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

Poppy Day

ANZAC Day is probably New Zealand’s most special day.  Like Remembrance Day in the UK, it's the day we remember the sacrifices our soldiers made in two world wars.  Like Remembrance Day, the symbol of ANZAC Day is the poppy.  Not only that, but it's often said that the battle of Gallipoli, which it originally commemorated, was when New Zealand grew up, whatever that means.

Wellington's superb art deco war memorial in Buckle Street.  The tomb of an unknown soldier is here.

 In the past, particularly during the Vietnam War, ANZAC Day was sometimes controversial.  This year the poppies themselves have been a bit controversial, because instead of being made in the sheltered workshops in Christchurch, the poppies on sale are mostly made in China.  Some people have suggested we should make our own, but give a donation to the Returned and ServicesAssociation anyway.

So what are they like, these ANZAC Day poppies?  Of course they're symbolic, not intended to be botanically correct, but how well do they do from a botanist's point of view?

The first ANZAC Day poppies I remember were from the late 50s or early 60s.  There were two kinds.  Either you could get a nice fabric 3D poppy for your donation, or you could get a slip of paper with a picture of a poppy printed on it.  The 3D poppies were interesting, because, although they were the right shade of red, they didn't look a lot like a real poppy.  Rather, they were more like a Californian poppy (Eschscholzia californica) in shape, with a conical corolla.  

Californian poppy, Eschscholzia californica, Central Otago, New Zealand.

Later (maybe in the 1980s or 90s) they changed the design, to this:

In many ways, this is a more realistic poppy.  It has opposite petals and a dark black spot in the centre of the flower.  But, when you look at a real poppy it's not really like that.

Papaver rhoeas, Sete, southern France.

First, there are four petals, not two. Four is an unusual number of petals in flowers.  Monocots usually have three or six, and Eudicots mostly five.  Some Lamiales have four, mostly it seems because two of the five petals have fused together, as in Veronica.  The evidence for that interpretation is in the two sets of vascular bundles that supply the enlarged posterior petal and the quite common occurrence of a divided petal (strictly speaking, in families that have the petals united at their bases into a tube, each is called a corolla lobe rather than a petal). A few families are characterized by truly having four separate petals, e.g., Onagraceae (evening primroses, Fuchsia, etc) and Brassicaceae (the mustard family).  And of course the poppies and their relatives mostly have four petals. 

Long-headed poppy, Papaver dubium, Roca Grosso, Catalonia.

 Poppy flowers are interpreted as having two whorls of two petals, rather than one whorl of four like Brassicaceae.  This is because (1) the outer pair overlaps the inner pair, (2) the calyx has just two opposite sepals and development of calyx and corolla are often related, and (3) in a few poppies, like P. bracteatum, there has been a duplication of a whorl to give 6, not 8, petals.  [Note added later: Point 3 isn't well argued, because those poppies have two whorls of three petals, not three whorls of two.  So rather than have a whorl duplicated, each of the two whorls has increased the number of its parts from 2 to 3.  However, if it had been a single whorl of four petals, such an increase would give five (i.e., 4+1) rather than 6 (i.e., 2x(2+1))].  So the ANZAC poppy is botanically correct in having a pair of opposite petals, but wrong to have just one pair. 

Sepals fall early in poppies, as the flower opens (in Californian poppies they’re joined as a cap that pops off the opening flower), so you wouldn't put sepals on an ANZAC poppy.

The black spot in a poppy isn’t in the centre of the flower, but there’s one at the base of each petal.  The spot is a variable feature.  Sometimes, especially in cultivated Shirley poppies, it’s absent, or even white.  The makers of ANZAC poppies have used the black dot as a constructed feature that holds the corolla onto the pedicel (stalk) of the flower, but they would have done better by making it green to represent the ovary and stigma.  In the true poppies (Papaver), the stigma is a large flat or conical disk on top of the ovary, and the receptive surfaces (where the pollen adheres) radiate out like the spokes of a wheel. 

