Sea worm’s tail develops its own eyes and brain, then detaches and swims away to mate

I honestly don’t remember when or how I found out how human reproduction works.  By the time my mother made my father have “The Talk” with me — because that is how that dynamic generally works, errr, I’ve been told — he asked me if I knew how that went down, I gave him a two-sentence explanation, and that awkward little exercise was allllll taken care of.  Nothing more to see here.

But imagine my surprise if my father had said: 

Well, actually, that’s not how it works.  Let me tell you the truth.  When you turn 18, you’ll notice that your left foot will begin to change.  It will get a little thicker, change color a little, and start moving around by itself.  Gradually it will develop its own eyes and ears, a brainlike structure, and also a series of new toes coming out the sides.  By the time you are 19, your left foot will detach and scoot away on its own.  It will go and mate with the detached left foot of some nice girl, and before long, little human babies will result.  But don’t worry.  Soon your left foot will grow right back and be just like it was before.

While evolution didn’t quite work out that way for humans, it could have, because that’s pretty much what happened for a sea worm called Megasyllis nipponica.  When it becomes mature, part of its tail grows its own eyes, antennae, and brain, and it breaks off from the body as an autonomous swimmer to go and perform its one function: mating.  And that is indeed where new sea worm babies come from.  

This process is called “stolonization”, “schizogamy”, or “epitoky”.  Here is a sea worm developing one of these mating minions that will soon detach from its tail.  We catch it here when it has developed some of its own neural structure and is starting to twitch around independently:

The section that looks like it wants to swim away is called the stolon, and it will develop its own eyes and antennae while it is still attached to the main body:

Then it will break away and somehow know how to go and join a swarm of other stolons, where it will release either sperm or eggs, depending on whether it came from a male or female.  Then offspring will result, but because the parents are too busy partying and regenerating their butts, the sea worm babies will just have to figure things out for themselves.

Now, aside from saying, “Man, that is really weird”, you might also wonder how on Earth a head spontaneously forms in the middle of an animal’s body.  Well, great minds think alike, because researchers at the University of Tokyo also had this same question.  They took a look at what genes turn on in order to accomplish this odd feat, and they describe their findings in the November 22 issue of Nature Scientific Reports.  

Before we get into that, though, I should point out that a phenomenon like this isn’t as inconceivable in human beings as you might think.  For example, there is a relatively rare condition called sacrococcygeal teratoma in which a human fetus develops a growth that comes out of its tailbone.  This appendage is the result of uncontrolled growth (that is, cancer) of germ cells.  

These growths are generally benign, but they are very weird.  Because germ cells can develop into any other kind of tissue, these growths often contain structures like teeth, brainlike neural clusters, and even fully developed eyes.  Below we see a piece of such a tumor, and this piece has a fully formed and functional eyeball within it.

This particular teratoma also had masses of liver cells, and eerily enough, immature neural tissue, the makings of a primitive brain.  

Evolution didn’t wind up favoring this sort of thing in humans, but it is an event that clearly can happen if the right switches go off, and if it had conferred enough of an advantage to humans, it’s something that could very well have stuck with us into maturity, as it did in the sea worm.  (But I must say that on balance I am thankful that no little mini-man ever grew out of my butt.)

In fact, in odd instances, there are mutants of Megasyllis nipponica that grow eyespots on all of their posterior segments.  Evolution is very malleable:

So anyway, what did our researchers find?

First of all, they had a somewhat surprising result with a group of genes called Hox.  Those occur in a few clusters and generally set the development of headlike, torsolike, and taillike features along the length of a developing embryo.  This is widely true throughout the animal kingdom.  Here you can see how these genes are arranged in four clusters on the chromosomes of mice and where those genes are turned on (produce RNA, that is) to determine the general layout of the developing body:

So our researchers looked for the different Hox RNAs along the length of the sea worm.  When a stolon is forming, you might think that the “red” (anterior) Hox genes would get turned on around the newly forming head, but no.  They found that the “purple” (posterior) Hox genes stay on in the developing stolon, and the other Hox types stay off, and thus the stolon remains designated as “tail”.  So when a stolon head starts forming, it really is an island of “head” popping up in a sea of “tail”.

So if the Hox genes aren’t turning on or off differently, and so that isn’t what’s guiding this head development, then what is?  There’s another well-known group of “head determination” genes in the animal kingdom that specifically get turned on only within a developing head.  So the Tokyo researchers looked for the RNAs corresponding to those genes.  They found that some of these were indeed present at the front end of the stolon, especially the RNAs from a gene called Six3, one that is known best in humans specifically for eye and forebrain development.  

Now the question remaining is, if you turn on head-development genes like Six3 somewhere on a worm’s body, is that sufficient to start forming eyes or a head?  I’m sure that’s what they’ll try next.  You can’t really do stuff like that in a human being for ethical reasons, but it’s this kind of thing that can help us understand better how we might one day regenerate lost parts of humans like functional eyes.

I thought this story was especially timely because head development, believe it or not, has been prominent in the news lately.  Back on November 1 it was reported in Nature that researchers at Cal-Berkeley, Stanford, and a couple other places had investigated why the starfish does not seem to have a head.  The explanation, unexpectedly, was that the starfish does indeed have “head determination” genes turned on — such as Six3 — but it does not have torso or tail genes turned on at all, meaning that in developmental terms, it is basically just a giant head!  Maybe this shouldn’t have been too surprising, because we know that starfish do have eyes at the end of each arm that they can actually use to see with.

All of this goes to show that evolution has a large bag of tricks to draw from, and it does crazy experiments and produces weird prototypes.  But sometimes those weirdos go on to show that they are advantageous, and weird becomes the new normal.