Sperm racing: the tortoise and the hare.

When I explain plant evolution, I often use vertebrate evolution as an analogy for some of the key innovations that happened in land plants.  Like the land vertebrates' ancestors (fish), the ancestors of land plants lived in water (they were green algae).  Once they conquered the land, the earliest land plants (the bryophytes) were like the amphibians: they can live on dry land, but they need water for mating.  The seed plants acquired a kind of internal fertilization, because they use pollen grains to deliver their sperms right to the stigma or the ovule, where a pollen tube can take it the last few millimetres to the egg.  In this, the seed plants resemble the mammals.  However I take pains to stress that this is an analogy.  These plants are doing similar things to the animals for similar reasons, but in completely different ways.

Bryophytes are rather simple small plants that tend to grow in damp places.  There are three main groups: mosses, liverworts, and hornworts.  Mosses are probably the most familiar, because they're common on damp banks and in shady lawns.

A moss (Leptostomum inclinans), liverwort (Aneura sp.) and hornwort (Phaeoceros carolinianus).

When you look at mosses, it's easy to get the impression that they're delicate and don't stand up well to environmental stress.  They live in the shade and damp, and they have a range of adaptations that protect them from drying out.  These include a dense felt of rhizoids that trap water against the stems, small overlapping leaves, sometimes able to curl up to prevent drying, and fine hair-points to the leaves that cut down drying wind flow over the cushion.

But if you thought mosses were the weaklings of the plant world, you'd be wrong.  They can be tough, and some can be completely dried down and yet can still revive when wetted.  One of the easy ways to grow mosses is to dry them out, grind them into a powder, and sow the dry dust into a damp plant pot.  The cells will rehydrate and start to grow new moss plants.  They do this by protecting the internal structures of their cells from irreversible damage when they are dried, or by having mechanisms for quick recovery once they're rehydrated. 

Leucobryum candidum, a common forest floor moss in New Zealand.

Their weak point has always been seen as the stage in the life cycle when mating takes place.  Algae mate under water; they can shed their sperms into the water to swim off in search of eggs.  Seed plant sperms are protected from drying inside the pollen grain or the pollen tube.  But moss sperms must live in a surface film of water in a damp moss cushion, or run the gauntlet of a short shower of rain.  They're not released until there's significant water present and the assumption has always been that the vulnerable sperm cells are short-lived and must swim to an egg quickly.  They must achieve fertilization, or soon die trying as conditions dry out.

Now Sarah Eppley of Portland State University, her graduate student Erin Shortlidge, and PSU plant physiology professor Todd Rosenstiel have looked at the tolerance of moss sperm to the stress of drying out (Shortlidge et al. 2012), and we'll have to change the way we look at mosses.  They set up some experiments using the sperms of three common mosses, Bryum argenteum, Campylopus introflexus, and Ceratodon purpureus.

Bryum argenteum, Campylopus introflexus, Ceratodon purpureus (from Malcolm et al. 2009, with permission)

After sampling the plants in wild populations, they established cultivated populations under uniform conditions.  They learned to recognise the male structures (antheridia) and find out when each antheridium was ready to release its sperms.   

In some mosses the male structures, antheridia, are clustered in hundreds at the tips of the branches (Bill Malcolm, photo)

They could use this knowledge to collect and purify sperms from the moss plants for their experiments.  Using different dry-down rates and different lengths of time for drying and before rehydration, they were able to measure the sperms' tolerance to desiccation by looking for tell-tale signs of cellular damage.  The results show that in all populations of all three species, a similar proportion of sperms can survive desiccation and rehydration.  Usually, it's about one in five or one in six sperms that survive.  It doesn't make a lot of difference how quickly they're dried, although there was more variability in the slowly-dried samples.

Tufts of silvery Bryum argenteum, growing with Syntrichia sp.

In plants and animals that can survive almost total dehydration, one of the commonest ways to protect the cell structures from damage is by using sugars.  Trehalose is a sugar that's protective, especially in animals like brine shrimps or in fungi.  Shortlidge et al. (2012) added sucrose, another sugar that's common in bryophytes, to some treatments to see if it made a difference.  That's interesting too, because sucrose and other simple sugars are used by the female moss as a chemical trail that sperms use to find their way to the eggs, so if sucrose helps, maybe the females are helping the sperms not only to find their way, but to survive their journey.

Moss sperms approach the neck of a female archegonium (from Iowa State University)

If sugar was added at the time of rehydration of dried sperm, the result was no different from the control (where no sugar was added); it didn't enhance protection.  But if extra sugar was present during the earlier drying-down phase, a higher proportion of sperms recovered.  Also, more sperms recovered in higher doses of added sugar than in lower doses.  This suggests the sugar helped protect the cells from damaging effects of drying, but its presence in cells during the recovery stages perhaps made little difference.

This research is interesting because it shows mosses aren't as fragile and vulnerable as we might have thought, even at mating time.  It also suggests some interesting possibilities to study evolution and natural selection at the time of moss mating.  If individual mosses have variable sperm, and this paper suggests they do, why would they produce some vulnerable sperms and some tough and resistant ones?  In animals, it's been shown that there's a tradeoff between vulnerability and life span, so if there's a race among sperms to fertilise an egg, it'd be an advantage to have fast, but vulnerable sperms (like the hare in the old story of the tortoise and the hare).  But if plants are far apart, weather is unpredictable, or there's little competition among sperm donor mosses, then having slow long-lived sperm (like the tortoise) might be an advantage.  It seems mosses might be hedging their bets by producing both kinds at once.

But hang on a minute!  If you've studied the moss life cycle, you'll remember that the moss plant is a haploid with one set of chromosomes.  Its sperms are formed by mitosis, not meiosis as in animal sperm.  That means all its sperms are genetically the same, so how can they vary in physiology?  One way might be that although the dividing cells get identical nuclei, there might be differences in the cytoplasm or in mitochondrial activity.  In animals, it's been shown that sperm can help each other by offering co-protection, helping siblings to complete, and helping each other to move in water.  If that's the case in mosses, it makes absolute sense for a sperm to sacrifice itself for a genetically identical sperm from the same plant, because it's as closely related to its sibling's offspring as it would be to its own offspring.

About 60% of mosses, like this Polytrichadelphus magellanicus, have separate male (right) and female (left) plants (not to the same scale).

Although mosses are simple plants, their sex lives are only now coming to be understood, and they're increasingly becoming used as models for studying general ideas about the evolution of reproductive biology.

REFERENCES

Malcolm, B., Malcolm, N., Shevock, J., & Norris, D. (2009).  California Mosses.  Micro-Optics Press.

Shortlidge, E., Rosenstiel, T., & Eppley, S. (2012). Tolerance to environmental desiccation in moss sperm New Phytologist, 194 (3), 741-750 DOI: 10.1111/j.1469-8137.2012.04106.x