In flowers, the stamens are found between the petals and the ovaries, and in poppies there are a large number of them.  The stamen filaments are black, so they do add to the central black area.  Multiple stamens are hard to represent in a fake flower that has to be cheap, but a tuft of dark plastic strands would do the job.

Poppy flowers are fragile, and the petals and stamens soon fall.  They can be made to last a day or two in a vase if you pick them as buds before the stalk straightens (below on the left hand side) and singe the cut ends of the stalks (the milky latex sizzles and maybe it seals the vascular bundle although I don’t know why that would make the poppy last longer).  Later, the ovary swells and becomes the seed capsule.  Tiny pores open in a ring below the lobes of the stigma, and the seeds are shaken out through these.

Corn poppy, Papaver rhoeas, Kaikoura, New Zealand.

The poppy day poppy is modeled on the corn poppy, Papaver rhoeas, but other poppies are very similar.  In New Zealand the long-headed poppy, P. dubium, is quite common, along with a couple of other similar red-flowered species.  Iceland poppies have a range of flower colours.  These are all annual poppies, but there are perennials too, like the oriental poppies P. orientale and P. bracteatum and the orange-flowered P. atlanticum.

Papaver atlanticum, Dunedin Botanic Gardens, New Zealand.

Another poppy has a longer association with warfare.  The opium poppy (Papaver somniferum) is the only natural source of morphine, used for thousands of years as a pain killer.  All poppies have related alkaloid compounds.  Most of these are very unpleasant and dangerous poisons, but thebaine and codeine are used in medicine.

Opium poppies, Wiltshire, England.

Mugwort sex

Mugworts sound like something out of Harry Potter, maybe a sort of school for Muggles, but no, they're plants, and they have interesting sex lives.  

The mugwort genus Artemisia is a large one, with about 400 species, according to Mabberley's Plant Book (Mabberley 2008).  The genus belongs in the Asteraceae (sunflower family) and within that family it's classified in Tribe Anthemideae, along with chrysanthemums, cotulas, tansy, and yarrow.  Many Anthemideae have aromatic leaves, for example chrysanthemum leaves give the evocative smell of florist's shops.

Yarrow, Achillea millefolium.

 Artemisia is no exception.  They're highly aromatic, like the soft silver A. arborescens that's grown as a low hedge in old New Zealand gardens.  A. absinthium is wormwood, which contributes mostly flavour to absinthe and A. dracunculus is the herb tarragon.

Silvery Artemisia arborescens at Makara, New Zealand.

Artemisias are wind pollinated, and this makes their flowers rather different from many in the family. 

Like the rest of the Asteraceae, Artemisia flowers are tiny florets clustered together in flower-like heads.  In many tribes, there are two types of florets: rays, the petal like outer ones, and disk florets, the tubular inner ones.  In Artemisia, the rays are very reduced, especially the corolla, which instead of being a long strap-like structure is just a very short tube that surrounds the style.

A typical daisy capitulum showing disk florets (top left and centre) and a ray floret (right)

 In many Asteraceae, ray florets are female, that is they have no stamens, while the disk florets have both an ovary and stamens.  However in the disk florets, the five anthers are joined in a tube that surrounds the style, and the pollen is swept out of this tube by the stigma as the style elongates.  Thus, even if the disk florets function purely as males, they have to retain a functioning style and stigma to sweep out their pollen.

Female ray floret (left) and hermaphrodite disc floret (right) of Artemisia vulgaris (from Garnock-Jones 1986).  Note the pollen-sweeping brushes on the ends of the hermaphrodite stigma, and the reduced corolla of the female floret.

Insect-pollinated flowers can combine male and female functions easily.  Their male function is dispersing pollen, which is done by attracting an insect such as a bee.  Their female function, receiving pollen, is also achieved when an insect visits.  Sometimes a single visit is enough, but in other plants the two functions are separated in time and two or more visits might be needed.

