healthHealth and Medicine

Toenail Fungus Gives Up Sex To Infect Humans

Onychomycosis, or toenail fungus, caused by the microorganism Trichophyton rubrum. Marcel Jancovic/Shutterstock

If you’re concerned that a nasty case of athlete’s foot is hurting your dating game, take some comfort in the fact that the fungus causing the unsightly condition is also not getting any.

A study led by Duke University indicates that the microscopic organism Trichophyton rubrum lost its ability to mate as it evolved to live on the human body.


Thought to infect up to a quarter of the world’s population in the form of athlete’s foot, ringworm, jock itch or nail disease, T. rubrum is undoubtedly well-suited to munching on our dead skin cells while evading our attempts to kill it. But the process of acquiring highly specific adaptations often comes with a high cost – loss of genetic diversity.

Scientists already knew that all members of the string-like species are very closely related and that they appear to reproduce by asexual cloning only, despite the continued existence of two different mating types, or sexes. Yet since T. rubrum is so common and widespread, the researchers wondered whether or not occasional co-mingling is still occurring, away from the watchful eyes of microbiologists.

Their investigation, now published in the journal Genetics, analyzed the genomes of 135 T. rubrum samples collected from various continents. Interestingly, the results showed that all but one fungi belonged to the same mating type. Moreover, each individual filament was 99.97 percent identical to any other randomly selected individual; two phenomena that don't occur in a normal sexually reproducing organism.

An electron micrograph of Trichophyton, commonly known as toenail fungus. Wenjun Li and Joseph Heitman/Duke University, and Valerie Lapham/North Carolina State University

"Such incredibly high clonality across isolates from around the world is remarkable, and suggests that this organism is very well adapted to humans," said senior study author Dr Christina Cuomo in a statement.


To definitively test the microorganisms' sexual proclivities, the specimens were then placed in Petri dishes filled with a variety of opposite-sex individuals from a compatible subspecies. The match-making researchers even tried to get the mood just right by setting up culture conditions with varying lighting, petri dish orientation, and dish insulation.

After five months of waiting, however, it became clear that no amount of coaxing would do the trick. None of the T. rubrum had spore-bearing appendages, meaning no sexual reproduction had taken place.  

This is good news for people, but bad news for T. rubrum. Without sexual reproduction, evolution occurs at a staggeringly slow pace. Consequently, the fungus is unable to gain resistance to new treatments at the pace that we can develop them. Promisingly, several next-generation antifungals appear much more efficient than our older stand-by medications.

Just don’t expect the irritating infection to go away on its own.


"It is commonly thought that if an organism becomes asexual, it is doomed to extinction," said senior study author Joseph Heitman. "While that may be true, the time frame we are talking about here is probably hundreds of thousands to millions of years."


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