Don't go conjuring up images of giant, city devouring monsters, or crime fighting turtles; those aren't the kinds of mutants we learn from. I'm talking about mutations that arise during the development of an organism and how scientists are able to use them in order to learn about the process of embryonic development. We all know that a sperm fertilizes an egg, and an embryo develops, eventually giving rise to a new organism. What you may not realize is that there are a lot of different ways to get from the first cell to a juvenile organism. In the course of my research, I've come across two interesting developmental abnormalities that serve to illustrate two of the major modes of development.
Which came first, the mouth or the anus?
(Protostomes vs. Deuterostomes)
That's a rather simple question, from an evolutionary perspective. Looking at some of the basal (a neutral way to say primitive) digestive systems like those of cnidarians (including jellyfish and corals), we see that they have one opening for both entry and exit. Organisms evolved a second opening somewhere along the line: one entry and one exit. So in a way, they evolved at the same time.
We’re not talking about evolution, though; we’re talking about embryonic development. In animals that have complete digestive tracts (two orifices), there are two major clades: protostomes and deuterostomes. A hollow ball of cells called a blastula forms early on during development. Through a process called gastrulation, a divot is made in the blastula, which then becomes an opening (blastopore). The blastopore will eventually develop into the mouth of a protostome, whereas in a deuterostome it becomes the anus. The opening eventually connects to the other side of the blastula and soon the internal organs start to develop.
What can an abnormality teach us?
Placida dendritica (the sacoglossan sea slug that I work with) produces veliger larvae which, look like a tiny helmet with tentacles (called cilia) and a mouth at the opening. Occasionally, I will come across an egg mass where gastrulation never occurred. Instead of a normal veliger, a large spherical larva develops with the lining of the digestive tract on one half, a row of cilia in the middle, and a shell on the other half. The internal organs are still present in some form inside the organism, though not in the correct places.
I have no idea what causes this to happen (yet), but it does tell us something about how an organism develops. For one thing it, illustrates that gastrulation doesn't create the internal body cavity that holds the organs. If that were the case, the organs would be on the outside. The placement of the cilia gives the indication that the blastopore develops into the mouth. Imagine pushing the digestive lining back into the larva (as if to mimic gastrulation). In addition to having that disgusting mental image, you can picture that the cilia would now surround the opening that you created, just as they would in a normally developed larva.
Do cells have a destiny?
(Indeterminate vs. Determinate development)
I’m going to take this in a different direction that Kenny did in his earlier post about microbial evolution. This is not evolutionary destiny, but developmental destiny. It turns out there’s two answers to this question as well.
For deuterostomes (including you and I) the answer is no. We have indeterminate cell development. This means that any given cell has the potential to develop into a complete embryo (at least at first). If a zygote were to break apart during the early stages of development, two (or more) complete and identical organisms would arise.
On the other hand, the answer is yes, in protostomes. They undergo determinate development. From a very early stage of development, the cells are locked in to a particular path of development. As cell division continues, each daughter cell will be even more limited than the previous cell. If such a zygote were to break apart it may still develop, but it wouldn't create two separate organisms. Instead, there would be two separate pieces that would contain (nearly) all of the pieces of the whole veliger.
Okay, so what can mutations teach us here?
Looking back at P. dendritica, there is another abnormality that occurs from time to time. I will look at an egg under a microscope and occasionally see that there is a shell with some internal organs inside, and a small ball of cilia spinning around beside it. What has happened here is the cluster of cells destined to become the mouth and cilia were separated from the rest. Both parts continued to develop "normally" and now you have two separate parts of one organism, thus proving that P. dendritica undergoes deterministic development.
I should point out that what I've shown here is nothing new. Karl Grobben first described the groups protostomia and deuterostomia in 1908. It's possible that he saw similar abnormalities that gave him the clues he needed. I don't know for sure, since his work is written in German (if anyone is aware of a translation I'd love to know about it). I do know that scientists often learn more when things take an unexpected turn than when they go exactly as expected. That's the exciting part about science; you never know what will lead to a new discovery!