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Toenail fungus lost ability to reproduce sexually as it adapted to infect humans, new study finds

What’s the least sexy thing in the world? If you think it’s toenail fungus, you’re right on multiple levels.

A study by Duke scientists recently discovered that the fungus responsible for conditions like athlete’s foot and toenail infections may have lost its ability to reproduce sexually because it has adapted over time to infect human hosts. The findings were published in the online journal Genetics, on Feb. 21.

The researchers found that the organism Trichophyton rubrum belonged almost exclusively to a single mating type and would not mate with a different type—even when the scientists tried creating a sexy environment by lowering the lights, covering Petri dishes in plastic or flipping them upside down.

“Part of the effort was to sequence the genome,” said senior study author Joseph Heitman, James B. Duke professor and chair of molecular genetics and microbiology at the School of Medicine. “In this case, we wanted to take one species and really drill down on the broader population. To me, the most surprising finding is just how clonal they are.”

The study focused on the genetic makeup and sexual tendencies of the fungus by looking at 135 different Trichophyton rubrum samples from around the world. Researchers determined which mating type the fungus had and were surprised to discover that all but one of the samples had the same mating type.

Even after placing the Petri dishes in different conditions for five months, there was no evidence of coiled appendages that might contain spores, suggesting that the fungi were not sexually reproducing.

The researchers then sequenced the fungi genome and found that the populations were near perfect clones of each other with little variation. In fact, any two genomes were 99.97 percent identical. Other fungi such as Cryptococcus are only 99.36 percent identical, which means they are 21 times more diverse than the toenail fungus.

“This suggests that this species has adapted to human skin and does not need, or can’t mate with, another mating type to create diversity,” said Ted White, dean of the school of biological sciences at the University of Missouri at Kansas City and an author of the study.

Not being able to sexually reproduce is a problem, right? However, though these results mean that the species cannot diversify and may eventually become extinct, this probably will not happen for millions of years.

Knowing that this species may be asexual highlights potential vulnerabilities that could be targeted by new antifungal medications.

This is especially important given that about 25 percent of the world’s population is afflicted by fungal infections, most of which are caused by Trichophyton rubrum. Walking barefoot in showers and locker rooms or sharing personal items can led to the infections.

“Athlete’s foot is a unique human infection,” White said. “It remains dormant for long periods of time in the skin and then can be activated in times of stress or altered immune response.”

He explained that even if antifungal medication can control athlete’s foot for a period of time, people could still be infected again by surfaces like public shower floors.

“Most of the drugs that we have are over the counter, and even some of the prescription ones, keep the infection at bay but don't clear it,” Heitman said.

White noted that understanding the fungus may lead to new therapies that prevent the fungus from causing disease or even a vaccine that would boost the immune system so that the fungus can never cause disease again.

In the future, the researchers hope to learn about the interactions between the clonal fungus and the human immune system.

Heitman noted that dermatologists are becoming much more interested in the skin’s microbiome, or the microorganisms which reside on the skin.

However, one possible struggle in this research is convincing the public and scientific community of the importance of studying fungi. Even though athlete’s foot is the most common fungal infection and may be one of the top ten infections in humans, it rarely kills.

“Because it rarely kills, the National Institutes of Health has made it clear that these fungi are not a priority,” White said. “Therefore, obtaining funding from the NIH to continue the studies will be difficult.”


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