Fungus therapy

By Athayde Tonhasca

About 12,000 years ago, mankind took a mighty leap forward by adopting agriculture; peoples in different parts of the world abandoned nomadic, hunting-gathering existences to take up farming and animal husbandry. Cities multiplied, populations grew dramatically, and civilizations flourished. Not bad for the bipedal primate Homo sapiens, but agriculture was already old news – in fact, at least 40 million years old – for some insects.

Agriculture, or the practice of producing crops, has long evolved as the way of life for about 80 species of leaf-cutter ants, 330 species of termites and 3,400 species of ambrosia beetles: these insects get their food by cultivating fungus gardens. Ants and termites collect plant material to provision their fungi, which convert the vegetable substrate into nitrogen-rich fungal biomass. Ambrosia beetle fungi extract nutrients directly from the host plant. These farming insects propagate and control the growth of their fungi, weed out contaminants and pests and take spores with them to start new colonies. And without their gardeners, these fungi quickly die.

Leaf-cutter queen and workers on their fungus garden © Christian R. Linder, Wikipedia Creative Commons

Recently, a bee was found to belong to this insects’ farming union: the South-American stingless bee Scaptotrigona depilis. Its larvae feed on a fungus in the genus Monascus, bits of which adult bees transfer between brood cells and take to newly founded nests. Without the fungus, few larvae survive. The need to eat a fungus seems puzzling because inside each brood cell, a larva floats in a pool of abundant, nourishing food. The reason appears to be protection rather than nutrition. The fungus may produce chemical compounds that defend the larvae and their food from harmful fungi and bacteria. Monascus fungi are used to preserve meat and fish in Southeast Asia because of their antibacterial and antifungal properties, so the hypothesis is plausible.

A. A Scaptotrigona depilis egg floating on the semi-liquid brood food; B. 1-day old larva: fungal mycelia growing from cell wall onto larval food; C. 3-day-old larva: dense fungal mycelia on cell wall © Menezes, C. et al. 2015. Current Biology 25: 2851-2855

These tropical fungus gourmets may seem of little relevance to our pollinators, but they suggest that cases of insect-fungus symbiosis – from the Greek syn (together) and biosis (living) – are more common and relevant than what we know. Some Aspergillus, Penicillium, Cladosporium and Rhizopus fungi protect the honey bee (Apis mellifera) against diseases such as chalkbrood and contribute to the fermentation of pollen to produce ‘bee bread’ (the main food of larvae and workers, comprising a mixture of pollen and honey). Fungi are known to produce chemicals that work against other fungi, bacteria and viruses. In fact, honey bees that feed on Fomes and Ganoderma mushrooms have reduced levels of infestations of some destructive viruses such as the deformed wing virus and the Lake Sinai virus. We have much less information on beneficial fungi in relation to other bee species. 

The stingless bee Scaptotrigona depilis © Cristiano Menezes, Agência FAPESP

Most bees nest and store nutrient-rich food underground, which makes them vulnerable to pathogens and parasites. Many of these bees – and the honey bee as well – are protected to some extent by gut microbiotas, and bacteria are the better known components of these symbiotic fauna. In time, we may find that fungi have a greater protective role than is currently recognized.