Back in July of 2011, Scientists at the University of Chicago have been able to transplant genes coding for limb and digit development from fish into mouse embryos. What is really interesting, is that when these genes are capable of activating the development of limbs in the mouse embryo, specifically the development of wrist and proximal digits in Zebrafish. This has lead the team of researchers to conclude that the mechanism for limb development is conserved, even though these two animals are separated by almost 400 million years of evolution.
Evolutionary biology has taught us that mammals evolved from reptiles, which in turn evolved from fish.
This suggests that fish, before any mammals with their fingers were present on the earth, possessed the capacity, even though inhibited, to code for limbs and digits.
Picture of fish genes activating the development of the distal limb in a mouse embryo.
This research was inspired by the finding of a "missing link" between fish and four legged tetrapods, called the Tiktaalik. Even though the Taktaalik had fins, they also had the skeletal structure of limbed land animals.
It was the land-dwelling-esque skeleton structure of the Tiktaalik that urged researchers to investigate the physical and genetic homologies between fish and limbed animals.
In the past scientists did not believe that genes from fish could activate limb development in mice. A previous study had been done in the past with pufferfish genes leading to no success. This lead many scientists to believe that limbs and their genes were specific to tetrapods. We now know otherwise.
So is the opposite true too? Can mouse limb and digit gene switches (CsB) activate development in fish. Indeed it can! CsB's from zebrafish have been shown to be able to activate the development of the distal ends of zebrafish fins. The article by Science Daily does not go into specifics about this development, but based on the activation of wrist and digit development in zebrafish from mice genes, I would guess that since the development is in the ends of the zebrafish fins, it is the development of digit-like structures.
So what are the implications? In the future, scientists will explore the bodies and genes of fish and tetrapods to see how gene expression changed to produce the anatomy of a fully land dwelling animal.