To get sperm, female mosses attract microarthropods with sexy smells


Moss species used in the paper to identify the sweet smell that attracts sperm-carrying springtails to female mosses (Ceratodon purpureus). Image via Wikipedia.

Way back, before mosses and their brethren evolved, all sex happened in the water. This made things pretty easy for plants and other species that don’t have fancy internal fertilization like us. All they had to do was let their sperm go forth, into the ocean, and eventually they would swim to a suitable female plant and, voilà! Sex.

But mosses, liverworts, and hornworts (a group of plants called bryophytes) brought sex to the land–but how did they adapt to the lack of water for sperm transport?

As you probably know, flowering plants often have helpers to do their sexytime work for them. Pollinators (like bees, for example) carry pollen (which contains sperm) to a female flower, eliminating the need for water to carry the sperm. The plant gets away with this because it usually offers something to the pollinator in return: nectar, a place to live, or it might just trick it with sweet sweet smells (like those dirty orchids).

But moss doesn’t grow flowers. It’s not even all that pretty to look at. So how could it trick anyone into carrying its sperm around?

At first, scientists thought maybe bryophyte sperm only traveled in the small amount of water around it, from rain or whatever, and so it couldn’t get very far. This would lead to their being little patches of closely related mosses in a small area (like 10cm small).

A springtail. Image via insects.tamu.edu.

This turned out not to be true, and moss sperm can actually travel quite a bit farther–with the help of microarthropods, like springtails and oribatid mites.

The question then becomes: do these tiny bugs carry sperm just by accident, and only randomly happen to bring it to a female moss? Or might the plant somehow entice the little guys to bring the sperm hither, to a waiting female moss? Microarthropods are already known to use smells to communicate with each other and to find food–might there be smells that the moss uses to tell them where to go?

This brings us to a paper in press at Nature: Todd N. Rosenstiel and colleagues at Portland State University in Oregon wanted to know if female mosses emit smells that springtails like. If so, what are those smells?

As you might have already guessed, they did indeed find some sweet smells in these mosses. Female mosses emitted more than 3 times as many “volatile organic compounds”, or individual scents, as did males. Interestingly, many of these smells are ones that we would normally think of as floral, because they’re found in flowering plants.

Okay, so there are different smells for male and female mosses–but can springtails tell the difference? And do they even care?

This is why they’re called microarthropods–those itsy bitsy specks next to the penny are springtails.

In 2 different tests for preference, springtails preferred female mosses (or their smells) over males about 60-70% of the time.

Okay, so female mosses emit smells that springtails seem to like. Does having springtails around actually help the mosses have sex? To answer this, the researchers performed experiments with mini-habitats of mosses. They varied the amount of water and whether or not the habitat had springtails in it. Adding either a spray of water or springtails significantly increased the number of habitats with sporophytes (the plants produced by sex between mosses). If neither water spray or springtails were added, very few plants reproduced (about 5-10%). If one was added, this number went up to about 20%.

What about adding both a spray of water and some springtails? In this scenario, about 50% of the mini-habitats ended up with sporophytes.

These results show that microarthropods, like springtails, can definitely aid reproduction in ancient, non-flowering plants like moss. Since both microarthropods and bryophytes are ancient, it is possible that this relationship is also ancient–and much, much older than the more famous relationship between flowering plants and their pollinators.

There are still plenty of questions that remain. For example, which smells in particular are the springtails attracted to? Are any other species attracted to the same scents? And what is the springtail getting out of the arrangement? Is there are reward (like, do they eat the sperm?), or are they just clueless saps hauling moss sperm around for their mossy masters?

Next time you see a little green moss, which unless you have a special interest honestly doesn’t look like much, just think about all the kinky interspecies sexual escapades going on. Isn’t nature beautiful?

Reference: 
Rosenstiel TN, Shortlidge EE, Melnychenko AN, Pankow JF, & Eppley SM (2012). Sex-specific volatile compounds influence microarthropod-mediated fertilization of moss. Nature PMID: 22810584

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