Category: farming and gardening

The first fungus gardens, adaptations, and innovations

Ted Schultz and colleagues link the emergence of ant farming to an asteroid impact and the domestication of the fungal crop to a period of strong climate change.

You can forage for food, but you can also grow it to make sure it is available. About 250 species of ants from South, Central, and North America do the latter, growing a fungus in their nests for food. The evolutionary history of this ant-fungus relationship was largely known. Now, Ted Schultz and colleagues compare the evolutionary tree of fungus-growing ants with that of cultivated fungus varieties and refine the picture.

It is fascinating how they link the history of ant farming to two major events on Earth.

Pioneers

The fungus-growing ants have a common ancestor that started agriculture. This happened 66 million years ago in wet tropical forests of South America. Shortly before that, an asteroid had hit Chicxulub in Mexico with enormous consequences. Dust in the atmosphere blocked sunlight for months, plants died and many species of plants and animals, including dinosaurs, became extinct.

But fungi that live on dead material, such as fallen leaves, flourished, and some ants took advantage of this. They could not digest organic material themselves, but they allowed a mushroom-like fungus that could do the job to grow in their nest by providing it with detritus. The fungus broke down the material, and the ants consumed breakdown products as well as fungus. All 250 species of ants that currently have a fungus garden in their nest descend from these pioneers.

Soon, ants picked up a second mushroom-like fungus. Virtually all cultivated fungus varieties today – and there are several hundred of them – descend from these two early crops.

Domestication

From the beginning, the farming ants could not do without their crops; they would starve. But conversely, the fungi did not need the ants. They also lived outside ant nests, and fungal crops exchanged genetic material with their wild relatives. Outside they formed mushrooms, within ant nests the ants prevented this and only allowed fungal threads to grow. This is known as lower ant agriculture. Today, roughly a hundred species of ants exist that practice lower farming, with many semi-wild fungal varieties.

But it did not stop there. At some point, there were ants that domesticated their crop. That means that the cultivated fungus became dependent on the grower and can no longer live in the wild. And it produces nutrient-rich food bodies especially for the ants, the so-called gongylidia. These ants and their fungi are inseparable, and a young queen that wants to establish her own colony does not leave the maternal nest without a piece of fungus garden between her jaws. This is called higher ant agriculture.

This agricultural transition only occurred when lower ant agriculture had already existed for 36 million years, now about 27 million years ago. Why didn’t it happen earlier, and why did it suddenly happen then?

Global cooling

The researchers point at the so-called Terminal Eocene Event 33.5 million years ago that preceded the transition. The Earth cooled down rapidly and many species became extinct, although the extinction was not as massive as 66 million years ago. In South America, part of the wet tropical forests made way for landscapes that were seasonally dry, such as savannas.

Some of the fungus-farming ants moved to drier areas. The fungi in their nests retained the same growing conditions, but they lost contact with wild relatives, which lived in wet forests only. Because the cultivated fungi no longer exchanged genetic material with their wild relatives, the ants could select freely for characteristics that were beneficial to themselves, and not necessarily to the fungus. And so, a domesticated crop developed.

Innovation

Finally, 18 million years ago, a new form of higher ant farming appeared: there were fungus growers that started to provide their gardens with pieces of fresh leaves instead of leaf litter. These leaf-cutter ants form complex colonies of millions of individuals; they manage to keep their gardens in perfect order. They all grow the same fungus species, Leucoagaricus gongylophorus, a descendant of the very first fungus with which ant farming ever started.

Schultz does not mention whether there was a special reason for these ants to switch to fresh leaves as a substrate for their fungus. There are now more than fifty species of leaf-cutter ants.

Alternative agriculture

Two other agricultural systems branched off from lower ant agriculture. About 30 million years ago, a group of ants switched to growing fungi in unicellular form – that is: a yeast – instead of in multicellular thread form (there are unicellular and multicellular fungi). This is remarkable, because the mushroom-like fungi of which the cultivated crops are derived grow exclusively in multicellular form. Even the fungi cultivated as yeast never occur in yeast form in the wild.

