Genetic instructions for developing limbs and digits were present in primitive fish millions of years before their descendants first crawled on to land, researchers have discovered.
Genetic switches control the timing and location of gene activity. When a particular switch taken from fish DNA is placed into mouse embryos, the segment can activate genes in the developing limb region of embryos, University of Chicago researchers report in Proceedings of the National Academy of Sciences. The successful swap suggests that the recipe for limb development is conserved in species separated by 400 million years of evolution.
“The genetic switches that drive the expression of genes in the digits of mice are not only present in fish, but the fish sequence can actually activate the expression in mice,” said Igor Schneider, PhD, postdoctoral researcher in the Department of Organismal Biology and Anatomy at the University of Chicago and lead author on the paper.
The results contradict a previous finding that a developmental switch from pufferfish DNA was not capable of gene expression in the limbs of mice, suggesting that tetrapods evolved a novel developmental system. But the new experiments suggest that the genetic switch controlling limb development was in fact present deep in Earth’s evolutionary tree.
“There previously was the idea that these switches had to be generated from scratch de novo, but no, they already existed, they were already there,” said Marcelo Nobrega, MD, PhD, assistant professor of human genetics at the University of Chicago Medical Center and another author of the study. “Maybe the key was expressing a gene earlier or later or in a specific territory, but it was just a modification of a program that was already encoded in the genomes of fish almost half a billion years ago and remains there to this day.”