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December 04, 2005

Hox OS

This is an interesting article about Hox genes and how they turn on and off the same genes at different times and places to create most of the diversity among animals, or vertebrates at least, that we see today:

Evo devo’s first big finding is that all animals are built from essentially the same genes. In the past few decades, biologists collected thousands of genetic mutations that disrupt the normal course of an embryo’s development. (Most of this work involved the humble fruit fly.) By characterizing how particular mutations derail the growth of embryos, biologists were able to figure out which genes control key steps in animal development. In one of the biggest breakthroughs, biologists worked out how fruit-fly embryos decide which of their ends should be the head, which the tail, and what should go between. Part of the answer involves what are called Hox genes. Different Hox genes get expressed in different parts of a fly’s body, and each Hox gene tells that body part what appendage it should grow. A Hox gene expressed in the head, for example, might tell the head to grow antennae, while a Hox gene expressed in the body might tell the body to grow legs. If you tinker experimentally with Hox genes, you get the stuff of B movies: mutant flies, for example, that have legs, not antennae, growing out of their heads.

But the truly surprising thing about Hox genes turns out to be evolutionary. All animals have Hox genes, and nearly all animals use their Hox genes to determine which appendage should go where along the axis that runs from head to tail. Given that the major animal groups, among them arthropods (now including insects), mollusks (snails), annelids (worms), and chordates (human beings), were in place at the start of the Cambrian period, Hox genes must be at least half a billion years old.

What’s more, plenty of important genes turn out to be this old. We now know that several hundred genes, including the Hox genes, are needed to lay out an animal’s basic design. Carroll calls these the “tool kit” genes, and they’re the central characters in his story. Nearly all tool-kit genes are present in all animals, and they do much the same thing in all animals. The same gene, for example, that triggers eye development in fruit flies also triggers eye development in mice. Indeed, genetically engineered flies will happily build eyes if supplied only with the mouse gene. (They build fly eyes, not mouse eyes.) Similarly, a gene that affects pigmentation in birds like the chicken and the bananaquit also affects pigmentation in mammals like the jaguar and you. Indeed, changes in bird-plumage color often involve the same gene that causes red hair in humans. This surprising genetic conservatism across nearly all animals is evo devo’s key empirical finding: swans, swallowtails, and socialites are all built from the same genes.

Posted by Mike at December 4, 2005 03:36 PM

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