When walking through woodland wilderness, your gaze will probably be drawn up to the majestic treetops instead of down to humbler organisms, such as moss and lichens, that carpet the forest floor.
But new research suggests that deceptively simple lifeforms like Cladonia stellaris, commonly known as the reindeer lichen, should not be overlooked, because it turns out these organisms are having a lot more sex than scientists initially expected.
This dirty lichen secret was revealed by the “first population genomic study” on C. stellaris, in which scientists compared the DNA sequences of 122 specimens across Québec, Canada, according to a study published on Friday in the American Journal of Botany.
The results exposed an unusually high level of genetic diversity for a species that was previously thought to reproduce almost entirely through asexual cloning, and hints that reindeer lichen exhibit “cryptic sexuality,” which is sex that is difficult to observe or quantify.
“We were basically not expecting to see that they have this gene exchange in the genomic data, so we have to assume there’s a lot of cryptic sex going on,” said Felix Grewe, the co-director of the Grainger Bioinformatics Center at the Field Museum in Chicago and a co-author of the study, in a call.
Lichens are a form of symbiotic partnership between fungus and algae: The algae lives inside the protective structures of the fungus, while the fungus feeds on carbohydrates forged by the photosynthetic algae. These “composite organisms” can reproduce both sexually, which involves the exchange of genetic information from two parents to produce a spore, and asexually, which occurs when one individual severs a twig (or “thallus”) that can grow into a clone of its parent.
Grewe and his colleagues, including lead author Marta Alonso-García, did not expect the reindeer lichen to be totally asexual, as many lichens spread spores to some degree. But they assumed that C. stellaris would be overwhelmingly asexual, because it is simpler for both the fungal and algal components to reproduce that way.
“It seems to be a much more complicated process for lichen to reproduce if they both go along that sexual way,” Grewe said.
The downside of asexual reproduction is that clones don’t travel as far as spores, which leads to asexual lineages becoming cloistered into specific geographic regions. For this reason, the team expected to see separate lichen families, mostly descended asexually, in different parts of the province.
“Our assumption was, from literature and from all this background, that they asexually reproduce, and that we would see kind of a genomic differentiation, in the northern part and the southern part” of Québec, Grewe noted. “Over time, if there's just no intermixing anymore, they have built their own species.”
Instead, the reindeer lichen families were mixing genes across all the sampled areas, which shows that spores—and therefore sex—play a much larger role in the propagation of this species, and its geographic distribution, than was previously understood. Alonso-García also identified spore-bearing sexual structures in about 10 percent of the sampled lichens, providing physical evidence that the populations are, indeed, getting frisky with each other.
“There's some form of cryptic sex, which is not really visible but they still have sexual reproduction going on, which then causes them to really cross the barriers from the southern Québec area to the northern lakes and woodlands,” Grewe said.
The study also found that the lichen populations that re-emerged after a wildfire tended to be closely related to the population that had been there before the earth was scorched, rather than descendents of spores seeded from more distant populations. This finding, in addition to the cryptic sex revelation, could yield broader insights about forest ecology and conservation in the province.
Beyond those applications, the researchers hope to see what other surprises are hidden in the DNA of lichens, a group of lifeforms that seems so basic and yet contains multitudes.
“What I have always liked is that there's just so much that is not known about lichens,” said Grewe. “It's just really so much to discover.”