January 22, 2014
Fish Have The Genetic Blueprints For Fingers
Brett Smith for redOrbit.com - Your Universe Online
A new study from a team of Swiss researchers published in the journal PLOS Biology has found that our fish ancestors had the genetic machinery for fingers, but these structures did not develop until the evolution of limbs in amphibians.
“The basics of the regulatory mechanism are there in the fish,” said study author Denis Duboule, a geneticist at the University of Geneva and Federal Institute of Technology in Lausanne. “Everything is there, you just need to click it, and then it goes into the genes."
Both fish and land-dwelling animals have groups of Hoxa and Hoxd genes, which are necessary for the formation of both fins and limbs during embryonic development. In the study, researchers examined the make-up and actions of these gene clusters in embryos from both mice and zebrafish.
"If you have this complex of genes, on one side there is sort of a control tower for the forearm, and on the other side there is a control tower for the digits,” Duboule said. “This is what we call enhancers: regulatory sequences which control gene expression."
The study team found a similar three-dimensional DNA organization in both sets of genes clusters, an indication that the main genetic mechanism behind tetrapod limbs was already present in fish. When the researchers inserted the fish Hox genes into transgenic mouse embryos, they were only functioned in the mouse arm but not in the digits. This led the team to conclude that the fish DNA lacks necessary genetic material necessary for digit development.
"We realized that by taking either the one on the right or the one on the left, we got the same result: They both made forearms,” Duboule said. “So there is no way we can see, so far, that these control towers work in making digits. Our conclusion, therefore, is that everything you need to make digits is there in the fish, but it's not properly used."
The Swiss scientist suggested that regulatory genetic material was repurposed by evolving amphibians.
"We share our genes with all vertebrates, and the genes have been used and reused and reused,” he said. Duboule indicated that the study of embryonic development could offer a storyline for this evolutionary tinkering."Our limbs, our arms and our legs, grow from the tip in embryogenesis – they don't grow from the bottom,” Duboule noted.
The researchers said their future studies will focus on determining what has changed in the DNA elements of fish versus tetrapods.
"By now we know a lot of genetic switches from the mouse 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."
The team said they are also focused on creating a transgenic fish – as has been done with mice.
"It's not the end of the argument,” Duboule said. “The end of the argument is the day you can produce a fish with digits - the day where people can sit around the table and say: Hey, these are digits."