While it has long been known that the first amphibians were descendants of ancient fish, the exact genetic mechanisms for development of limbs were previously unknown. However, a recent study indicates that the required genes for the transition were already present in the ancient fish. All that was needed was the activation of these genes. What resulted was an explosion of four legged vertebrates on land. Every single quadruped animal alive today owes its existence to the expression of these genes.
The study was conducted by a team of researchers led by Denis Duboule of the University of Geneva in Switzerland. According to their findings, the necessary genes required for the evolution of limbs and digits were already coded into the genomes of our ancient fish ancestors. The activation of these genes then resulted in the evolution of fins and radials to limbs and digits.
The research group published its findings on the online journal PLOS Biology and the data it gathered might finally answer one of the long-standing questions in evolutionary biology. Since Darwin's time, geneticists and evolutionary biologists have been trying to figure out how exactly ancient fishes bridged the formidable gap between swimming in oceans and walking on land.
"The basics of the regulatory mechanism are there in the fish," says Duboule, a geneticist from the University of Geneva. "Everything is there, you just need to click it, and then it goes into the genes."
By comparing the genetic mechanisms that controlled the development of limbs and fins in terrestrial animals and fishes, the scientists were able to deduce the genetic details of the mechanism. The genes responsible for the embryonic development of different body parts in mammals are celled Hox genes. The team looked at two types of Hox genes. HoxA and HoxD genes are responsible for the development of fins and limbs respectively and both mammals and fishes have these two types of genes. The study indicates that a bimodal 3 dimensional DNA structure is responsible for the expression of HoxD genes and causes the formation of limbs.
When studying HoxD genes in zebrafish embryos, the researchers were surprised to detect the presence of similar bimodal 3 dimensional chromatin architecture. This means that even before the advent of amphibian evolution, the control mechanisms necessary for limb development was already present in fish. "We found the same bimodal chromatin architecture in fish embryos, indicating that the regulatory mechanism used to pattern tetrapod limbs may predate the divergence between fish and tetrapods," the researchers said.