And 21 million years ago, another group of fungus growers exchanged the usual mushroom-like fungi (from the family Agaricaceae) for coral fungus species (from the family Pterulaceae), which do not break down leaf litter, but wood.

Would there be a reason for these two shifts also? It would be great if a reason was found.

Willy van Strien

Photo: Cyphomyrmex ant species that cultures yeast. ©Alex Wild

See also: leaf cutter ants supply their crop with food according to demand

Sources:
Schultz, T.R., J. Sosa-Calvo, M.P. Kweskin, M.W. Lloyd, B. Dentinger, P.W. Kooij, E.C. Vellinga, S.A. Rehner, A. Rodrigues, Q.V. Montoya, H. Fernández-Marín, A. Ješovnik, T. Niskanen, K. Liimatainen, C.A. Leal-Dutra, S.E. Solomon, N.M. Gerardo, C. R. Currie, M. Bacci, Jr., H.L. Vasconcelos, C. Rabeling, B.C. Faircloth & V.P. Doyle, 2024. The coevolution of fungus-ant agriculture. Science 386: 105-110. Doi: 10.1126/science.adn7179
Branstetter, M.G., A. Ješovnik, J. Sosa-Calvo, M.W. Lloyd, B.C. Faircloth, S.G. Brady & T.R. Schultz, 2017. Dry habitats were crucibles of domestication in the evolution of agriculture in ants. Proceedings of the Royal Society B 284: 20170095. Doi: 10.1098/rspb.2017.0095
De Fine Licht, H.H., J.J. Boomsma & A. Tunlid, 2014. Symbiotic adaptation in the fungal cultivar of leaf-cutting ants. Nature Communications 5, 5675. Doi: 10.1038/ncomms6675
Schultz, T.R. & S.G. Brady, 2008. Major evolutionary transitions in ant agriculture. PNAS 105: 5435-5440. Doi: 10.1073_pnas.0711024105
Villesen, P., U.G. Mueller, T.R. Schultz, R.M.M. Adams & A.C. Bouck, 2004. Evolution of ant-cultivar specialization and cultivar switching in Apterostigma fungus-growing ants. Evolution 58: 2252–2265. Doi: 10.1111/j.0014-3820.2004.tb01601.x

Leafcutter ant prevents fungus garden from starving

The fungus-breeding ant Acromyrmex ambiguus checks its crop and intervenes if it threatens to starve, Daniela Römer and colleagues show.

Leafcutter ants grow fungus on plant material in their underground nests with chambers. The ant workers cultivate their crop with care, according to research by Daniela Römer and colleagues on Acromyrmex ambiguus: if the fungus garden in one of the nest chambers deteriorates because of lack of food, they will bring it more pieces of fresh green leaves.

Ants are unable to digest plant leaves, but, thanks to their crop, fungus growing ants still are herbivorous. The fungus breaks down plant material and converts it into digestible, nutritious globules. The larvae are completely dependent on that food, the workers also eat it.

It was already known that leafcutter ants, which live in North, Central and South America, take excellent care of their crops. They have to, of course, because if the fungus dies, all their effort to grow it is wasted and the larvae have no food.

Even distribution

Acromyrmex ambiguus is such leaf cutter species; it lives in nests holding thousands of chambers in which fungus grows and brood is raised. Römer wanted to know how the workers distribute the leaf material they collect – i.e., food for the fungus – equally among those chambers.

The experiments she conducted once again show how skilled the small farmers are. The researchers have a colony of Acromyrmex ambiguus in the lab. It is housed in an artificial nest with a number of nest chambers that are filled with fungus, about thousand workers, and ant brood. For the experiments, they placed three of those nest chambers serially, each with its own access tube connected to a main corridor. Chambers with fresh leaf material and with water and honey for the workers were connected to one end of the main corridor. At the other end was a box to which the ants could bring waste. Video cameras recorded the workers’ behaviour.

In experiments in which the fungus in every nest chamber was in good condition, the workers distributed the pieces of leaf evenly among those three chambers: they delivered a similar amount to each one.

