A Genetic Event Underpinning The Transition From Fins To Limbs

The transition from water-dwelling fish to land dwelling tetrapod was no simple affair. They had to revolutionise many of their organ systems from the way they excreted to the way they respired. One of the greatest challenges they faced was gravity.

In water weight is of little concern. It is for this reason that whales and sharks have been able to attain their immense size. On land, size is quickly limited by the structural properties of bone and flesh. The first tetrapods had to adapt their skeletons to make them more robust. More importantly they had to make the change from fins to limbs to navigate their new environment.

Catsharks were the principle of focus of the genetic study, along with mice

The palaeontological record shows this transition in great detail. Yet the molecular revolution in the 1980s and a new understanding of the genes which underlie both ontogeny and phylogeny - evolutionary developmental biology - gives us a different perspective.

The most recent study, published by researchers from Tokyo Institute of Technology and the Centre for Genomic Regulation in Barcelona, highlights a key genetic change which underlies this transition.

The forelimbs of the tetrapod evolved from the pectoral fins of ancestral fish. These fins contain three or more basal bones connected to the pectoral (shoulder) girdle. However, most of basal the bones located in the anterior side of the limb (the thumb side in humans) were lost in early tetrapods, while only the most posterior bone remained as the humerus - currently the bone in our upper arm.

Pectoral fins of catsharks also contain three basal bones as seen in the ancestral fish. In development, proteins are responsible for designating the anterior and posterior regions or fields, as they are termed. The team examined the development of fins in catsharks and limbs in mice. Their results showed that the development of these homologous, yet very different structures, stemmed from differences in the balance of the anterior and posterior fields. The protein Gli3 controls this balance.

A diagram demonstrating how the balance in the anterior and posterior
fields in limb buds gives rise to fins in fish and legs in tetrapods

Its concentration determines the expression of other developmental genes, for example by triggering the expression of Alx4 and Pax9 genes in a small area of the anterior field whilst triggering the expression of Hand2 in a large area of the posterior field.

A genomic and developmental comparison between the mice and the catsharks revealed that the Gli3 expression was intensified in the anterior field of the former and the posterior field of the latter. This difference in the balance of posterior and anterior fields giving rise to fins and limbs. Additionally, the results showed that the catshark genome lacked a sequence found in mice and other tetrapods, which is responsible for preventing Gli3 expression in the posterior part of tetrapod limb buds. The restriction of Gli3 to the anterior may result in the loss of skeletal structure in this field.

The pectoral girdle and limb of Tiktaalik, displaying the single bone connection

The final piece of evidence for the importance of a posterior-anterior field shift came when the researchers artificially 'posteriorised' the pectoral fin buds of catsharks. This resulted in the loss of the anterior skeletal elements, producing a single bone connected to the pectoral girdle - in other words a homolog of the humerus in all tetrapods.

The results of the genetic study show a neat link to the fossil record as well. The transition fossil between fish and tetrapods - the 375 million year old Tiktaalik roseae - displays this same arrangement of a single bone connected to the pectoral girdle, suggesting that the arrangement did indeed stem from a shift in the posterior-anterior field during limb development.

Evolutionary development is an incredible subject area as it allows us to reconstruct the genetic events underlying evolutionary events which took place hundreds of millions of years ago. As well as the developmental and mechanistic insight it offers, the results of these genetic studies may give us an indication of what to look for in the fossil record, as highlighted between the consonance of the anatomy of Tiktaalik and the altered catsharks in the experiment. This fusion of genetic and palaeontological evidence will ultimately yield a rich, more diverse picture of life.