The Secret Sex Lives of Lichens

Many biologists who make the journey to Antarctica go to study the continent’s most iconic animals, including emperor penguins and massive humpback whales. But one team, from the Grainger Bioinformatics Center at the Field Museum in Chicago, has a different to-do list. They want to check out the sex lives of patches of greenish-brown “dust” clinging to rocks.

These lumps, which at first glance resemble moss, are found around the world but are easy to overlook. Odds are, you’ve walked by some on a tree or cemetery headstone, or even sat on a smattering taking up residence on a park bench. These complex living things are known as lichen. They are a living partnership between fungi and algae or bacteria that get their energy from sunlight. They can be frilly, brightly colored, or in the case of the team’s research focus, dusty green and brown clumps of Lepraria, a genus of lichen. Despite their lackluster appearance, these lichens may be hiding a sexy secret: They may be more sexual than previously thought, according to a new paper in the journal BMC Genomics.

When it comes to lichen sex, things can get complicated. Some species reproduce sexually, in an erotic intertwining of subterranean thread-like networks, ultimately forming a “fruiting body” that bursts up through the earth and releases spores out into the world, much like a mushroom. This is a pretty risky way of reproducing for an organism that needs its spores to land somewhere they can pair up with an algae, says the paper’s lead author Meredith Doellman, a postdoctoral researcher in the Field Museum’s Grainger Bioinformatics Center.

To be safe, lichens have a back-up method of reproduction—a plan B, if you will. They can asexually split themselves, by budding off and regrowing new individuals genetically identical to the parent, like a sea star. While some lichen species reproduce only sexually, many others are capable of both sexual and asexual reproduction. Very few seem to be totally asexual. While that route is a better guarantee of procreation than sending your spores off into the wind, it doesn’t allow for the mixing of genes from two individuals, which is what drives the evolution of a species over time and keeps a population healthy.

“For lichens, it’s kind of an advantage to be asexual, because they don’t always have to find new algae [or bacteria],” says the paper’s senior author Felix Grewe, the Grainger’s director. “But then again, you need some form of sex to to maintain your genome.”

That’s what makes the new Lepraria paper so intriguing. Researchers have been studying the genus for more than 200 years. In that time, no one has ever found any sign of the structures other lichens use for sexual reproduction. Lichenologists have long assumed they were completely asexual. While there are a handful of other asexual lichens in the world, Lepraria was considered special for being the only completely asexual genus. “Lichenologists would have bet a lot of money on them being asexual,” says Grewe.

It was Lepraria’s reputation for being asexual that inspired Doellman, Grewe, and their colleagues at the Field Museum to study the genome of this unique group. “There aren’t very many purely asexual organisms, so we thought that this would be a really good opportunity to sequence and characterize an asexual genome,” says Doellman.

The team was shocked at what they found. “We didn’t think that it would have any of the genes involved in sexual reproduction, or at least not working ones, but it did,” Doellman says. “Surprise, they were all there!”

The genes associated with sexual reproduction weren’t just present, they were intact and functional, which means the lichens were engaging in some form of sexy time, even without the typical sexual organs. The scientists have a few theories for why this could be. The lichens could be what’s called parasexual: They still recombine genetic information from two different individuals, but in a way that’s different from traditional lichen sex. “This has been shown in a variety of different fungi, and it’s probably pretty important,” says Ian Medeiros, a postdoctoral fellow in the botany department at the National Museum of Natural History, who was not involved in the study. “It’s never been shown in any lichen, but there’s no reason why it couldn’t happen.”

While the real sex lives of Lepraria remain a mystery, this discovery broadens what we know—and don’t know—about the lichens all around us. At the end of the day, it’s a reminder of how essential sex is. Organisms are going to get it done, one way or another—even if in secret.