A Fungus That Causes Athlete's Foot May Be Cloning Itself

The fungi responsible for toenail infections and athlete's foot may reproduce by making clones of itself. According to a new paper published Wednesday in Genetics, the genome of the fungus responsible for these irritating conditions, Trichophyton rubrum, shows some telltale signs of asexual reproduction—which could be good news for people who want to develop new drugs to stop it.

There’s a reason sex is good, genetically speaking. Sexual reproduction leads to offspring that have different genetic codes from their parents. Keeping variety in a population is useful because it means that people will have different responses to potentially lethal threats. People, for example, may have different, genetically determined susceptibility to infections—so even if a plague strikes, it’s unlikely to actually kill us all. But by definition, organisms that reproduce by making clones of themselves don’t have this variety.

Nature gets that. “Most organisms are sexual reproducing,” Duke researcher Dr. Joseph Heitman told Newsweek—and that includes most fungi. “The conventional evolutionary theories are that asexual organisms are doomed to extinction. We don’t find very long branches in the tree of life where things are asexual.” But the fungus might want to keep things exactly as they are—after all, it's gotten very good at infecting human skin.

To figure out exactly what these fungi are doing, scientists have a few options. They can look for physical evidence—but that’s easier said than done. “It’s often hard to catch them in the act in laboratory conditions,” Heitman said. So after he and his colleagues failed to find the fungus in flagrante, they looked for genetic evidence, too.

Fungi don’t have X and Y chromosomes like humans, but they do have something similar, called the mating type locus. “It serves a similar function, in that there’s typically two mating types,” Heitman said. “For Trichophyton rubrum, they’re essentially all of one mating type.” (Of the 135 fungi included in the paper, 134 had the same mating type.)

But just because the fungi are mostly of one mating type doesn't mean that they can't still mix it up, genetically. So Heitman and his colleagues also looked for signs that genes have been recombining as they would in sexual reproduction—which did not exist.

Genes that the fungi need to sense and respond to pheromones that trigger the sexual cycle did still exist. So there could still be toenail fungi that are reproducing sexually. “It’s very hard to prove that something is strictly asexual,” Heitman said. “I think it is important to note that this is a hypothesis. There are certainly further and additional studies that need to be conducted.” But based on the work he's done, the fungus does seem to be just producing clones of itself.

If other studies come to similar conclusions, it could mean that drug companies should have a look at the genome to see if there are any useful hints for making more effective drugs. "It's extremely hard to get rid of it," Heitman said. But in theory, knowing more about the way the fungus works could reveal new pathways to target. “Having the genomic blueprint for the organism is the starting point."

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