August 2006 Archives

Several months ago, I participated in my first extended research cruise. I spent two weeks in the Sea of Cortez, dissecting jumbo squid, extracting statoliths, artificially inseminating squid eggs, sorting plankton samples, untangling fishing lines, and watching Firefly. I slept very little, learned a great deal, and had the time of my life.

Although at some point I may go into great depth about the various scientific insights I gained on the boat, this post isn't about that. It's about a word that I learned on the boat. Maybe the best word I've ever learned.

demarsupiate

I learned this word from my bunkmate, who used it very casually in a conversation one day. We were discussing amphipods, and one amphipod in particular that happens to be a jellyfish hitchhiker (I don't recall the exact species, but it must have been a relative of this adorable beast). I believe I made inquiries about the reproductive habits of the animal. I was told that females brood their young until they land on an appropriate jellyfish, at which point they demarsupiate.

That's when I halted the conversation and demanded an explanation of this marvelous new word.

Apparently, after copulation and fertilization, female amphipods release their eggs into a brood chamber on their bellies. Thus protected, amphipod babies develop into young that closely resemble their parents, and when they are ready, their mother releases them into a suitable environment. Thus, to demarsupiate is to release offspring from a brood chamber, a.k.a. marsupium.

Amphipods clearly do not have a monopoly on marsupiums/marsupia, or on the most excellent verb, to demarsupiate. Many crustaceans retain their larvae for some time in a brood pouch* before demarsupiating. Their fellow arthropods, scorpions, ought to be able to demarsupiate, as should a number of fishes**. One fish that throws me into a bit of a semantic quandaray, however, is the seahorse.

When I introduced our lab intern to the word demarsupiate she immediately inquired if it could be applied to the male seahorse. Now, I've always heard seahorses described as being "pregnant" and then "giving birth", and I think this may be the more accurate terminology. Truly marsupial animals (and here I'm considering arthropods and fish as well as mammals) give birth to their young, brood them for a time, and then demarsupiate. Birthing and demarsupiating are temporally separate processes. Female seahorses deposits their eggs into the male's pouch, but it would be hard to describe that as "giving birth"--it's more like ejaculation. Furthermore, the male isn't just holding the eggs in his pouch the way female crustaceans hold onto their young, he's actively enriching their environment with oxygen and nutrients. So, despite my enthusiasm for the word "demarsupiate" and my burning desire to use it in every appropriate context, I am doubtful that it can accurately be applied to seahorses.

However, we are left with abundant scenarios in which it can be used, and with relish! Marsupials, of course, can demarsupiate, but I think even we placentals can demarsupiate at times. At the end of the day, I bet these parents will be glad to demarsupiate.

The last, and best, part of this post is that I may be the first person ever to use the word demarsupiate on the web.

(be warned, the footnotes are freaky today)

* This reminds me of an unrelated but awesome story about a barnacle that parasitizes crab brood pouches. The female barnacle finds a crab and injects herself into it (no joke). Once inside, she essentially takes over the animal's nervous and reproductive system. As she grows, she extrudes her own gonad into the crab's brood chamber. Male barnacles come to mate with her, and she starts to produce young. Meanwhile, the happily oblivious crab host cares for its parasite as though she were caring for her own eggs. But what if our lady barnacle happens to infect a male crab? All is not lost! She simply scrambles his brain, effectively feminizing him, so that he creates a brooding area and cares for his parasite just as a female crab would.

** Many cichlids, for example, are mouth-brooders, which sounds pretty weird at first, but then . . . no, actually it just gets weirder. See, mouth-brooders can be divided into larvophiles and ovophiles. The larvophiles make a little nest where the female deposits her eggs and the male fertilizes them. Mom guards the eggs while they develop, and as soon as they hatch, she takes the fry (baby fish) into her mouth to brood them for a while.

But! Female ovophiles deposit their eggs on the ground, then immediately slurp them up into their mouth (still unfertilized, mind you). Males of these species have "egg spots" on their anal fin that are supposed to look like eggs, so the female tries to slurp them up along with her eggs, and instead she gets a mouthful of milt (fish ejaculate) which fertilizes the eggs in her mouth. The eggs then develop and hatch inside her mouth. I believe this is the only animal, ever, to achieve fertilization through oral sex.

At last!

I ended the previous post by linking to Nicole King, a biology superstar who has a genius award under her belt and whom I was lucky enough to see here at the marine station. She gave a great seminar, the kind which left me thinking--momentarily, at least--"Forget cephalopods. I want to study choanoflagellates!"

Before I explain why they're so awesome, let's go back to classification for just a moment. Remember the domain Eukarya? It gets broken down into "supergroups" that separate out a lot of things that used to just be lumped into Protista (i.e. little single-celled beasties). If you're really burning to see a list of all the supergroups, Wikipedia has one--scroll down until you find the domain Eukaryota*. The only thing that's really worth noting is that Kingdom Plantae is in an entirely different supergroup than Fungi and Animalia (which are both in the supergroup Opisthokonta). Just as we were all surprised to find that Eukaryotes are more closely related to Archaea than to Bacteria, it's a bit unexpected that Animalia are closer to Fungi than to Plants.

