Eggs compete for sperm in an ancient plant species

Trimenia moorei. Photo by J. Piaza.

Ever heard the saying “the exception that proves the rule”? Well, I never really understood what this is supposed to mean, but in biology, there are very few rules. Most everything is exceptions. One of the “rules” is that males compete for females. I’m sure you can think of examples off the top of your head; just think male rams butting heads. Many of my posts are about male-male competition (golden-collared manakins, singing mice, peacock spiders, parasitoid wasps…). And don’t even get me started on sperm competition (there’s tons in the literature about that). But what about female-female competition?

Well, don’t worry. There’s plenty of girl-on-girl competition for mates in the animal kingdom (including in humans). But here’s a story of female competition with a twist: eggs that compete for sperm. That’s right boys, the monopoly on gamete competition is over. But in contrast to sperm competition, where sperm from several different males compete for fertilizations, in some plants it’s the eggs (or egg precursors, called gametophytes) from a single flower that compete for fertilization. Julien Bachelier and William Friedman from Harvard University published their results on egg competition in the journal PNAS this month (abstract here).

Before I get into the research, let me just say this: plants are crazy. They commonly do things that respectable animals never would, like being polyploid all over the place (strawberries have 8 sets of chromosomes!). They hybridize with other plants willy-nilly, blurring the already tenuous definition of “species“. For these reasons, among others, I generally don’t trust plants (read: I know very little about them), so I try to avoid writing anything about them. I just get confused. So bear with me here.

First off, a quick plant sex primer (yes, mostly from Wikipedia). Keep in mind that when I say “plants” I’m talking about flowering plants (angiosperms). And for everything I say here, I’m sure there are plenty of exceptions.

Parts of a flower. Via Wikipedia.

Flowers are the reproductive organs of the plant, and they can have either male or female parts, or both. The male parts are called stamens. These are long stalks inside the flower that will produce the pollen which, if they’re lucky, will go on to make sperm.

The female parts are the ovary, style, and stigma. A lucky pollen grain will land on the stigma and start growing a tube down the style, toward the ovary. When it gets there, two sperm will enter one of the ovules and fertilize an egg. One of the sperm is used to make the actual embryo (the seed); the other sperm fuses with the extra female nuclei and becomes the endosperm. It’s kind of like egg yolk for plants–the endosperm will nourish the developing seedling.

Okay, so I said that the sperm enter the ovule and then fertilize an egg. So what the hell is the ovule? This is actually a structure that houses the female gametophyte (technically, the next “generation” of the plant. Remember alternation of generations from intro biology? Yeah, me neither). The gametophyte is another structure, made up of haploid cells (each cell has only 1 set of chromosomes). One of those cells is the egg. The other ones have supporting roles.

Ovules in the ovary. Image via Wikipedia.

Now, in the “normal” course of things, each ovule will contain exactly one gametophyte. It will develop in such a way that it strategically locates itself at the tip of the ovule, right where it will meet the sperm so it can get busy as quickly as possible. Many pollen tubes might compete with each other to reach one of those ovules, and plenty of research has been devoted to plant sperm competition. The fastest growing pollen tube wins the race. The “female” just sits and waits for a winner (though, she might help to push the race in favor of one gentleman…).

Bachelier and Friedman found that this “normal” course of events is by no means the only way things are done in the plant world. They had read scattered reports of “weirdo” plants where more than one gametophyte will start to develop inside an ovule.

They wanted to know if this was truly just some weirdo thing that happens in some weirdo plants, or if it might actually be the original way that plants did things. And if it was, could it be a way for eggs to compete against one another? To answer these questions, they looked at a species of plants that comes from a very ancient lineage (meaning they are likely to be more similar to the ancestor plant). They thought to look at ancient plants because most of the records of potential egg competition were from a few ancient plant groups.

The species they focused on was Trimenia moorei. What they found was egg competition was rampant in this species–the rule rather than the exception—and that it looks remarkably like sperm competition. Not just 2 or 3, but up to 20 gametophytes can start to grow in a single ovule in this species.

And by grow, I mean GROW. They don’t start off conveniently placed to passively accept sperm as it comes down the style. No head starts for these ladies. The gametophytes start out far from the site of fertilization and, just like pollen, grow long tubes to get there. The tubes don’t just look like the ones used by pollen; they work like them, too. Pollen tubes contain a special carbohydrate called callose to help them race to the finish. The female gametophytes in Trimenia eat up callose from the surrounding female tissue as they grow.

But what is the point of this competition? Isn’t the female just competing against herself? When it comes to evolution, it’s the genes (or, rather, the different variants of genes) that are in competition with each other. The gene variants that make the most “fit” offspring will become more prevalent in the population–that’s natural selection, distilled.

The gametophytes produced by a plant are not identical. The cells that the gametophytes come from are diploid, meaning they have two sets of chromosomes. When each gametophyte is being made, the genes on the two sets of chromosomes get re-shuffled to make entirely new, unique chromosomes. For each gametophyte, one set of re-shuffled chromosomes will become the egg, and that’s the set of chromosomes that gets passed to the next generation.

What it comes down to is that all of the eggs come from the same mom, but each one is very different–just like in humans, where the same man and woman can have lots of kids, and none of them are exactly alike (not including identical twins).

The big question is why don’t more plants have egg competition? Most of them do have sperm competition. The authors suggest that either the ancestor of plants operated with egg competition, or that soon after flowering plants started to spread, egg competition was selected for (because it led to better offspring). They don’t know, however, why it stopped working for plants. Maybe it takes up too much energy? There is a lot of extra tissue and fuel consumption involved in having eggs compete for sperm. And the plant still needs to have energy left over to make seeds once a winner is chosen. Like many things in evolution, it comes down to a cost vs. benefit analysis.

Well, that’s my best guess anyway. For the time being, this goes into my file of crazy stuff plants do.

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2 thoughts on “Eggs compete for sperm in an ancient plant species

  1. Pingback: Mama’s boys: How fig wasp mothers protect their sons | Molecular Love (and other facts of life)

  2. Pingback: Mama’s boys: How fig wasp mothers protect their sons | Molecular Love (and other facts of life)

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