Boost of gene activity may help explain how arms and legs evolved

By Tina Hesman Saey

Web edition: December 12, 2012

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LOOK MA, HANDS

Throwing a simple genetic switch was enough to turn one of this zebra fish embryo’s fins into a primitive limb with a bulge where a future hand or foot might have grown.

Credit: Freitas et al/Developmental Cell 2012

Evolving limbs from fins may be as easy as taking the activity of a few genes and kicking it up a notch, a new study suggests.

Many evolutionary biologists have thought that four-limbed creatures evolved from finned ancestors. Forming limbs involves turning on body patterning genes in two precisely timed phases, first in the back legs and then, later, in the part of the limb that will become the hand or foot. Fish have those genes but turn them on only once, or at very low levels during the second phase.

Working with zebra fish, researchers in Spain added a genetic switch called an enhancer to turn up activity of a gene called Hoxd13 at the tips of developing fins. Fish usually lack these genetic switches, but adding them and making more Hoxd13 than usual produced rudimentary limbs that had more cartilage and less fin material, the researchers report in the Dec. 11 Developmental Cell. Ancestors of four-legged creatures may have acquired these enhancers, leading to limb development.