Opisthokont translates as "rear pole", a reference to the posterior flagellum that many animals and fungi use to propel themselves. Choanoflagellates are also opisthokonts, and they also have this distinctive flagellum. Here's the hierarchy, for those of us who like to have it laid out visually:

Eukarya
    Plants
    Opisthokonta
        Fungi
        Animalia
        Choanoflagellata
    Lots
    Of
    Other
    Nonsense

Recall the coolness of multicellularity, and how it lets eukaryotes get really big? The best part is that eukaryotes didn't figure out multicellularity just once. We're pretty sure that it evolved multiple times. That means ancestral plants, animals, and fungi each figured out independently how to be multicellular. Not only that, but even within the plants, there are thought to be multiple multicellular lineages. I think at last count there are something like a dozen independent origins of multicellularity, and even this number is thought by some to be a significant underestimate.

However, if we just look at Kingdom Animalia, it's supposed to have arisen from just one multicelluar ancestor. That makes things simpler. So let's examine the origins of animal multicellularity.

The simplest group of animals is the sponges, Phylum Porifera. Compared to the extensive differentiation of tissues and organs that goes on in other animals, sponges are little more than glommed-together cells. But they do have different cell types. One of the cell types that they have is called a "collar cell", because it has a single flagellum (there's that opisthokont) surrounded by a little collar of microvilli. Collar in Latin is choano so these cells are also referred to as choanocytes.

Ah, you knew I was getting around to choanoflagellates sooner or later! There is, as you may guess, a morphological basis for the linguistic similarity between choanocytes and choanoflagellates. Observe:

Choanocyte (also available in German)
Choanoflagellate

It doesn't take a genius to speculate that these two types of cells--one its own self-contained organism, the other a participant in the multicellular miracle that is a sponge--have a common ancestor. The question is, how common? Do choanos and animals share enough that we can look towards their similarities to give us hints about the evolution of multicellularity?

In my heirarchical diagram above, I've placed fungi, choanos, and animals on the same level, implying that they are all equally related to each other. That's highly unlikely. There have been conflicting theories published about who is most closely related to whom, but King has provided strong molecular evidence that the choanos and animals are more closely related to each other than to fungi. Given that common ancestry, she starting looking in choanos for genes that are characteristic of animals--and in particular, genes that animals use for multicellularity.

What's that mean? Well, for multicellularity to work, it is fundamentally necessary that cells be able to communicate with each other. Just as in any society with division of labor, individuals must be able to express their needs and trade their products. Cells communicate by means of signaling molecules and receptors. Receptors are transmembrane proteins, which means they have a bit on the outside of the cell and a bit on the inside of the cell. Signaling molecules sent from other cells can bind to the outside bit of a receptor, which starts a biochemical chain reaction inside of the cell, so that it can respond to the signal.

(Signaling pathways are endlessly complex, and careers have been built on figuring them out. In fact there's a whole freaking journal about them.)

Noting that receptors are proteins, let's sidestep for a moment and remind ourselves of Biology's Central Dogma:

DNA --transcription--> RNA --translation--> protein

All of your proteins are encoded in your DNA, but not all of your DNA gets translated into proteins. Forgetting about junk DNA for the time being, not even all of your functional genes get translated into proteins. Some of them are only turned on at some times, and some never get turned on. So what scientists have figured out how to do is to look at an organism's RNA** to see only those genes that the organism is actually transcribing and turning into proteins.

King & Co. looked at choano RNA, and found out that they have a whole mess of transcribed genes (and thus proteins) that have only ever been found in animals before. And such proteins! They found receptors in choanoflagellates that are used in animals for adhesion and signaling between cells. Smells like multicellularity!

It's not quite that simple, of course. Choanoflagellates aren't multicellular, and they aren't the ancestors of animals. But they may provide some hints about where animal "multicellular" genes came from, before the evolution of multicellularity. Some of the receptors might have been used for prey recognition. And some of them, more excitingly, may have been used for colony formation. Now colonies, as I mentioned before, aren't multicellular organisms. But they could well be the first step. And there are some colony-forming choanos . . . which is just about the coolest thing ever. Proterospongia has a misleading name, since it is not the ancestor of sponges. But it may be the best approximation we've got as to where this whole animal thing got started.

(Really, King's article says everything I just tried to, and better. Forgive my poor mutterings, and go read that, if you've the time.)

* No one really seems to care whether you say Eukarya or Eukaryota. My cherished textbook, Campbell's Biology, uses Eukarya, and so do I.

** Messenger RNA, or mRNA, to be precise. RNA comes in different flavors, and it's mRNA that plays the key role in the central dogma. rRNA, for example, forms part of the structure of the ribosome, which is the little cellular machine that translates mRNA into proteins . . . I trust I make myself obscure.