The Biggest Mushroom?

© Else C. Vellinga
Original publication: Mycena News, May 2004

“How big can a mushroom grow?” – my favorite question from the children at the fungus fair! There is always a clump of Gymnopilus spectabilis to show how huge our local mushrooms can grow, but still that is half the size of the record holder, Termitomyces titanicus. Caps more than 3 feet wide are no exception! And these fungi are good to eat too!

Unfortunately we have to travel quite a bit to collect those giants. Termitomyces species only grow in Africa and South East Asia. Furthermore, they are exclusively found in connection with termites and their nests, living a pampered existence where all the material for them to grow on is faithfully schlepped in by the termites. Their lifestyle is similar to that of the leucocoprinoid fungi cultivated by the leaf cutter ants and relatives in the New World. Besides the titanic T. titanicus, there are some 30 to 40 Termitomyces species, most of them smaller, starting at half an inch. Throughout their range, Termitomyces species are sought after for food, and are widely sold on markets.

Not all termite species rely on fungi. All members of one subfamily (Macrotermitinae) do, but the other termites have found another way to break down their favourite foods (wood and other dead plant tissues) by harbouring bacteria and protists in their hindguts to do the work. Without the fungi the Macrotermitinae can not live, and without the termites, Termitomyces species are not able to survive – an obligate mutualistic relationship for both parties.

The fungi are grown on combs, special structures made by the termites. The termites eat fresh plant material which passes quickly without much decomposition through the intestinal canal, and that material is moulded together to form a substrate for the fungi. On these combs, the fungi form white blobs ( in jargon they are called myco-têtes, which is French for fungal heads), containing asexual spores, and the termites eat this part of the fungus. Different termite species use the fungi and the comb in different ways, some eat the fungus for food, and do not eat anything else, others eat the fungi for their enzymes and eat plant or comb material as well. In some cases, only the lignin degrading enzymes are used, in other cases also the enzymes which decompose cellulose. It is nature at its best: for everything a solution, and no two systems exactly the same.

By using this extra step in their plant diet, the termites get food which is much richer in nitrogen than if they only eat plants.

The fungus combs not only serve as food, but also as regulators to keep the temperature and humidity in the termite nests or heaps constant. Thereby they provide a stable environment for the eggs and developing larvae.

Of course, the combs full of easily accessible plant material is attractive to lots of other fungi, but the termites ward these intruders off with specific fungicides in their saliva. In abandoned nests Xylaria species immediately take over from the Termitomyces.

Because of this cultivation strategy, termites build huge heaps and have big colonies. The same is true for the leaf cutting ants with colonies of millions and millions of individuals. Farming is a good strategy: the human population, too, exploded after the invention of agriculture around 10,000 years ago. The mutualistic relationship between fungi and termites is much older than that, 30-50 million years older, by present estimates. It evolved only once, in Africa, followed by several waves of emigration to Asia.

Some species never produce fruitbodies and in their nests, before the winged termites set out on their flight to form a new colony, either the females or the males, depending on the species, first eat spores from the white blobs, and later excrete a bolus to inoculate the combs in the new nest. However, most of the species of fungal symbionts do produce fruitbodies, and these appear outside the nest at just the right time. When a new nest has been constructed but is still fungus-free, workers leave the new nest and gather the spores with which they will inoculate the combs.

As always there are still many questions. In the first place, of course, is: how did this mutualism evolve? What triggered the termites to change from the trusted bacteria and protists within their bodies to this much more complicated system of fungus culturing? Secondly, there is a huge disparity in the numbers of species involved, not more than about 40 fungi but about 330 fungus-growing termites. Cultivar-sharing or competition for the symbionts must be going on; is there also competition on the combs by different Termitomyces species? For the species that produce fruitbodies, how do worker termites recognize Termitomyces as THE mushroom to gather spores from?

Questions, questions, at least, we know the answer to that one from the beginning: termites grow the biggest mushrooms on earth!

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