Because the soil layer was so thin, most hyphae, which usually grow and spread underground by releasing spores, were easily seen, giving the researchers an opportunity to observe where connections were being made in the mycelium. Early hyphal coverage was not too different between the X and circle formations. Later, each showed a strong hyphal network, which makes up the mycelium, but there were differences between them.
While the hyphal network was pretty evenly distributed around the circle, there were differences between the inner and outer blocks in the X arrangement. Levels of decay activity were determined by weighing the blocks before and after the incubation period, and decay was pretty even throughout the circle, but especially evident on the four outermost blocks of the X. The researchers suggest that there were more hyphal connections on those blocks for a reason.
“The outermost four blocks, which had a greater degree of connection, may have served as “outposts” for foraging and absorbing water and nutrients from the soil, facilitated by their greater hyphal connections,” they said in the same study.
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Fungal mycelium experiences what’s called acropetal growth, meaning it grows outward in all directions from the center. Consistent with this, the hyphae started out growing outward from each block. But over time, the hyphae shifted to growing in the direction that would get them the most nutrients.
Why did it change? Here is where the team thinks communication comes in. Previous studies found electrical signals are transmitted through hyphae. These signals sync up after the hyphae connect into one huge mycelium, much like the signals transmitted among neurons in organisms with brains. Materials such as nutrients are also transferred throughout the network.
Tracing the lineages of agricultural ants to their most recent common ancestor revealed that the ancestor probably lived through the end-Cretaceous mass extinction—the one that killed off the dinosaurs. The researchers argue that the two were almost certainly related. Current models suggest that there was so much dust in the atmosphere after the impact that set off the mass extinction that photosynthesis shut down for nearly two years, meaning minimal plant life. By contrast, the huge amount of dead material would allow fungi to flourish. So, it’s not surprising that ants started to adapt to use what was available to them.
That explains the huge cluster of species that cooperate with fungi. However, most of the species that engage in organized farming don’t appear until roughly 35 million years after the mass extinction, at the end of the Eocene (that’s about 33 million years before the present period). The researchers suggest that the climate changes that accompanied the transition to the Oligocene included a drying out of the tropical Americas, where the fungus-farming ants had evolved. This would cut down on the availability of fungi in the wild, potentially selecting for the ability of species that could propagate fungal species on their own.
This also corresponds to the origins of the yeast strains used by farming ants, as well as the most specialized agricultural fungal species. But it doesn’t account for the origin of coral fungus farmers, which seems to have occurred roughly 10 million years later.
The work gives us a much clearer picture of the origin of agriculture in ants and some reasonable hypotheses regarding the selective pressures that might have led to its evolution. In the long term, however, the biggest advance here may be the resources generated during this study. Ultimately, we’d like to understand the genetic basis for the changes in the ants’ behavior, as well as how the fungi have adapted to better provide for their farmers. To do that, we’ll need to compare the genomes of agricultural species with their free-living relatives. The DNA gathered for this study will ultimately be needed to pursue those questions.
Enlarge/ Fungus samples are seen on display inside the Fungarium at the Royal Botanic Gardens in Kew, west London in 2023. The Fungarium was founded in 1879 and holds an estimated 380,000 specimens from the UK.
It’s hard to miss the headliners at Kew Gardens. The botanical collection in London is home to towering redwoods and giant Amazonian water lilies capable of holding up a small child. Each spring, its huge greenhouses pop with the Technicolor displays of multiple orchid species.
But for the really good stuff at Kew, you have to look below the ground. Tucked underneath a laboratory at the garden’s eastern edge is the fungarium: the largest collection of fungi anywhere in the world. Nestled inside a series of green cardboard boxes are some 1.3 million specimens of fruiting bodies—the parts of the fungi that appear above ground and release spores.
“This is basically a library of fungi,” says Lee Davies, curator of the Kew fungarium. “What this allows us to do is to come up with a reference of fungal biodiversity—what fungi are out there in the world, where you can find them.” Archivists—wearing mushroom hats for some reason—float between the shelves, busily digitizing the vast archive, which includes around half of all the species known to science.
Enlarge/ Fungarium Collections Manager Lee Davies inspects a fungus sample stored within the Fungarium at the Royal Botanic Gardens in Kew, west London in 2023.
In the hierarchy of environmental causes, fungi have traditionally ranked somewhere close to the bottom, Davies says. He himself was brought to the fungarium against his will. Davies was working with tropical plants when a staffing reshuffle brought him to the temperature-controlled environs of the fungarium. “They moved me here in 2014, and it’s amazing. Best thing ever, I love it. It’s been a total conversion.”
Davies’ own epiphany echoes a wider awakening of appreciation for these overlooked organisms. In 2020, mycologist Merlin Sheldrake’s book Entangled Life: How Fungi Make Our Worlds, Change Our Minds, and Shape Our Futures was a surprise bestseller. In the video game and HBO series The Last of Us, it’s a fictional brain-eating fungus from the genus Cordyceps that sends the world into an apocalyptic spiral. (The Kew collection includes a tarantula infected with Cordyceps—fungal tendrils reach out from the soft gaps between the dead insect’s limbs.)
