Why You Should Care About Acoelomorph Flatworms

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Greg Laden is guest-blogging this week while Sarah is on vacation. You can find his regular blog at Scienceblogs.com and Quiche Moraine.

Darwin proposed that all species arose from a single common ancestor, and that this process involved almost uncountable branching events over eons of time. Working backwards, this means that an analysis of all of the living species should provide a "family tree" of life, showing, for instance, how all the monkeys are related to each other, and how the monkeys fit into the broader mammalian tree of life, and how the mammals fit as a branch on the vertebrate tree of life, and so on.

This is, of course, one of the main things scientists since Darwin have been working on, first using the physical appearance of living animals and fossils, and later using DNA. With DNA, however, it becomes difficult to unravel the details of the tree of life the farther back in time you look. This is because as parts of the DNA code change over time, it can randomly change back to an earlier code, which confuses the situation. This can be overcome by using a very large amount of data and a great deal of computer power and applying some powerful theories.

An international team of researchers has just come out with such a study of early bilaterians (bilaterally symmetrical animals, such as humans, fish and worms) that solves a long standing question in biology: Where in the evolutionary tree of life do we put a particular group of worms called the Acoelomorpha?

These very small flatworms are like the bilateral animals in many ways but lack some of the most important features that bilateral animals have ... such as a gut. All bilateral animals have a gut lined with a specific kind of cell that facilitates digestion. Acoelomorpha, which is an entire phylum including about 350 species, "digest" food in an entirely different way. Some species take food into their body via a mouth, but that food does not enter a proper gut. Instead, pieces of food enter a sack full of special cells which then surround pieces of the food. The food is then broken down inside the cells. In some species, there is not even a space for the food to go into, though there is a mouth. In these species, the food is more or less shoved between the body cells of the organism where it is then digested.

Because of the lack of some of the key features of other bilateral animals, it has been difficult to place these creatures with certainty on the tree of life, so over the years this branch has been moved now and then from one place to another.

Casey Dunn at Brown University and sixteen colleagues from around the world claim that they have finally grafted Acoelomorpha where it belongs on the tree of life. Using a detailed and extensive analysis of DNA, they have placed Acoelomorpha just outside the other bilateral animals, as a sister clade to all other bilaterians (but still within the bliaterian group).

This is important for several reasons other than just putting Acoelomorpha in its proper place.

For one thing, it places the first split in the linage of bilaterians in its proper place. This, in turn, allows a better reconstruction of the last common ancestor of the bilaterians. Reconstructing the last common ancestor of any group of species is very important because differences between that ancestor and all of the subsequent species represent evolutionary events (or sequences of events). For example, Acoelomorpha lack a gut lined with special cells, lack two sexes, have sperm with two tails instead of one and have muscle tissues that are different from later bilaterians. One of the best ways to understand the evolution of key features of bilaterian guts, sexual reproduction and muscles would be to directly compare the early forms of these adaptations, as represented by Acoelomorpha, with the later forms.

Also, this finding might say something important about the evolution of the early bilateral animals. If it can be confirmed that Acoelomorpha truly existed back then as gut-free, using the method of enveloping its food that it is known to use today, then this indicates that a key evolutionary event at the origin of bilateral animals may have related to a change in how food was used as an energy source. It could be that the invention of the bilaterian gut is the very reason for their evolutionary success.

It is possible that this strange gut-free form of digestion, or any of the other traits that are unique to Acoelomorpha, evolved within that group early on in Acoelomorha history. The mere fact that a trait is simpler in one kind of animal than another does not guarantee that it represents the ancestral form. (For example, tapeworms pretty much lack a brain but evolved from ancestors that had brain-like structures.) Additional analysis would be needed to make it more certain, for instance, that this method of digestion represents the original, pre-bilateral (pre-gut) adaptation. But it probably does.

The work was published in the Proceedings of the Royal Society B.

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