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The Body Plan of Modern Life

Although they look like blobs, sea anemones and other cnidarians have a basic anatomical plan called a body axis. That is, they have a top, defined by the mouth, and a bottom. Now it appears that nature came up with this design using homeobox genes--a key group of developmental genes involved in organizing animal body plans. The findings support the idea that the appearance of these so-called Hox genes 550 million years ago sparked the evolution of diverse forms of early life.

The Hox genes play a major role in guiding development of fruit fly and vertebrate embryos. Because evolution resembles development played out over hundreds of thousands of years, many biologists have wondered whether Hox genes were an engine of diversity. A pair of researchers--John Finnerty of Boston University and Mark Martindale of the University of Hawaii, Honolulu--decided to test this by looking for Hox genes in cnidarians. These creatures resemble the fossils of some relatively simple creatures that existed before an explosion of new life-forms at the beginning of the Cambrian era. Both cnidarians and sponges lack a key feature of most modern animals: bilateral symmetry, a body distributed in mirror-image halves along a line drawn from head to tail.

After searching for Hox genes in two sea anemone species and reported in cnidarians by others in the literature, Finnerty and Martindale analyzed 13 candidate genes. Seven turned out to be either Hox or Hox-like genes. But whereas vertebrate Hox gene clusters, groups of the genes at one chromosomal location, can have up to 13 genes, the sea anemone has only about four genes in its cluster. Overall, the researchers concluded in a presentation at the annual meeting of the Society for Integrative and Comparative Biology earlier this month, Cnidaria "seem to be at an intermediate stage of evolution" between organisms with no Hox genes and bilaterally symmetrical species.

Hox genes, it would appear, "are absolutely required for the establishment of the body axis in all animals" more complex than a sponge, which lacks a true body axis, says Finnerty. Later in evolution, the genes may also have helped organize tissues along the axis into discrete and ever more complex body regions. "We are starting to get enough comparative data that we can fill in the details of the evolution of the clusters," says Finnerty, whose report was one of several presented at the meeting on the existence of Hox genes in different types of organisms. And that, says John Postlethwait of the University of Oregon, Eugene, is what makes cnidarian work "great."