November 1, 2011 | 2
If you had to guess which organism possesses sperm with 40,000 tails, what would you guess? Elephant? Whale? Chuck Norris? Would you have guessed that it belongs to a plant?
This is the sperm of Zamia roezlii. It has a flapper dress-like fringe of tens of thousands of flagella to turbo-charge its way to eggs.* That a plant should possess sperm with such horsepower is all the more surprising given that most plants have sperm with no tails at all. A pollen grain is actually a tiny haploid plant called a male gametophyte. In most flowering and coniferous plants a pollen grain produces two tailless sperm who travel down a burrowing pollen tube to reach the female gametophyte buried inside a cone or flower — no tails needed.
Ancestral plants tend to have swimming sperm because land plants evolved from aquatic green algae, where the swimming — and fertilization — was easy. A few ancestral plants like mosses still have sperm with tails and rely on a watery film on the surface of the plant for the sperm to get from Point A to Point B. But they are such lousy swimmers that some mosses resort to Plan B: they make special leaves folded into little splash cups, and when a raindrop lands in them, their architecture blasts sperm inside skyward in an attempt to Fed-Ex them to the vicinity of a receptive female. So turbocharged swimming plant sperm is definitely an oddity.
To which plant does this impressively-endowed sperm belong? A cycad.
OK, so what’s a cycad? This is a cycad.
Here’s what they typically look like in profile. These are mature, juvenile, and seedling cycads of the same species.
Cycads are old. Way old. They were the dominant plants during the Mesozoic, the age of the dinosaurs, as seen in countless ancient murals. At least that was the dogma for many, many years. But recently a widely reported studied was published that undermines that idea — if you look at it in a certain way. But before we come to that, let’s look at what cycads are and at their surprising biology.
Sometimes called sago palms, cycads are in no way palms. Palms are flowering plants. Cycads, like pine trees and juniper bushes, are gymnosperms — “naked seed” plants. In other words, they make seeds, but they don’t make flowers or fruit as a way of bringing those seeds into the world or sending them on their way. Seeds were a great evolutionary innovation for plants, because they allowed them to package a little plant embryo that has already begun to develop — perhaps even a little root and a few small leaves — with a little stored food in a dessication-resistant shell.
A seed is a way of giving a young plant a jump start on life and a little food for the road, not unlike the relatively contemporaneous innovation of amniote eggs in land vertebrates. Microscopic spores — the previous dispersal method still employed by mosses, lycopods, and ferns today — are much more prone to drying out or landing in a place far too inhospitable for germination. Seeds help increase the chance of success.
A cycad plant is a column or trunk of old leaf bases surmounted by a cluster of new leaves. Cycad leaves are wonderful to touch, if you ever get the chance. Strappy, leathery, tough, and often blue-green, there’s something delightfully primeval in the way they feel (although, it should be said, the much-older mosses have delicate, thin leaves**).
Cycads also have an interesting symbiosis with cyanobacteria, also called blue-green algae, just like some lichens. They make special “coralloid roots” that grow upward in a very un-root-like fashion. In return for feeding, housing, and hoisting their cyanobacterial symbionts above-ground inside these structures, the cyanobacteria provide the cycad with that most limiting of bio-nutrients: fixed nitrogen.
In addition to having super-charged sperm, the pollen of cycads has another interesting habit that they share with their close relative, Gingko. When their pollen arrive on a female cone (usually via weevil, beetle, or wind), they start to grow into an ovule. Once there, they send out a long tube into a part of the ovule called the nucellus and send out a haustorial structure that sucks up nutrients from the
host female. Haustoria are also used by parasitic fungi that penetrate their plant hosts.
In cycads, the haustorial pollen tube ends up destroying most of the tissue into which it penetrates, although this tissue is a) not part of the future seed and b) technically expendable because once a cone is fertilized it is not used again. But imagine, if you will, that every time human sperm landed in a uterus, they implanted in the uterine wall, sucked up nutrients and grew into little organisms of their own for a while, and only then released the gametes that would go on to fertilize the egg. That should give you the general, somewhat unnerving idea.
