The mysteries of moss sex

Walk down any street in Portland, and you’ll see it. It covers roofs, tree trunks, sidewalks and fences. It’s part of what makes Portland look so magical, like a fairy tale forest full of history.

Moss.

Rocky Cookus/VANGUARD STAFf

A SPRINGTAIL, in close up, wades through moss leaves, aiding in the fertilization process.

What we don’t realize is that moss is one of the oldest plants on earth, and that for the past 400 million years it has been growing and reproducing with assistance from one of the world’s oldest land-dwelling animal species.

Like modern pollination relationships between flowering plants and bees, moss has long been manipulating tiny invertebrates called microarthropods to transfer sperm from male to female moss in order to sexually reproduce.

“This work excites me because it reminds me that mosses and insects have likely been dancing partners for a very, very long time,” said Todd Rosenstiel, a professor of biology at Portland State.

Mosses can be sexually separate, meaning that there is a physical distinction between male and female, and like other species it is the male that carries sperm. Originally it was thought that mosses had to live in a wet environment in order to successfully reproduce—places with consistent rain, like here in Portland—because the water would allow the sperm to swim from the male moss to the female moss.

Erin Shortlidge/VANGUARD STAFf

Erin Shortlidge, a PSU graduate student, looks closely at moss in Lassen Volcanic National Park.

What research is finding, however, is that microarthropods like springtails and mites are carrying the sperm from the male to female mosses.

“Mosses and microarthropods have been coexisting for about 450 million years—that is more than 300 million years longer than the flowering plants and their pollinators emerged,” said Erin Shortlidge, a PSU graduate biology student whose dissertation work focuses on the factors that influence sexual reproduction in mosses.

Erin Shortlidge/VANGUARD STAFf

WALK OF FAME: A tiny springtail walks across firemoss leaves, helping the plants reproduce.

The fact that researchers can see that there might have been an interaction between plant and animal life much earlier than previously thought raises an important question: “Could this interaction between plants and arthropods be one of the keys to plant life on earth?” asked Dr. Sarah Eppley, a PSU biology professor.

Using fire moss, a species of moss commonly found in Portland, Eppley and her team confirmed that, while a moist environment does improve moss fertility rates, mites and springtails will also deliver sperm to female mosses in both rainy and drier environments.

By walking through the moist moss, the springtails and mites dip their feet in droplets of water containing sperm. The sperm then latches onto their feet and legs, stealing a ride to the female moss, where the tiny arthropods will unknowingly deposit the sperm.

This discovery helped answer the question of how moss manages to reproduce in areas with minimal moisture, or in harsh environments where sperm may have trouble swimming to the female moss.

The lingering question was why the springtails and mites do this—how can they tell the difference between the male and female moss and why do they travel between the two?

“We got into the idea that there might be volatile chemicals—smells. That was the question: Why do the bugs go to the reproductive part of the moss, the part that has the sperm or the ovule?” Eppley said.

They discovered that the arthropods are drawn to the female moss by scent. In the lab they were able to separate male and female mosses, allowing springtails access to both guided only by scent, they consistently chose the female moss.

Using gas chromatography, technology that color-codes vapors, Eppley and her team were able to see the chemical compounds emitted by the male and female mosses and found that the female moss emits a much stronger odor than the male.

“That says there is this difference between males and females—the chemistry proves it’s true,” Eppley said.

This discovery sheds new light on the spread of plant life on earth. So much so that the scientific community chose to publish Eppley, Shortlidge and Rosenstiel’s research in the highly esteemed science journal Nature.

“[Mosses] are more than just rooftop annoyances and a scenic place to plant your gnome collection—they are this little, living, green thread that links us back to the earliest origins of
plants on Earth,” Rosenstiel said.

The next step in this research is to find out if this moss-arthropod relationship applies to multiple species of moss, and if it occurs in every environment. Shortlidge recently received a Doctoral Dissertation Research Improvement Grant from the National Science Foundation and a field research grant from the American Philosophical Society, which will help fund her study of mosses in harsh environments like Lassen Volcanic National Park in California.

“This work should remind us that plants have been using scent to manipulate animal behavior for a very long time,” Rosenstiel said. “After all, why do we give scented roses and lilies on Valentine’s Day if not to manipulate animal behavior?”