Vertebrates' cousin shares key genes.

Whether a scientist or a romantic, one cannot help but marvel at the incredible diversity of Earth's organisms. Worms seem nothing like elephants; eagles bear little resemblance to octopuses. Yet research reported this week reveals how nature improvises with certain genetic material, called homeobox (hox) genes, to create this variety.

The number and locations of hox genes and their DNA sequences in a wormlike, seemingly headless creature called amphioxus place this animal as a missing link -- genetically speaking -- between animals with backbones and animals without. Furthermore, the genetic makeup of amphioxus indicates that multiple copies of hox genes and of sets of these genes may drive the evolution of ever more complex animals, says Peter W.H. Holland of the University of Reading in England.

For more than a century, biologists have debated the status of amphioxus as a close relative of vertebrates, basing their arguments on morphological and biochemical studies. While at the University of Oxford in England, Jordi Garcia-Fernandez and Peter Holland joined in by tracking down this animal's hox genes. They first found nine hoxlike DNA fragments by using a technique called polymerase chain reaction. Further screening revealed one more, for 10 in all, they report in the Aug. 18 NATURE.

The protein products of hox genes control the activation of other genes, ensuring that various body parts develop in the appropriate places. "These are the blueprint genes," explains Nicholas D. Holland, a zoologist at the Scripps Institution of Oceanography in La Jolla, Calif.

In cells, genes belonging to one chromosome are strung like beads on a necklace, often with other bits of DNA interspersed. In nematodes, a single chromosomal necklace contains all the hox genes. In all the animals now studied, the order of the beads reflects the order in which each gene is expressed along the developing embryo's head-to-tail axis; however, mammals and other vertebrates possess four such chromosomes, each with its own set of hox genes.

To determine whether the amphioxus hox genes lay on one chromosome, Garcia-Fernandez carried out a "chromosome walk," a tedious analytical technique for determining the order and position of genes. He and Peter Holland found that, indeed, these genes do cluster. Also, surprisingly, the DNA sequences of these 10 matched closely the sequences of mammalian hox genes, even though the lineages of vertabrates and amphioxus separated 520 million years ago, says Garcia-Fernandez, now at the University of Barcelona in Spain. Because mammalian clusters can contain 13 hox genes, the two scientists hope to find three more in amphioxus.

Like nematodes, amphioxus uses a single set of hox genes. But unlike fellow invertebrates' genes, amphioxus' set closely resembles those of vertebrates, says Peter Holland.

The DNA of the modern-day amphioxus does in fact reflect the makeup of the invertebrate ancestor of all vertebrates, comments John W. Pendleton, a molecular biologist at the Oregon Regional Primate Research Center in Beaverton. As a result, "I think the status of amphioxus as an archetypal primitive chordate [an animal possessing a primitive spinal cord and neural tube at some point during its development] will be more accepted," he adds.

In mammals, the hox genes at each position in each of the four sets resemble those at comparable positions in the other sets, suggesting that the multiple sets arose as duplicates of the original, Peter Holland explains. Fish, birds, amphibians, and other vertebrates also have multiple sets. He wonders whether other groups of genes have expanded similarly in vertebrates. Moreover, some hox genes seen in amphioxus look as if they arose first as duplicates of other hox genes.

Thus this discovery in amphioxus indicates that the evolution of more complex organisms proceeded in parallel with the increasing complexity of hox genes. "The idea is that by gene duplication, you could suddenly make really major steps in evolution," Nicholas Holland explains.

Hox genes in particular increase the flexibility of the developmental process. "And development is the currency of evolution," says Pendleton.

"If you fiddle around with genes at [the hox] level, there are all sorts of opportunities for massive and very rapid advances," Nicholas Holland adds.