Wind pollinated flowers have a problem.  Their best strategy for dispersing pollen is for the flowers to be high up and facing downwards, whereas their best strategy for receiving pollen is to be low down and facing upwards, to receive the pollen as it floats down from above.  This is thought to be the reason why many wind-pollinated plants have separate male and female flowers.

Some wind pollinated daisies have separate male and female heads, but in Artemisia vulgaris they're combined, with outer female ray florets and inner hermaphrodite disk florets.  At flowering, the heads hang down, so the pollen can simply fall and be carried away by the breeze.  But the outer female flowers are also hanging down, which is not ideal.  They get around this handicap by having styles that curve around to point upwards on the outside of the head, a much better position for catching pollen.

Several flower heads of Artemisia vulgaris, showing the stigmas of ray florets curving around the outer bracts to face upwards, while the central disk florets disperse their pollen downwards.

 Both kinds of floret set fruit, but the female florets set more fruit, typically 2-3 times as many as the hermaphrodites.

Overall, the investment in male and female structures is quite uniform throughout the populations.  This means there is no tendency for some plants to produce more female florets than others do.  However, when it comes to fruiting, it's clear that the larger plants, those that are taller and have more stems, produce more fruit per 100 florets than small plants do.  Even though their flower ratios are much the same, the smallest plants produce very few fruits.  We don't know if they also father very few fruits, but given that they provide a short platform to disperse pollen from, that seems possible.  However overall, the larger a plant is, the higher its fruit output, i.e., the more female it appears to be.  Even with quite low percentages of florets producing fruits, a large plant can produce 3–5 million of them in a season.

The significant relationship between increasing fruit output (expressed as femaleness where a score of 1 is an exclusively female plant and 0 is an exclusively male plant) and plant height (left) and number of stems (right) in a population of A. vulgaris from Denmark (from Garnock-Jones 1986)

Other species have different strategies.  The coastal A. maritima has lost its ray florets, retaining only the hermaphrodite disk florets, whereas in A. campestris the disk florets are strictly male because they never set fruit.

Later in the season, when it's time to disperse the one-seeded fruits, the flower heads have turned upright.  The fruits have no plumes for wind dispersal, so maybe they need a vigorous shake from a gust of wind to dislodge them, a strong enough gust to blow them a short distance away from the parent plant.  If the heads stayed hanging, the fruits might simply fall to the ground underneath.

Heads of A. vulgaris at fruiting time; their stalks have straightened and now hold the heads erect.

Artemisia shows that even when plants are hermaphrodites (or cosexual to use the specialist term), individuals can still vary in their reproductive functions.  That's one of the ways that plant sex is interesting.  New Zealand botanist (and my PhD supervisor) David Lloyd used another member of Anthemideae, the largely New Zealand genus Leptinella, to work a lot of this out in the 1970s.

References.

Garnock-Jones, P.J. 1986.  Floret specialization, seed production and gender in Artemisia vulgaris L. (Asteraceae, Anthemideae).  Botanical Journal of the Linnean Society 92: 285–302

Mabberley, D.J. 2008.  Mabberley's Plant Book (3rd ed.). Cambridge University Press.

New New Zealand conservation blog

I don't write much about conservation on Theobrominated.  It's not that I don't care about it; I do.  But I'm not an expert and my main passions lie elsewhere.  But here's a new blog about New Zealand conservation by two passionate experts, so go on over there and have a look.

Meanwhile, to get you in the mood here are a couple of rare plants.

Veronica scrupea

Veronica scrupea is known from only a few populations in the Seaward Kaikoura ranges, where it grows on shattered argillite.  There's actually a lot of suitable habitat, so I don't think this is actually a rare plant, but those sites are impossible to get at because the rock is to crumbly to climb.

Unnamed Ranunculus, N. Otago.

