For Mosses, Bigger Is Not Better

After more than a century of speculation by biologists, a lab test has shown that mosses have their own animal-courier system for sperm that’s similar to pollination, researchers say.

Mosses don’t package their male gametes in pollen, as flowers do, but rely instead on swimming sperm. Textbooks state that moss sperm need to swim or splash to a female moss tuft. However, an experiment with a common moss species shows that sperm hitchhike on mites and tiny insects, says Nils Cronberg of Lund University in Sweden.

Those couriers aren’t just scuttling around randomly. In lab tests, they preferred visiting fertile moss tufts instead of sterile ones, Cronberg and his colleagues report in the Sept. 1 Science.

“This paper is pretty exciting,” says Lloyd Stark of the University of Nevada, Las Vegas, who’s studied moss reproduction. The work “opens up a field that has been closed off up to now.”

Moss reproduction offers many surprises. While most animals and flowers carry a double set of chromosomes, the plush, mossy cushions on stream banks and sidewalks have just a single set.

In about half of moss species, male and female organs grow on separate plants. When moss sperm reach a plant that bears an egg, an offspring typically sprouts as a wiry filament right on top of mom. Changing course from the previous generation, this one has a double set of chromosomes and reproduces without sex. It forms spores that waft away to start another moss cushion elsewhere.

One of the most challenging steps in this cycle, the sperm transfer between immobile parents on land, has intrigued biologists. Occasionally, female mosses have sprouted the next generation with no males apparent within 10 centimeters or more. That’s suspiciously far for sperm to swim, and biologists have mused that insects might have given the sperm a lift, says Cronberg. However, the scientists couldn’t rule out other explanations, such as a lucky raindrop splash or an undetected male nearby.

The chance to test the idea came when Rayna Natcheva of the Bulgarian Academy of Sciences in Sophia grew clones of males and females of the widespread moss Brjum argenteum. The clones, part of another experiment, had no intermingling of males and females.

The researchers paired moss cushions at various distances apart and surrounded them with plaster that absorbs all moisture. For some male-female pairs, the researchers dropped in several members of an arthropod species commonly found living with wild mosses, either the springtail Isotoma caerulea or the mite Scutovertex minutus.

After 3 months, mosses set 2 or 4 cm apart hadn’t reproduced unless arthropods were present. Mosses frequented by springtails reproduced more abundantly than did those inhabited by mites, which lumber around more slowly. Cronberg speculates that sperm somehow stick to insects’ or mites’ cuticles during transit.

In a separate test, both mites and springtails tended to move toward fertile moss shoots of either sex as opposed to shoots with no reproductive organs. Why this happens isn’t clear, says Cronberg.

“It’s a simple and elegant experiment that answers a simple and important question,” says bryologist Dale Vitt of Southern Illinois University in Carbondale.