The mystery of secretly sexual lichens

The patches of lichen you’ve probably seen growing on tree trunks and park benches may be easy to overlook, but they’re actually some of the strangest living things in the world. Although they are sometimes confused with moss, lichens are miniature ecosystems composed of a fungus and algae or bacteria that can produce energy from sunlight, living together in a single body.

They don’t seem to follow the same biological rules as most of their peers, and scientists continue to discover new things about them. Concrete example: in a new study published in the journal BMC genomicsResearchers were shocked to discover that a type of lichen called Lepraria, long thought to be asexual, still has the genes that govern sexual reproduction. These lichens, contrary to what scientists have believed for decades, could have a secret sexual life that no one has been able to observe.

“Lepraria basically looks like greenish, grayish, brownish dust. It’s probably what you would typically think of as a lichen growing on a bench or rock: a little mossy, but not a moss,” says Meredith Doellman , a postdoctoral researcher in the Grainger Bioinformatics Center at the Field Museum and lead author of the paper.

“Scientists have spent over 200 years studying these things and they swear that none of the lichens that make up the genus Lepraria ever produce structures that allow sexual reproduction. So they assumed that these lichens were asexual.”

Fungi make up the majority of a lichen’s body, and lichens rely on their fungal parts to reproduce. Fungi can reproduce asexually by fragmentation or budding from the parent body, but they are also capable of sexual reproduction. Sex with mushrooms is complicated. The short version is that when the underground threadlike network of two compatible future fungal parents merge and share genetic material, they come together to build an above-ground structure called a fruiting body. (Mushrooms are probably the best-known fungal fruiting bodies.)

The role of the fruiting body is to disperse spores, which are like the fungal equivalent of seeds. These spores are dispersed by wind, water and animals, eventually landing somewhere where they can grow into fungal networks and start the process again.

Sexual reproduction in lichens follows a similar pattern. “A typically sexually reproducing lichen mates with another individual and produces fruiting bodies called ascomata. These ascomata release spores into the air and settle down to become new lichens,” says Doellman.

In two centuries of examining Lepraria lichens, scientists have never found ascomata. And although there are many asexual lichens in the world, Lepraria has long been considered a special genus of lichens without sexual reproduction. Most of the time, there is an asexual lichen species and a sexually reproducing sister species. Lepraria, as a genus composed entirely of asexual species, appears unique.

It was this hypothesis that led Doellman and his colleagues to the Field Museum research project.

“We thought we had a situation where we could do some interesting comparative genomics and show that Lepraria, unlike its closest cousin, Stereocaulon, had lost the ability to have typical fungal sex,” says Doellman. Scientists in the Field Museum’s Pritzker Lab took samples of Lepraria and Stereocaulon DNA collected around the world, from the Chicago Botanic Gardens to Antarctica.

“We assembled their genomes, annotated the genes, and looked for genes generally known to be involved in the cellular process of meiosis that only occurs during sexual reproduction. We looked for genes involved in the formation of fruiting bodies,” says Doellman. . “We expected to see that in Lepraria, these genes would be degenerated, no longer functional, or completely absent. But instead, we found the full complement, and they all appeared intact, functional, and almost exactly like their sisters. in Stereocaulon.

Evidence of sexual reproduction in Lepraria upends years of scientific observations.

“I was very, very surprised,” says Felix Grewe, director of the Grainger Bioinformatics Center at the Field Museum and lead author of the paper. “No lichenologist in the world would assume that these lichens have sex, and yet they have the genes for it.”

Although researchers have discovered that Lepraria has the genes associated with sexual reproduction, they still have not found fruiting bodies. “If they do occur, they are very rare. They have found a good way to hide from us,” says Grewe.

Another possible explanation is that Lepraria actually only reproduce asexually, but have retained the genes necessary for sex because those genes are useful for something else.

“It’s possible that they’re doing something like sexual reproduction, but that’s not the case. A sort of parasexual reproduction where they’re still recombining the genetic information, but in a different way,” Doellman says. “For future research, we could see if there are different mating types, and we could look at the genetics of Lepraria at the population level to see if it is consistent with asexual reproduction.”

The mystery of Lepraria’s sex life could help shed light on the broader picture of lichens’ identity as a partnership between a fungus and algae or bacteria capable of photosynthesis. For a fungal spore to turn into a new fungus, it must land in a hospitable environment.

For a lichen spore to transform into a new lichen, it must both land in a hospitable environment and capture the photosynthetic algae or bacteria it needs for nourishment. (For most, if not all lichens, the fungal partner has evolved such that it can no longer survive on its own without a photosynthetic partner to feed it.)

Sexual reproduction therefore constitutes a risk for lichens. This can be very lucrative, in terms of genetic diversity and evolutionary potential, which is why almost all known lichens do it. But if the lichen spores don’t land in an environment where they can easily grab a photosynthetic companion, then they’re in trouble.

“I see the advantage for a lichen to reproduce asexually by separating from its parent. You may not be able to spread that far, but you can take your photosynthetic partner with you,” says Grewe. “But sexual reproduction has other advantages. Numerous studies show that the cellular processes involved in sexuality contribute to the long-term stability of the genome, for example by repairing breaks in the genetic code.”

Although it’s not yet clear how Lepraria uses its surprisingly sexy genes, this study is “another piece of the puzzle to understanding how lichens work,” says Grewe. These humble dust-like organisms could help scientists better understand genes, sex and evolution itself.