Leave it to an obscure, puny parasite to upset biologists' thinking about what makes a worm. According to a new DNA analysis, Buddenbrockia plumatellae--which looks and acts like a worm--belongs with jellyfish, sea anemones, and hydras on the tree of life. The findings challenge conventional wisdom about the evolution of animal body plans.
Until now, all worms supposedly belonged to the "higher animals," with earthworms, tapeworms, and their relatives fitting in nicely with arthropods, mollusks, and vertebrates. The cnidarians--jellyfish and their kin--fall outside this group, because they lack a clearly defined gut and nervous system. Enter Buddenbrockia, a microscopic species that is one of about 2000 myxozoans, animals that live inside and mooch off of other aquatic organisms. Researchers have long debated where on the evolutionary tree myxozoans belong. Some DNA data suggest they are higher animals. Yet other DNA data, as well as the presence of modified stinger cells, indicate they are cnidarians. Most myxozoans are lazy blobs. But not Buddenbrockia. Shaped like a worm, it wiggles furiously thanks to four bands of muscles that run the length of its body. This design makes it--and its fellow myxozoans--a good candidate for membership in the higher animals.
Intrigued by this weird animal, Peter Holland, an evolutionary developmental biologist at the University of Oxford in the U.K., postdoc Eva Jiménez Guri, and colleagues compared 50 of Buddenbrockia's genes with the same genes from 60 other species, including sponges, insects, crustaceans, cnidarians, and vertebrates. The researchers used more DNA and more sophisticated computer programs than previous studies. Buddenbrockia turned out to be a cnidarian, albeit a strange one, the group reports in today's Science.
Some researchers think that an elongated body shape evolved once, in the common ancestor of higher animals and cnidaria, but was abandoned by cnidarians. But Holland thinks this body plan evolved twice: once in the ancestor of higher animals and again independently in Buddenbrockia.
Another implication of the finding concerns crawling. Researchers have thought that crawling required bilateral symmetry--right and left sides that are mirror images--as seen in earthworms and other worms. Buddenbrockia is not symmetrical this way, indicating that this type of symmetry is not a prerequisite for a mobile lifestyle. "Weird animals can help us work out what is possible in evolution and what is not," Holland says.
John Finnerty, an evolutionary developmental biologist from Boston University in Massachusetts, agrees. "It is especially significant that the [wormlike] body plans were achieved independently” in the cnidaria and in the higher animals, he says. "The cnidarians show us that a crawling habit need not be associated with bilateral symmetry." Finnerty would have preferred to see a greater variety of cnidarians included in the analysis to pin down which of the gelatinous creatures Buddenbrockia is closest to. Nonetheless, the work nails down the cnidarian connection, he says. "This paper may resolve the issue once and for all."