A Shared Tool Kit

A few minutes into her Frontiers of Science lecture, Pamela Mitchell flashed a picture on the overhead screen of what looked like three identical embryos. "One of these is a chicken," she prompted, "one a mouse, and one a human. Can anyone tell me which is which?"

Mitchell, associate professor of biochemistry and molecular biology at Penn State, paused politely. Her audience stayed quiet. At this early stage, the embryos were impossible to tell apart.

To Darwin, she noted, this similarity suggested that diverse organisms were "constructed on the same pattern," eventually diverging from this common plan to develop features that were species-specific. Within each developing individual creature, in short, was a demonstration of evolution writ small.

Confirming Darwin's hunch would take over 100 years. Even after the emergence of modern genetics in the middle of the last century, many thought the mysteries of development would remain unfathomable, that the pathway from single egg to complex organism was likely to be different for different types of animals. Eventually, however, an increasingly detailed understanding of the way genes are structured set the stage for a truly profound discovery by a small group of developmental biologists.

Fruitfly brain
Courtesy Pamela Mitchell

Developing central nervous system (brain and ventral nerve cord) of a fruit fly larva. The green cells are neurons; those that express the gene AP-2 are colored yellow or orange. The red cells are neural progenitors that express AP-2.

Beginning in the 1970s, Edward Lewis at Caltech and Christiane Nusslein-Volhard and
Eric Wieschaus at the European Molecular
Biology Laboratory
in Heidelberg, Germany, working with the fruit fly Drosophila, conducted painstaking analyses of the effects of mutations they induced in successive generations of flies. By linking specific genes with their functions in the developing body, they were able to isolate and identify a small set of master genes that control the early stages of development.

As Mitchell explained, these genes fall into two groups: transcription factors, which direct the transfer of genetic information inside the cell, and signaling proteins, which regulate communication between cells. Lewis, Nusslein-Volhard, and Wieschaus were able to show how these genes interact, turning on and off at precise intervals, integrating information to determine and carry out the fruit fly's body plan. Lewis even managed to show that the genes he studied, now called Hox genes, are organized along the chromosome in the same order, head to tail, as the body parts they control. In essence, Mitchell said, the genes identified by these pioneering researchers make up a developmental tool kit.

Their work, eventually rewarded with a Nobel Prize, led other developmental biologists to an even more amazing discovery: Master genes directly analogous to those found in the fruit fly exist in all higher animals, including humans. The basic tool kit for development, in other words, has been conserved across species during millions of years of evolution. In developmental terms, "We're still using the same logic," Mitchell said.

This powerful confirmation of Darwin's early insight, she added, "was the birth of the field we call EvoDevo." The ability to compare the genomes of various species also presented biologists with an important new puzzle. For example: While a fruit fly's genome has 13,000 genes, a human being's has only double that amount—about the same number as a mouse. How could species so obviously different seem so alike at the level of their DNA?

"The number of genes has not increased very much in higher animals," Mitchell acknowledged. "What has increased is the total amount of DNA. While the fruit fly has only 136 million base pairs; humans have 3 billion." What's happened across the evolutionary span, Mitchell explained, is that our genes have accumulated a lot more regulatory information. "It's not the increase in genes that creates complexity; it's the increasingly sophisticated ways in which these genes are being used."

In her own lab, Mitchell is studying one of the tool-kit genes, AP-2, in both fruit flies and mice. AP-2, she explained, codes for a transcription factor protein that controls development of the limbs, face, and parts of the brain. Mitchell and her collaborators have shown that mutations in this gene in mice cause severe craniofacial defects and underdevelopment of the limbs. When her experiments on the analogous gene in fruit flies resulted in similar deformities, "It was a great surprise," said Mitchell. Who would think that the mouse and the fruit fly would use the same genes to build their faces?"

Fruitfly leg
Courtesy Pamela Mitchell

A highly magnified view of two tiny leg primordia (leg imaginal discs) from a fruitfly larva that have been immunostained with fluorescent antibodies to identify early-born sensory neurons (green nuclei) and cells that express AP-2 protein (red nuclei). The latter are precursor cells for joint tissue between leg segments and also provide essential survival signals for adjacent epithelial cells that are precursors of the "inter-joint" regions. In mutant flies lacking functional AP-2 protein, leg joints fail to develop and outgrowth of inter-joint regions is severely reduced.

Currently Mitchell is focusing her laboratory's attention on the fruit-fly leg, trying to pin down regulatory elements in the DNA of the AP-2 gene that specify when and where the gene is activated during build-out of the multi-segmented limb. Already the lab has found that the gene contains multiple regulatory "switches" for activation during leg development; notably, each switch activates the gene in a different part of the leg. These switches, called enhancers, contain recognition sequences for different combinations of DNA-binding transcription factors. Some combinations of factors cooperate to activate the linked gene; others cooperate to repress it. Ultimately, gene activity is determined by changes in the availability of different factors.

The insights gained into AP-2's workings in Drosophila, Mitchell hopes, will further understanding of the analogous gene in humans. "We're just scratching the surface of understanding this logic of developmental switches," she emphasized, "but already the pattern is clear. We see the same strategies being repeated again and again across
species that are many, many evolutionary steps away from one another. It's variations on a theme."

Pamela J. Mitchell, Ph.D., is associate professor of biochemistry and molecular biology in the Eberly College of Science; pjm23@psu.edu.This article is based on a talk given by Mitchell as one of the 2006 Penn State Lectures on the Frontiers of Science.

Last Updated November 06, 2006