Thursday, 12 August 2010
Some pictures of my flatworms: Dendrocoelum lacteum (left) and Dugesia lugubris (right). If the picture looks a bit odd, its because it was taken through the plastic of the tank with a macro lens, and what your seeing is the undersides. The lighter patch in the middle of each worm is the pharynx, which is popped out when the animal wants to suck up some food. Flatworms don't have a through digestive system, so its also where the waste leaves when digesting is done.
Despite their similarity, these species are not close relatives. Dendrocoelum is in the family Dendrocoelidae, a group of active hunters with very simple eyes and an 'adhesive organ' (sucker) at the front for holding their prey, while Dugesia is in the family Dugesiidae, a more opportunistic predator that lacks a sucker but has a much more complex eye. These species used to be all lumped together in a larger group, the Paludicola, defined as all triclad planarians (triclads are flatworms whose intestine divides into three parts) that live in fresh water. They were believed to have descended from terrestrial flatworms, who were themselves descendents of marine flatworms.
About 10 years ago, a series of molecular studies completely rewrote the book on flatworm taxonomy and gave us a very different view of their evolution. The freshwater triclad flatworms are now believed to have evolved from marine flatworms (which when you think about it, makes more sense than if they had come from terrestrial species), and at some point around 100 million years ago, the ancestor of the Dugesiids and all terrestrial flatworms split off from the ancestor of Dendrocoelum. In the process they gained a rather unique mutation - a second version of their ribosomal RNA genes. All animals have multiple copies of their ribosomal RNA genes, but because all these copies are next to each other on the DNA strand, the mechanisms of DNA copying ensure that they are all identical copies. At some point, the ancestor to the Dugesiids and land flatworms underwent a mutation that moved some copies of their ribosomal RNA genes to a different location in their chromosomes, where they could evolve in a different direction. Its not really clear what these second ribosomal RNA genes do, but they are expressed and they can be found in all of the descendents, so they must be doing something important, but different from the original ribosomal RNA. My guess is that they may only be expressed in certain tissues or at certain times of development, but I don’t think anyone has really looked into it.