Undernourished

But in several trials, the researchers had starved the fungus in the center chamber by disconnecting it from chambers with pieces of leaf during two days before the experiment. The workers in that chamber had been unable to provide food for the fungus. As a result, the normal greyish-green top layer of the fungal mass had vanished.

In these experiments, the ants brought much more pieces of leaf to the center chamber than to the other two. They probably noticed that the fungus crop in this chamber was in bad condition and demanded more food because of the smell it emitted. So, they tried to save the dying fungus garden with extra care.

Workers of Acromyrmex ambiguus, and probably also those of other leafcutter ants, are farmers that monitor how their crop is doing. If its condition deteriorates, they react appropriately.

Willy van Strien

Photos: two different fungus growing Acromyrmex species
Large: Acromyrmex octospinosus. Deadstar0 (Wikimedia Commons, Creative Commons CC BY-SA 3.0)
Small: Acromyrmex balzani. Alex Wild (Wikimedia Commons, Creative Commons, Public Domain)

Source:
Römer, D., G.P. Aguilar, A. Meyer & F. Roces, 2022. Symbiont demand guides resource supply: leaf‑cutting ants preferentially deliver their harvested fragments to undernourished fungus gardens. The Science of Nature 109: 25. Doi: 10.1007/s00114-022-01797-7

Common ragworm caches seeds and consumes the seedlings

Common ragworms bury seeds of cordgrass for future use: when the seeds have germinated, the worms eat the sprouts. Sprouting seeds is a newly discovered gardening strategy in animals, as Zhenchang Zhu and colleagues point out.

The seeds of cordgrass, Spartina species, are protected by husks that make them inedible for the common ragworm, Hediste diversicolor. Yet these worms take the trouble to drag the large seeds to their burrows and pull them inside. This is deliberate behaviour, according to Zhenchang Zhu and colleagues. The buried seeds will germinate and the worms can eat the sprouts. In fact, the seeds are a nutritious dietary supplement for the ragworms, which feed mainly on low-quality sedimentary organic matter, because they are high in protein and vitamins.

Waiting

Common ragworms live in the seabed of intertidal flats. Each animal inhabits a self-made burrow in the sand or the mud.The species is native to the north-east Atlantic.

In experiments, the researchers noticed that the ragworms wouldn’t eat intact cordgrass seeds. But if sprouted seeds were offered, they did eat them. Experiments also showed that the worms grew much better when given a diet including cordgrass sprouts than on a diet without sprouts.

It is striking that they cache the seeds, because that behaviour pays off only in the long term. The cordgrass produces seeds from October to March, and these seeds will germinate from April until July. So, the worms have to wait a few weeks or months before their stored food supply will be usable.

Seed dispersal

Sprouting seeds to consume the seedlings is a form of agriculture. More examples of agriculture in animals exist, such as the well-known fungus gardens of ants and termites. The sprouting strategy of common ragworms, however, differs from fungus gardening. While the fungus is the main food source for the ants and termites, the sprouts are a superior supplementary food for the ragworms: superfood instead of staple food.

Also, a mutual relationship exists between termites or ants and the fungus they grow: the animals are dependent on their crop, but the fungus is also dependent on its growers. Common ragworms and cordgrass, in contrast, have a predatory relationship, as the cached seeds are eaten after germination. The ragworms may help in seed dispersal, though. Buried seeds will not be displaced by water currents, retain their viability and can produce new plants when the ragworm that cached them dies or is eaten itself. This often happens, as common ragworms have many predators: birds like avocets and curlews, and fish like plaice and sole.

The authors suggest that common ragworms and their relatives may bury seeds of plants like seagrass and glasswort to sprout them as well.

Willy van Strien

Photo: Common ragworm. © Jim van Belzen

Source:
Zhu, Z., J. van Belzen, T. Hong, T. Kunihiro, T. Ysebaert, P.M.J. Herman & T.J. Bouma, 2016. Sprouting as a gardening strategy to obtain superior supplementary food: evidence from a seed-caching marine worm. Ecology 97: 3278-3284. Doi: 10.1002/ecy.1613