While the wider world is waking up to these fascinating organisms, scientists are getting to grips with the crucial role they play in ecosystems. In a laboratory just above the Kew fungarium, mycologist Laura Martinez-Suz studies how fungi help sequester carbon in the soil, and why some places seem much better at storing soil carbon than others.
Soil is a huge reservoir of carbon. There are around 1.5 trillion tons of organic carbon stored in soils across the world—about twice the amount of carbon in the atmosphere. Scientists used to think that most of this carbon entered the soil when dead leaves and plant matter decomposed, but it’s now becoming clear that plant roots and fungi networks are a critical part of this process. One study of forested islands in Sweden found that the majority of carbon in the forest soil actually came from root-fungi networks, not plant matter fallen from above the ground.
Enlarge/ Mature morel mushrooms in a greenhouse at an agriculture garden in Zhenbeibu Town of Xixia District of Yinchuan, northwest China’s Ningxia Hui Autonomous Region.
True morel mushrooms are widely considered a prized delicacy, often pricey and surely safe to eat. But these spongey, earthy forest gems have a mysterious dark side—one that, on occasion, can turn deadly, highlighting just how little we know about morels and fungi generally.
On Thursday, Montana health officials published an outbreak analysis of poisonings linked to the honeycombed fungi in March and April of last year. The outbreak sickened 51 people who ate at the same restaurant, sending four to the emergency department. Three were hospitalized and two died. Though the health officials didn’t name the restaurant in their report, state and local health departments at the time identified it as Dave’s Sushi in Bozeman. The report is published in the Centers for Disease Control and Prevention’s Morbidity and Mortality Weekly Report.
The outbreak coincided with the sushi restaurant introducing a new item: a “special sushi roll” that contained salmon and morel mushrooms. The morels were a new menu ingredient for Dave’s. They were served two ways: On April 8, the morels were served partially cooked, with a hot, boiled sauce poured over the raw mushrooms and left to marinate for 75 minutes; and on April 17, they were served uncooked and cold-marinated.
The mystery poison worked fast. Symptoms began, on average, about an hour after eating at the restaurant. And it was brutal. “Vomiting and diarrhea were reportedly profuse,” the health officials wrote, “and hospitalized patients had clinical evidence of dehydration. The two patients who died had chronic underlying medical conditions that might have affected their ability to tolerate massive fluid loss.”
Of the 51 sickened, 46 were restaurant patrons and five were employees. Among them, 45 (88 percent) recalled eating morels. While that’s a high percentage for such an outbreak investigation, certainly enough to make the morels the prime suspect, the health officials went further. With support from the CDC, they set up a matched case-control study, having people complete a detailed questionnaire with demographic information, food items they ate at the restaurant, and symptoms.
Mysterious poison
Forty-one of the poisoned people filled out the questionnaire, as did 22 control patrons who ate at the restaurant but did not report subsequent illness. The analysis indicated that the odds of recalling eating the special sushi roll were nearly 16 times higher among the poisoned patrons than among the controls. The odds of reporting any morel consumption were nearly 11 times higher than controls.
The detailed consumption data also allowed the health officials to model a dose response, which suggested that with each additional piece of the special roll a person recalled eating, their odds of sickness increased nearly threefold compared with people who reported eating none. Those who ate four or more pieces of the roll had odds nearly 22.5 times higher. A small analysis focusing on the five employees sickened, which was not included in the published study but was noted by the Food and Drug Administration, echoed the dose-response finding, indicating that sickness was linked with larger amounts of morel consumption.
When the officials broke down the analysis by people who ate at the restaurant on April 17, when the morels were served uncooked, and those who ate at the restaurant on April 8, when the mushrooms were slightly cooked, the cooking method seemed to matter. People who ate the uncooked rather than the slightly cooked mushrooms had much higher odds of sickness.
This all strongly points to the morels being responsible. At the time, the state and local health officials engaged the FDA, as well as the CDC, to help tackle the outbreak investigation. But the FDA reported that “samples of morel mushrooms collected from the restaurant were screened for pesticides, heavy metals, toxins, and pathogens. No significant findings were identified.” In addition, the state and local health officials noted that DNA sequencing identified the morels used by the restaurant as Morchella sextelata, a species of true morel. This rules out the possibility that the mushrooms were look-alike morels, called “false morels,” which are known to contain a toxin called gyromitrin.
The health officials and the FDA tracked down the distributor of the mushrooms, finding they were cultivated and imported fresh from China. Records indicated that 12 other locations in California also received batches of the mushrooms. Six of those facilities responded to inquiries from the California health department and the FDA, and all six reported no illnesses. They also all reported cooking the morels or at least thoroughly heating them.
