Penis worms, anuses, and evolution

Now that I have your attention, what is a penis worm?  Technically, they are invertebrate animals called priapulids which, if you know your Latin, has the word “penis” in there.  What exactly are they?  They are sausage-shaped, segmented worm-like animals with an extensible, toothy proboscis they use to capture prey.  Here is what a priapulid looks like:

Priapulus caudatus

A priapulus “worm” — the proboscis is pointing up in this picture.

What do these weird-looking and rather unknown animals have to do with evolution?  A whole lot, as it turns out.

The group of creatures we call Animals is incredibly diverse, and most of us have only a passing familiarity with a small fraction of these denizens.  It turns out that once you get past sponges, jellyfish and their kin, and a weird band of animals called comb jellies, there is a great clade of animals called the Bilateria.  Sounds formal, but basically the name means that this group of animals is bilaterally symmetrical – that is, these animals have symmetry about their midline and have right and left sides.  Look in the mirror – congratulations, you, too, are a member of Bilateria!

File:SymmetryOfLifeFormsOnEarth.jpg

The diversity of Bilaterian animals.

You may not appreciate it, but one of the key developments of bilaterian animals is forming a distinct mouth and anus.  This involves the formation of a gut tube, and like all tubes it has to start somewhere.  That somewhere is a puckered indentation called a blastopore that forms early during their embryonic development.  Yes, even we humans develop a blastopore as the beginning of our gut tract.

Among the bilaterian animals, there has traditionally been a split proposed that divides these organisms into two groups based on a fundamental difference in the way their digestive tracts develop.  In one group called Protostomes, the blastopore becomes the mouth, and the gut tract develops until it “punctures” the other end of the animal, forming the anus.  The word Protostome means “mouth first.”  Protostomes include a huge variety of animals such as insects, crustaceans, earthworms, mollusks, and most other “creepy-crawlies” you are familiar with.

In the other group, called Deuterostomes (meaning “mouth second”), the blastopore becomes the anus and the digestive tract stretches from hind to fore, eventually “punching” through the head region to form the mouth.  As I like to tell my students, Deuterstomes develop from the bottom up – you may now groan.  Something that may make you groan all the more is the fact that we vertebrates are members of the Deuterostomes.

File:Protovsdeuterostomes.svg

The development of the mouth and anus in Protostomes and Deuterostomes.

What does all this have to do with Priapulids?  Everything.  You see, the big evolutionary question is which came first, Protostome development or Deuterostome development?  Which is the original condition in the common ancestor?  Knowing this would inform our understanding of how other changes in development downstream from this evolutionary event were effected, and what we should predict to see in various animal lineages.

So, in a recent study by Martin-Duran and colleagues (2012) in which they followed the development of this engimatic worm, they found … drum roll … that priapulids develop as Deuterostomes, with the blastopore forming the anus.  Why is this shocking?  Because: priapulids share all the major DNA and anatomical characteristics with those of Protostomes!  In fact, they are nested among the members of the Ecdysozoa, the exoskeleton-bearing animals that include insects, nematodes, and crustaceans.  All of those animals have protostomic development … why would priapulids be any different in this regard?

Martin-Duran and colleagues (2012) suggest that we have it wrong when it comes to these evolutionary divisions of animals.  It turns out that we may be too hung up on what the blastopore forms.  Instead, Martin-Duran et al. (2012) suggest that it is the separation of the mouth from the anus that is the major adaptation to focus on in Bilaterian animals.  Given that a number of Protostome animals are similar to priapulids in having all the protostome characteristics except that their blastopore forms the anus, it seems that the Deuterostome condition was the most primitive.  In other words, the Protostome condition is derived, and probably developed independently several times.

http://www.sciencedirect.com/cache/MiamiImageURL/1-s2.0-S0960982212011396-fx1_lrg.jpg/0?wchp=dGLbVlt-zSkzV

Figure 4 from Martin-Duran et al. (2012) showing their evolutionary hypothesis for Bilaterian animal development.

What does all this mean?  It means that we now need to re-explore animal relationships and the fossil record to re-test whether the way we understand the evolution of the major animal groups is in need of re-tooling.  Who would have thought that a penis worm and the origin of its anus would have such an evolutionary significance?

References

  • Freeman, S. 2011. Biological Science, 4th Edition. Pearson.
  • Martin-Duran, J.M., Janssen, R., Wennberg, S., Budd, G.E., and Henjol, A. 2012. Deuterostomic development in the Protostome Priapulus caudatus. Current Biology, http://dx.doi.org/10.1016/j.cub.2012.09.037

One thought on “Penis worms, anuses, and evolution

  1. Pingback: Dead dinosaurs and reasons for hope | The Evolving Paleontologist

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s