"The emergence of paired appendages was a critical event in the evolution of vertebrates," Cohn said. "The fossil record provides clear evidence that the first fins evolved along the midline. The sequence of evolutionary events leading to the origin of limbs has been known for some time, but only now are we deciphering how these events occurred at a molecular genetic level."
Researchers isolated genes from the spotted catshark, a type of slow-moving shark from the eastern Atlantic Ocean. By studying the activity of a dozen genes in shark embryos, they determined shark median fin development is associated with the presence of genes such as HoxD, Fgf8 and Tbx18, which are vital in the development of human limbs.
They also used molecular markers for different cell types to determine which cells give rise to the median fins, finding that they arise from the same cells that form the vertebrae. These same genes dictate the emergence of symmetrical pairs of fins on the animal sides, showing a shared developmental mechanism in completely different locations, according to Renata Freitas and GuangJun Zhang, co-authors of the paper and graduate students in UF's zoology department.
Extending their genetic analysis to the lamprey - a living relic from the time before fish had paired fins - researchers found the same genetic cues in place.
"That we see these same mechanisms operating in lamprey fins tells us they must have been assembled in the median fins first, and later in evolution this entire genetic program was simply reutilized in a new position to build the first paired fins," Cohn said. "It tells us our own arms and legs have their evolutionary roots in the dorsal, caudal and anal fins of our fishy ancestors."
Many of these genetic mechanisms are involved in human birth defects, which provide insight into the evolutionary history of genes and their functions.
Source:University of Florida