The fungus that causes athlete's foot and other skin and toenail infections may have lost its ability to sexually reproduce as it adapted to grow on its human hosts, according to a Duke University team that analyzed 135 different Trichophyton rubrum samples, and found that nearly all belonged to a single mating type.
If this fungus can't sexually reproduce, it can't diversify, and if it can't diversify, that may mean its days on this planet are numbered, said Joseph Heitman, senior study author and professor and chair of molecular genetics and microbiology at Duke University School of Medicine.
The discovery that this species may be asexual—and therefore nearly identical at the genetic level—does highlight potential vulnerabilities that researchers could exploit in designing more effective antifungal medications. The findings appeared earlier this week in Genetics.
Though the findings suggested the fungi had become asexual, the scientists wondered if they would mate if given the opportunity. They placed the isolates in petri dishes along with a variety of potentially compatible mating types, under a variety of different conditions, and then waited to see if any magic would happen. After five months, the researchers looked at the petri dishes under a light microscope, scanning for the growth of coiled appendages that might contain spores. They didn't see any.
The team also sequenced the genome of the organism. They found that the organism is very clonal, with little variation from one genome to the next. Any two genomes of Trichophyton rubrum are 99.97% identical, which translates to three differences out of every 10,000 genetic letters. Other fungi such as Cryptococcus are only 99.36% identical.
"Such incredibly high clonality across isolates from around the world is remarkable, and suggests that this organism is very well adapted to humans," said Christina Cuomo, senior study author and a group leader for the Fungal Genomics Group at the Broad Institute of MIT and Harvard.
Because the fungus is so clonal and could also be asexual, its ability to adapt further may be more limited than some other fungi. Thus, any new strategies that researchers could develop against this species may have a higher chance of success than those targeting sexual species, which are more capable of mutating or amplifying drug-resistance genes.
Image: An electron micrograph of Trichophyton. New research shows the organism clones itself, making genetically identical daughters that may be vulnerable to new treatments. Image courtesy of Wenjun Li and Joseph Heitman, Duke University, and Valerie Lapham, North Carolina State University.