While studying the mechanisms of development in shark embryos, University of Florida scientists identified a spurt of genetic activity that is required for digit development in limbed animals.
"We've uncovered a surprising degree of genetic complexity in place at an early point in the evolution of appendages. Genetic processes were not simple in early aquatic vertebrates only to become more complex as the animals adapted to terrestrial living. They were complex from the outset," said Martin Cohn, Ph.D., associate professor with the UF departments of zoology and anatomy and cell biology.
"Some major evolutionary innovations, like digits at the end of limbs, may have been achieved by prolonging the activity of a genetic program that existed in a common ancestor of sharks and bony fishes," he said.
He said the same genes that produced ancient fins likely enlarged their role about 365 million years ago in amphibians struggling to adapt to swamps and terrestrial living, creating a distinct burst of development and more versatile appendages.
Using molecular markers to study the formation of skeletal cartilage in embryos of the spotted catshark, the team isolated and tracked the activity of Hox genes, a group of genes that control how and where body parts develop in all animals, including people.
They discovered a phase of gene expression in sharks that was thought until recently to occur only when digits began to form in limbed animals.
Co-authors graduate students Renata Freitas and GuangJun Zhang said the reason that despite this sharks and many other types of fish do not form more dramatic appendages during this late phase of Hox gene expression is because it occurs briefly and only in a narrow band of cells, compared with the more extended time frame and larger anatomical area needed to prefigure the hand and foot in limbed animals.
"We know when this particular Hox gene is mutated in humans, it results in malformations of fingers and toes. Until now it was thought these mutations were affecting a relatively recent innovation in the genetic process of limb development. Our results show that this phase of Hox expression is much more ancient and suggest that if the origin of digits involved a prolonged activity of Hox genes, a truncated period could result in defective digits," Dr Cohn said.
Dr. Cohn said the finding has shed light on how life developed on Earth, adding that the study might also potentially provide insight for scientists seeking ways to cure human birth defects.
The researchers further said finding the second phase in sharks, which have skeletons consisting not of bone but of cartilage, meant that the genetic processes necessary to muster fingers and toes existed more than 500 million years ago in the common ancestor of fish with cartilaginous skeletons and bony fish - more than 135 million years before digits debuted in the earliest limbed animals.
"The leap from aquatic life to terrestrial life is an extremely dramatic, important point in evolution that has captured the interest of many. Understanding how changes in gene regulation modify the body architecture is of extreme interest to scientists who are trying to find ways to improve human health by learning from developmental processes.
This work shows a late phase of gene regulation seems fated to the emergence of digits," said Marie Kmita, Ph.D., director of the Genetics and Development Research Unit at the Institut de Recherches Cliniques de Montreal, who was not involved in the research.