Sexual reproduction in flowering plants is often mediated by the birds & the bees (& other animal agents), but up until now the life cycle has appeared much simpler in plants like the mosses. Until fairly recently it was generally accepted that moss sex was a case of 'just add water': this released sperm from the male plants which could then swim in the film of water to where the female plants held their eggs. Of necessity this would mean that sperm dispersal could be only over quite short distances, of a few centimetres at most.
However, Todd Rosensteil and his colleagues (2012) decided to confirm the hypothesis that arthropods known as springtails could be involved in transferring sperm between male and female mosses. (Springtails and mosses evolved at the same time, during the Ordovician period.) They posed a number of questions: were springtails really acting as go-betweens in moss sex? If the answer was 'yes', how did the moss plants attract their little helpers? And, were the springtails important only if there was not much water around?
Using a common - & cosmopolitan - moss called Ceratodon purpureus, Rosenstiel and his colleagues first determined that female C.purpureus plants emit a significantly greater number of volatile organic chemicals (VOCs), which could act as signals to springtails, than male mosses do. They then carried out a number of experiments.
First of all, they gave springtails a choice between male and female moss plants - the tiny arthropods were much more likely to go for the female plants. (However, it's not yet clear why the springtails respond positively to this signal: do they get some sort of a food reward?) The same was true when the springtails were given no visual cues & were simply offered a choice between male and female moss VOC samples.
Then, they set up a series of 'microcosms' - miniature ecosystems containing moss plants, and where the presence of water and springtails could be manipulated. This time the research team used both C.purpureus and another moss species, Bryum argenteum, in which earlier work had shown that springtails were implicated in spreading sperm around. Some of their microcosms had only the mosses. Others were sprayed with water but had no springtails, or had springtails but no water spray. And some had both springtails and water. The results were fascinating.
When a female moss plant's egg is fertilised, the resultant zygote grows into a thin brown stalk with a capsule of spores on top: this structure is called a sporophyte. Unsurprisingly, mosses in the absence of both water and springtails produced very few sporophytes indeed. Both the 'springtail treatment' and spraying the mosses with water caused a marked increase in fertilisation, as measured by the number of sporophytes produced. But combining springtails and the water treatment saw the number of sporophytes more than double, compared to each treatment on its own. The researchers commented that
[t]hese results highlight the substantial role of microarthropods in facilitating fertilisation in mosses, presumably through enhanced sperm transport.
So maybe we really are looking at something akin to the relationship between flowering plants and their pollinators. And, given the potential antiquity of this arrangement,
it is important to consider the potential role that a plant-pollinator-like relationship may have had in shaping the evolutionary ecology of moss mating systems.
I will definitely be changing the 'additional reading' list for my first-years!
T.N.Rosenstiel, E.E.Shortlidge, A.N.Melnychenko, J.F.Pankow & S.M. Eppley (2012) Sex-specific volatile compounts influence microarthropod-mediated fertilisation of moss. Nature published on-line 18 July 2012, doi: 10.1038/nature11330