Diatom sperm go fishing for eggs

Close-up of a diatom. Image via Wikipedia.

Did you know that diatoms have sex? I didn’t. You know diatoms, those microscopic, silicon-encased, sorta algae things that live in the ocean…and basically anywhere else there’s water. They’re single-celled organisms that hang out at the bottom of the food chain and their dead, decomposed bodies (called diatomaceous earth) are used by humans for many things (like making nitroglycerine safer to handle).

I mean, look at that thing. It looks like a piece of glass. How exciting could its life really be? It turns out, many species of diatoms need sex to survive. And one species, Pseudostaurosira trainorii, has a really funky way of getting it on. The sperm from male P. trainorii cast out little threads that can act as fishing lines for grabbing eggs. Males and females of this species can also communicate, letting each other know when it’s time for sex. The research describing the secret sex life of these tiny creatures was published online in the journal PLoS One.

There are hundreds of species of diatoms, but they basically fall into two groups: the centrics (which evolved first) and the pennates (which evolved later). Centrics can be “male” or “female”, making either sperm or eggs. The sperm have a flagellum (sperm tail) so they can swim toward eggs. Most of the pennates, on the other hand, do not have tails and don’t even make sperm and eggs. Their gametes all look the same and are basically pretty boring, as far as I can tell.

There are two groups of pennates, however, that do make sperm and eggs. These are species that are more closely related to the centrics. But even though they make sperm, the sperm don’t have tails.

An obvious question is, where did the sperm tails go? Were they lost before the evolution of the pennates, or after?

The problem: no one has really bothered to look at how ancient pennates reproduce.

The authors of this paper wanted to fill this gap in diatom knowledge by looking at an ancient pennate species, P. trainorii. What they found was a totally new kind of sperm.

Casting off

Sperm of P. trainorii extruding a “thread”. Image from Figure 1 of Sato, et al. 2011.

The picture above looks like you might imagine most sperm to look. It has a head and a tail. But there are a couple of things that set this sperm apart from the rest.

First, that sperm has two nuclei instead of the normal one. Eggs also have two nuclei, so when an egg and sperm fuse, two nuclei have to be destroyed before a new diatom can be made. Why do it this way? I have no idea.

Second, that is no tail. Instead, the sperm extrude little “threads” that can’t actually move on their own. The sperm puts out a thread and then winds it back up to help itself move. The sperm put out threads in random directions, spinning around like tops, until they get near an egg. Check out movie S1 in the paper (scroll down after clicking the link).

If the sperm is lucky, it might hit the egg with its sticky thread and reel it in like a fish. The sperm and egg can then fuse and make a new diatomaceous baby.

Look at the bottom left of panel A. The top blob is a sperm that’s sticking out a thread. To the right is an egg (labeled e). If you follow this pair through panels B and C, you can see the sperm reel in the egg. Image from Figure 1 of Sato, et al. 2011.

If the sperm doesn’t get lucky and hit an egg with its thread, don’t worry; it can still fertilize an egg. When a sperm gets close enough to an egg, it becomes ameboid and moves directly toward the egg. Sometimes, two or more sperm will gang up on an egg and try to fuse with it. But once a single sperm has fertilized an egg, the other sperm will instantly go back to randomly casting out threads and searching for love.

Sexy shrinking

I want to backtrack for just a minute. Why do diatoms even need to have sex? They seem perfectly happy floating along as single-celled, asexual glassy things. What makes them switch from making clones of themselves to dividing into either sperm or eggs?

Russian Nested Dolls

Image via Flickr.

The answer: diatoms are constantly shrinking. Diatom shells are made up of two unequal halves. The big half overlaps the small one so that a tight seal is formed.

Each generation, the daughter cell that gets the smaller half has to make an even smaller new half. Eventually, the cells become so tiny they can’t divide that way anymore. To reset their size, they need to have sex. They do this by turning themselves in to sperm (if they’re male) or eggs (if they’re female).

P. trainorii clones can either make male or female gametes (sperm or eggs), but not both. That means, when they decide its time for sex, they have to find a suitable partner.

Sato and colleagues found out that each sex produces its own kind of signal, likely through a pheromone, that tells the other sex that its ready to get it on.

Sexy talk

The authors predict that this species makes at least three different kinds of pheromones, that are nicely summarized in the picture below.

The secret sex talk of diatoms. In pink, a female clone sends out a signal that she’s ready for lovin’. A male clone picks up the scent and undergoes gametogenesis to make two sperm. He then sends out the blue signal to let the female know he’s ready to go. She gets the message and makes two eggs of her own. It’s then up to the male to find her and finish the job. The sperm sends out random threads until it gets close to the eggs. It then goes ameboid, after sensing the third pheromone, and moves directly to the egg. Image from Figure 11 of Sato, et al. 2011.

Male clones were able to detect the presence of a female from a distance, through a filter that they can’t cross. Even though they couldn’t touch or get to the female, male clones were stimulated by something in the water and started the process to make sperm.

Females, in turn, could detect that a male had gotten the message. Also through a filter, a female clone could detect a “sexualized” male (i.e.: one that had turned itself into sperm) and this was enough to get her to go through the process of making eggs.

The third predicted pheromone wasn’t detected in the experiments. Even though eau de female clone¬†could convince a male to make sperm, it wasn’t enough to get him to form psuedopods and move directly toward the female. Eggs that had been killed by heat also couldn’t do it for a male. The authors predict that the third pheromone probably doesn’t last very long or go very far. Males might have to be very close to detect it.

I don’t know if this paper answered any questions about the evolution of pennate diatoms, since the threads described in the paper aren’t anything like the flagella of the centrics. But it’s still really cool. This is the first time threads like this have been shown to have a role in sex (a very distantly related type of algae, the Haptophytes, also send out threads, but these are used for catching prey, not for sex).

This is what I love about science. You never know what you’re going to find.

Sato, S., Beakes, G., Idei, M., Nagumo, T., & Mann, D. (2011). Novel Sex Cells and Evidence for Sex Pheromones in Diatoms PLoS ONE, 6 (10) DOI: 10.1371/journal.pone.0026923

One thought on “Diatom sperm go fishing for eggs

  1. Pingback: Sperm are Cool #1: Naked Mole Rats | Molecular Love (and other facts of life)

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