This buttercup is known from a single limestone outcrop on farmed land in North Otago.  It hasn't been named yet, but it has distinctive rigid leathery leaves.  One of the biggest potential threats to conservation in countries like New Zealand is that we simply haven't completed the inventory of what we have here.  Thus plants and animals can go extinct before anyone discovers they're at risk and is enabled to do something about it.

Is biology zoonormative?

A geneticist colleague once told me a story about hiking with another biologist.  The forest was quiet and still, from the emergent podocarps and the tawa canopy right down to the ferns and mosses on the forest floor (sadly often the case in New Zealand forest since introduced mammals ate most of our native birds).  The other biologist's reaction to this was to say, "It's so quiet; there's nothing alive here!" To what extent is our thinking and teaching in biology zoonormative?

Nothing alive here?

Zoonormative is a word I use to describe how biologists think that what animals, and in particular humans, do is normal in biology.  Botanists, mycologists, phycologists, and bacteriologists are constantly trying to explain to students that their understanding of how animals do things isn't really a good picture of how life as a whole, in all its glory and diversity, really works.  And if students can understand the true diversity of ways to make a living in this world, well, they should see animals in a more realistic light.  Nothing demonstrates zoonormativity more than sex.

Most people think sex is all about mating, or copulation, between a male and a female.  We talk about "having sex".  That's a very zoonormative point of view.  Sex doesn't need males and females.  It doesn't even need copulation.  All sex needs is meiosis and fertilization.  Meiosis is a special kind of cell division that halves the number of chromosomes in a cell.  Strictly, it doesn't just halve the number, but because chromosomes in a diploid cell occur in pairs, meiosis separates the two members of each pair of chromosomes into daughter cells.  It also mixes up the different copies of the genes on the two chromosomes of a pair.  Fertilization is the reverse of halving the chromosomes: two such haploid cells (gametes) unite to form a new diploid cell, the zygote, that has two sets of chromosomes, one set from each gamete.

In familiar animals, there are two types of gamete, eggs (large, resource-rich, and produced in relatively low numbers) and sperms (small, resource-poor, and produced in relatively large numbers).  By convention we call eggs female and sperms male.  But the gametes can be all the same.  Many fungi and some algae produce gametes that don't specialize as either male or female, so here we have a simple sort of sex that doesn't have males and females. (Strictly speaking there are usually plus and minus mating strains, such that a plus and another plus can't unite, but the point is they're not physically and behaviorally different.)

In the unicellular green alga Chlamydomonas, for example, the cells are generally haploid, so when mating occurs, two of these cells (gametes) simply (well, it's not simple really) join together to make a single diploid zygote, which is a functioning diploid Chlamydomonas.  When such a cell divides by meiosis, the haploid stage is restored.  Yeasts (unicellular fungi) behave in much the same way.  Such organisms are isogamous, meaning their gametes are the same.

In other algae, usually multicellular ones, there's specialization of the gametes as male (small, copiously produced, resource-poor) or female (large, few, resource-rich), and usually specialization of the structures they're produced in as well.  This is anisogamy, and it has evolved many times in separate lineages of animals and plants.  In Chara, cells of the algal body are haploid (one set of chromosomes in each cell), so it's easy for some cells to become gametes: they are already haploid and they simply specialize.  Specializing seems to help the gametes to find each other.  If one (egg) stays put and sends out a signal and the other one (sperm) can move and follow that signal, it's more efficient than both constantly moving in the hope of a random collision.  (Think of it like two friends downtown on a Saturday night; one texts the other and says, "I'm at Molly Malone's pub"; the other goes there and they meet).

When the gametes fuse and make a zygote, in many simple plants the zygote divides straight away, or immediately after a resting period, by meiosis to make haploid cells again.  Thus algae like Chara don't have a multicellular diploid stage; their only diploid cell is a zygote.  Back in the third paragraph (above), I introduced the term diploid.  I was tempted to add a couple of words to explain it, and those words included saying normal cells are diploid.  That's zoonormative thinking.  I hope you can see that being diploid certainly isn't the normal state for a Chlamydomonas or a yeast.