Mosses are a ubiquitous part of our environment, growing on trees, sidewalks, and in storm drains, coating rotting logs and forest floors. But even though we encounter them on a daily basis, it turns out that until just recently, biologists knew very little about how they reproduce. It turns out that mosses depend on itinerant visitors who help pass their reproductive goods from place to place. That is to say, when you come into contact with moss, you'll leave covered in moss sperm.
Mosses are notoriously bad at fertilization across even relatively short distances, but using an incredibly simple experiment, a group of researchers found back in 2006 that these plants have secretly been enlisting the aid of a host of microscopic insects for millions of years to help them reproduce. We’ve just never noticed before because the interactions taking place are so small. But in order to fully understand how these interactions work, we need to take a step back, all the way to the first land plants.
Four hundred and seventy million years ago, a seemingly innocuous group of algae called charophytes made their way from the depths of the world’s oceans into the shallower freshwater bodies and lakes and streams, and from there onto land. These were the ancestors of all modern land plants. The first group to evolve were bryophytes (the group that contains mosses, as well as the similar liverworts and hornworts).
The biggest problem these early terrestrial plants faced was the lack of a constant water supply. They had no complex vascular tissue to transport water and nutrients between cells, which meant that each cell had to obtain those resources solo. Partially as a result, the first plants were small and restricted to moist areas.
Lack of water also posed a significant barrier to reproduction. Like their algal ancestors, the first land plants produced motile sperm that required an unbroken film of water to swim to a nearby egg. In the oceans, this hadn’t been much of a problem. But on land, it further restricted where the first land plants could successfully reproduce.
Today, modern bryophytes still reproduce the exact same way as their early ancestors, and yet, with roughly 20,000 species, they’re the second largest group of land plants on the planet (second only to flowering plants) and can be found growing on every continent, including Antarctica. How did a group that relies so heavily on water for reproduction become so successful on land?
One way is by being exceptionally good at asexual reproduction. One of the oldest living organisms on Earth is a peat moss in Hawaii that has been asexually reproducing since a single spore landed on the islands 50,000 years ago.
The other option, of course, is to reproduce sexually. This is where the interactions with microscopic insects come into play. Bryophytes make up forests of Lilliputian size that can sustain entire ecosystems of microscopic life. They glint with brightly-colored mites that play dead at the approach of danger and teem with springtails that can catapult into the air at a moment’s notice by means of a highly-coiled appendage. The indelible water bear, the most resilient organism on Earth (and which looks like a miniature bear with eight legs), also wanders through these dense forests, as well as much larger arachnids, such as spiders and harvestmen.
In the midst of this panoply, the bryophytes produce tiny reproductive structures separately containing the sperm and eggs, which all these tiny insects inevitably trample through in their search for food. In doing so, they inadvertently transport the sperm from one plant to the egg of another. This is exactly what the researchers discovered in 2006, except they found that it doesn’t appear to happen inadvertently or occasionally but all the time, and it might even be critical for successful fertilization in some species.
Using one species of moss (Bryum argenteum), they put males and females at different distances to each other using petri dishes in a lab and made observations on how often fertilization occurred (based on whether sporophytes were produced) both in the presence and absence of springtails. Even when male and female mosses were placed together, the samples with microarthropods had higher rates of fertilization, and when males and females were separated by just a few centimeters, fertilization only occurred when microarthropods were present.
The large difference between variables seemed to imply more than just a passive benefit from mites and springtails roaming around, so the researchers set up another experiment in which they observed whether microarthropods preferred traveling to the reproductive structures or to the sterile leaves of the moss. Again, the results were unambiguous: the mites and springtails were overwhelmingly attracted to the reproductive structures and even seemed to prefer the female organs to those of the males.
Six years later, another group of researchers discovered why it was that these insects were attracted to the sexual organs of mosses. The plants were actively emitting volatile organic compounds to lure them in, and the female mosses were producing significantly more of these scents than the males were. The reasons for this discrepancy are currently unknown and are being actively studied at the University of Florida.
This might remind you of another group of plants that use scents to attract insects that aid in reproduction. Flowering plants do this all the time, and the co-evolutionary history they share with insects is one of the main reasons they diversified into the largest group of plants on Earth, a growth that initially baffled Darwin. But flowering plants are relatively young, geologically speaking, having been around some 125 million years or so. The discovery that many bryophytes may be doing the same thing means that the first instance of animal-mediated fertilization may have been widespread 300 million years before biologists thought it even existed.