Not only does the fish genome possess the main mechanism used to model limbs, but the DNA infrastructure for fingers and toes, too, a new study suggests. 

"A good metaphor for what has probably happened would be the process of 'retrofitting,' as is done in engineering to equip outdated machine frames with new technology," said Joost Woltering, from the University of Geneva's Department of Genetics and Evolution. "Only, in this case, it was a primitive DNA architecture which evolved new 'technology' to make the fingers and toes."

Published in the journal PLOS Biology, the study compared the structure and behavior of gene clusters known as Hoxa and Hoxd - instrumental in fin and limb development during the embryonic stage - in mice and zebrafish embryos.

The discovery of a similar 3-D DNA organization of both mouse and fish clusters suggests that fish contain a blueprint for limb formation, according to the scientists. However, when the zebrafish genes were inserted into the mouse embryos, they were active in the mouse arm, but not the digits. Based on their results, the researchers concluded that, "although fish possess the ... toolkit to produce digits, this potential is not utilized as it is in tetrapods," the University of Geneva wrote.

The scientists also wanted to know whether digits are essentially the evolutionary equivalent, or homologous, to a fish's fin structures. To find out, they inserted the parts of the genome that regulate Hox gene expression in fish fins into the mice embryos.

"As another surprise, regulatory regions from fish triggered Hox gene expression predominantly in the arm and not in the digits," said Denis Duboule, a University of Geneva professor of developmental genomics. "Altogether, this suggests that our digits evolved during the fin to limb transition by modernizing an already existing regulatory mechanism."

Going forward, the researchers hope to determine what shifted between the DNA elements in fish and land animals.

"By now we know a lot of genetic switches in mice that drive Hox expression in the digits," Duboule said. "It is key to find out exactly how these processes work nowadays to understand what made digits appear and favor the colonization of the terrestrial environment."