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Researchers discover evolutionary 'tipping point' in fungi

Scientists have found a “tipping point” within the evolution of fungi that throttles their growth and shapes them. The findings, published within the journal Nature, show how small changes in environmental aspects can result in large changes in evolutionary outcomes.

Fungi are nature's great composters. They wait throughout the forest floor to feed on fallen trees and autumn leaves, returning essential nutrients from these plants to the soil.

Although fungi often bring mushroom caps to mind, fungi even have underground “roots” called mycelia. Mycelia are made up of hundreds of interconnected, microscopic, finger-like cells called hyphae that grow in an intensive network. Hyphae worm their way up through the soil from their heads. To do that, they inflate themselves, just like the tall balloons used to make balloon animals.

Their elongated shapes allow the hyphae to search out and use nutrients throughout the soil. But not all hyphae are the identical shape: some have rounded ends, while others have pointed ends. The hyphae of water molds — fungi-like pathogens that cause crop damage — are particularly spiky.

“One of the major challenges in biology is identifying the specific evolutionary factors that determine the shape or form of an organism,” said Enrique Rojas, assistant professor of biology at New York University and senior writer of the study.

To understand the explanations for the various shapes of hyphae, Rojas and his colleagues combined theory and experiments to research fungi and water molds from across nature. They first used physics-based models of inflationary tip growth to find out all types of hyphae. Surprisingly, the shapes of hyphae present in nature assume only a small subset of the possible shapes.

The researchers hypothesized that the limited forms observed in nature reflected “survival of the fittest” and that many possible forms not seen in true fungi were, for some reason, weak evolutionary rejects. To explore this concept, they examined the expansion rate of hyphae with different shapes to create a fitness landscape for the hyphae.

“Our eureka moment was when we realized that the shape of hyphae is closely related to their ability to grow rapidly,” said Maxim O'Herway, a PhD student in NYU's Department of Biology and the study's lead writer. The writer is

A fitness landscape is sort of a topographic map that visualizes the evolution of an organism: each species moves through its fitness landscape by testing whether random mutations in its genes affect its growth rate, or fitness. Increase or not. A species stops its restless wandering only when a brand new mutation reduces its fitness — that's, when it's at peak fitness.

However, Rojas' team found that fitness landscapes will be much richer than a system of peaks and valleys. Indeed, they found that there's an overhanging cliff, or tipping point, within the fitness landscape for hyphae, and this acts as an evolutionary barrier, strongly constraining the types of fungal hyphae. Accordingly, they predicted that hyphae with morphologies near the tipping point could be particularly liable to small environmental, chemical, or genetic changes.

The researchers made their prediction by treating fungi near the tipping point with small amounts of chemicals that affected hyphal growth. They used a chemical that reduces the pressure contained in the hyphae and one other derived from a marine sponge that inhibits the hyphae's ability to move cellular components to the tip of the cell. Both treatments led to the identical dramatic effect: hyphae grew very slowly and with a wierd knob shape not present in nature.

“Our results illustrate the diversity of hyphal shape in a large, diverse and important species group,” Rojas said. “More broadly, they also demonstrate an important new evolutionary principle: that fitness landscapes can have instabilities, or tipping points, that impose strict constraints on traits as complex as biological form.”

The researchers imagine their findings have vital implications for our understanding of many ecological and evolutionary systems. For example, species whose evolution is subject to a tipping point could also be most vulnerable to a gradual increase in temperature resulting from climate change. Their findings can also aid in the event of latest antimicrobials against disease-causing fungi by identifying vulnerabilities of their growth related to an evolutionary tipping point.