This Fungus Borrowed From Ancient Bacteria to Defy Gravity

That mold that looks like a Dr. Seussian forest growing on the rotting strawberry in your fridge: It’s probably a pin mold, a remarkable example of some of nature’s most overlooked innovations.

It’s related to a common fungus called Phycomyces blakesleeanus, a larger one, famous for its sensing abilities. It can respond to wind and touch, grow toward light and detect and navigate around objects placed above it. It senses gravity too — with crystals that move around inside single, but giant, elongated, spore-containing cells that resemble Truffula Trees.

“You can put that thing in a microscope — you don’t need a high-powered microscope — and you just see these beautiful crystals,” said Gregory Jedd, a geneticist who studies fungi at Temasek Life Sciences Laboratory in Singapore. But he wondered where they came from.

So in a paper published Tuesday in PLOS Biology, he and his colleagues determined that the crystals were likely the result of a gene that the molds’ common ancestor borrowed from bacteria long ago. Their findings highlight how nature finds weird ways to turn accidents into strengths through evolution.

Although quite different from one another, humans, plants and some fungi share gravitropism, the ability to know up from down. It helps us survive. By sensing Earth’s gravitational pull, humans can move around without getting dizzy and plants and fungi know how to grow to obtain nutrients and reproduce.

This behavior is made possible by varying gravity sensors that many organisms carry inside their bodies. A calcium carbonate crystal deep inside your ear brushes against hairs when you move, signaling up from down to your brain. In some plants, balls of starch slide around inside special gravity sensing cells like beads in a maraca, telling a plant or tree to reorient if it tilts sideways.

Many fungi with parts that pop out of the ground are thought to also have gravity sensors. Because fungi only send out spore-filled fruiting bodies when nutrients are low, ensuring they point to the sky is critical to survival so spores can disperse.

But most fungal gravity sensors are mysteries — except the crystal matrix of Phycomyces blakesleeanus. These dense bodies fall through the cytoplasm of spore-containing cells, signaling them to keep reaching toward the sky as they grow.

To determine the origin of this crystal matrix, Jedd and his team isolated the proteins that built them, homed in on one called OCTIN and traced it to a single gene. By looking for related organisms throughout evolutionary history with similar proteins, his team determined that a common pin mold ancestor likely acquired the gene from a bacterium that shared the same soil hundreds of millions of years ago.

This happened randomly, through a process called horizontal gene transfer. It allows an organism to “pick up a piece of DNA from a completely unrelated species and potentially use it for adaptive purposes,” Jedd said. If the adaptation aids survival, the organism passes it on to future generations.

How this happened in the exchange between ancient fungus and bacteria was unusual. In the bacteria, the gene couldn’t have produced a gravity sensor because the protein structures it made were too small. But the researchers showed that the proteins were capable of self assembling. Following additional mutations inside the fungus, that ability may have resulted in the crystal matrices that now help it know up from down.

“Those little nanostructures could cluster together, and in that way they could attain a size that could make them primitive or rudimentary gravity sensors,” he said.

Instead of creating a shared trait, the gene, with a few mutations, had created a novel one.

Jedd said understanding OCTIN and other self-assembling proteins could help with developing drugs that could know exactly where and when to dissolve in the body.

But there’s another potential application: When your housemates hound you for being a fridge slob, try telling them you’re observing gravitropism at work. Maybe it will charm them.