Like most gymnosperms, cycads make both male and female cones, but unlike many conifers, they bear only male or female cones on a given plant. That is, there are girl cycads and boy cycads. The female cones make seeds and the male cones pollen. In the photo at top are female cones with seeds inside. When you break them open, you often find brightly colored seeds, like this, or this:
Because cycads are gendered, that means it’s also possible to end up all alone. Indeed, as Robert Krulwich reported recently on his blog, that has been the sad fate of a cycad found in Africa 100 years ago and sent off to the Royal Botanic Gardens at Kew. It’s a boy. And unless someone finds a female to keep it company, it will be the last of its kind.
Cycads have received a bit of attention lately. Many cycads contain carcinogens or neurotoxins or both. In Oliver Sacks’s Island of the Colorblind, he relates his fondness for cycads (a love I share) and his frustrating investigation into a mysterious devastating neurological disease among the people of Guam that may be linked to their habit of pounding up seeds to make cycad flour. Lately, suspicion has fallen on the practice of eating bats who may have concentrated cycad neurotoxins in their bodies.
That brings us back to the question of their antiquity. Cycad fossils date back to at least the Permian, and possibly the Carboniferous, 320 million years ago, the age of giant coal-forming swamps and disturbingly large dragonflies. But they flourished during the Mesozoic — the “age of the dinosaurs” — and probably were dined upon, and certainly trod upon by dinosaurs. They may also have been the first insect pollinated plants when beetles took up the habit of eating — and generally living in, wallowing in, and spreading around — cycad pollen, a tradition they continue today. But a recent study in Science reported here, here, and here finds that, according to their DNA, the 300 modern species of cycad evolved a paltry 5-12 million years ago.
OK, so the species alive today were not those alive during the Mesozoic. I accept that. That, in of itself, that is interesting news — their diversity declined in the wake of the Big Impact, and seem to have rebounded and re-speciated during some sort of worldwide climatic change. But the authors make a big point about us not being able to call cycads “living fossils” because they are not the same species we find in fossils.
Hold up there. They still look like ancient cycads. They still make seeds like ancient cycads. Their tailed sperm still betray their ancient origin, and the ancestral condition of all land plants, presumably just like ancient cycads. Survey says: They’re still cycads. And cycads are still old. And so, in my opinion, when you visit cycads at botanic gardens or see them in someone’s yard, it’s still OK to think of them as ancient, and imagine these extraordinary creatures among the dinosaurs.
**OPTIONAL BIONERD TECHNICAL DIGRESSION AND SOAPBOX ALERT**
You learn quickly in biology, as in our lives, nearly everything is a gray area. Life is a continuum of form and function. We try to impose neat conceptual order on it to make it easier for us to understand and discuss, but no sooner have we done it than we find some (or many!) creatures that throw wrenches in the cogs.
Take, for example, the term “species”. If you really want to light a match in a room full of ideological fuel, ask a group of biologists to define that term. Better yet, study life long enough and you’ll realize “species” has completely different meanings among various groups of living things, and especially bacteria and archaea, where inter-species sex is common and species-defining mechanisms like reproductive or geographical isolation that work fairly well in animals are often meaningless.
I’m not saying we should ditch the concept “species” or even get rid of binomial nomenclature, the double-named system devised by Carolus Linnaeus for calling species by name (think: Homo sapiens). I think it still has great value for humans to think of living things in terms of species — many of which are easily discernable and very “real” — and to give them names and not (forgive the term) dehumanizing barcode numbers, as some have proposed.
BUT, perhaps we should not be so finicky about the term “living fossil” with regard to species. Perhaps cycads have changed a little bit from their days of the dinosaurs. Perhaps they’re not the same “species”. If they look more or less the same and are doing more or less the same things in the same ways, that’s good enough for me. They are still recognizable ancient forms, and they’re still alive.
I, for one, love “living fossils” because it helps make things vivid for non-scientists. It helps people engage their imaginations. Because unless you know their story, and their history, cycads might just seem like a boring palm-thingy, and a cycad fossil just “a leaf”. When you read that cycads are living fossils, and that something almost identical lived among dinosaurs and giant ancient dragonflies, and that today they possess perhaps the most well-endowed sperm on the planet, and that you can still touch their living, breathing descendants today, your eyes may be opened that they are so much more.
*I’m sensing some latent flagella envy among some elements of my reading audience. Kinda reminds me of those guys with three or four outboard motors clamped to the back of their motorboats, if you know what I mean.
** Technically, “phyllids“. Only vascular plants (those possessing conducting tissues for water and sugars like xylem and phloem) have true leaves, according to botanists.