate to serve the termite farmers.

The new study identifies a special mechanism that prevents this from happening. All colonies from which multiple fungal samples were genetically analysed contained only a single fungal genotype in spite of gardens having been initiated from at least two and probably many more genetically different spores.

The research team found that genotypes that happen to be common in a garden become even more common at the expense of rarer genotypes. This happens not because common genotypes are better direct competitors, but because they have a higher chance of having an identical genotype as neighbor. Every time this happens, such genetically identical mycelia merge, which enhances the efficiency by which they produce asexual spores that the termites eat and deposit in new garden material of the colony.

This process of positive reinforcement eventually leaves every colony with a lifetime commitment to a single fungus, despite the population at large having many fungal genotypes.