Word: homeobox
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Over the summer, Erwin pondered this problem with two paleontologist friends, David Jablonski of the University of Chicago and James Valentine of the University of California, Berkeley. Primitive multicelled organisms like jellyfish, they reasoned, have three so-called homeotic homeobox genes, or Hox genes, which serve as the master controllers of embryonic development. Flatworms have four, arthropods like fruit flies have eight, and the primitive chordate Branchiostoma (formerly known as Amphioxus) has 10. So around 550 million years ago, Erwin and the others believe, some wormlike creature expanded its Hox cluster, bringing the number of genes up to six. Then...
That something extra, Duboule and his colleagues suggest in the journal Nature, is provided by a special set of genes that act as master architects in a surprisingly broad range of animals, from rodents to roundworms. These gossamer strands of DNA -- known as homoeotic homeobox genes, or Hox genes for short -- lay out the embryo from head to tail, controlling everything from the development of limbs and the wiring of the spinal cord to the patterning of the gut and urogenital tracts. "What's amazing," says University of Pennsylvania paleontologist Neil Shubin, "is that evolution of complex structures appears...
...genes pack such power? The DNA in all genes carries instructions for assembling proteins out of chemical building blocks called amino acids. What sets the proteins made by Hox genes apart is the biochemical motif known as a homeobox, a stylized string of 60 amino acids that enables Hox proteins to stick to DNA like strips of molecular Velcro and, in the process, activate still other genes. Hundreds of genes belong to the extended homeobox family, but those that are also homoeotic -- associated with changes in body parts -- are the most important. Though they are few in number...
Further studies are needed to convince scientists that Duboule and his colleagues have correctly solved the fins-to-feet riddle. Other factors could be involved as well, including homeobox genes that are not Hox genes (that is, they do not affect the overall structure of an animal). Last year Sean Carroll, a developmental biologist at the Howard Hughes Medical Institute in Madison, Wisconsin, showed that a homeobox gene involved in insect-limb formation also controls the genetic signals that paint spots on butterfly wings. In essence, says Carroll, butterflies use an old gene to perform a new trick. "Evolution...
...drawback for scientists is that nature's shrewd economy conceals enormous complexity. Researchers are finding evidence that the Hox genes and the non-Hox homeobox genes are not independent agents but members of vast genetic networks that connect hundreds, perhaps thousands, of other genes. Change one component, and myriad others will change as well-and not necessarily for the better. Thus dreams of tinkering with nature's toolbox to bring to life what scientists call a "hopeful monster" -- such as a fish with feet -- are likely to remain elusive. Scientists, as Duboule observes, are still far from reproducing...
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