Unique Wing-Like Fins Of Skates and Rays Evolved From Repurposed Genes, Researchers Say

Skates and rays get their striking, wing-like pectoral fins from repurposed genes, according to a new study from the University of Chicago. When taking a closer look at how embryonic skates developed their unique appendages, researchers found the rear portion of the sea creatures' fins are built by typical limb-development genes, but the front portion develops based on a different set of genes that are usually found in the shoulder areas of other species.

"What is surprising is that the extraordinary anatomy of skate fins comes about by simple tweaks to the processes that make the more normal-looking fins of other fish," Neil Shubin, a professor of Organismal Biology and Anatomy at the University of Chicago, said in a news release. 

For their study, researchers examined gene expression in the embryos of batoids, which is a group of cartilaginous fish including skates and rays that evolved from an ancient lineage of sharks. These creatures developed flat bodies and broad pectoral fins that are fused to their heads so that they could thrive on the bottoms of oceans and rivers. Their analysis revealed the pectoral fins of embryonic skates, at least early on, mirrored limb development in other species of fish and tetrapod. 

While limb development of tetrapods -- the first four-legged vertebrates -- is widely studied, much less is known about how fish acquired their remarkably diverse fins. The recent study, however, revealed a group of genes and growth factors are expressed in precise patterns in the embryo's budding appendage. These genes essentially create an apical ectodermal ridge (AER), along which cell proliferation stimulates limb elongation, organization and proper development. 

While most species only have one AER per appendage, skates develop two. This means one directs pectoral fin growth toward the tail, while the other directs growth toward the head, researchers explained. Furthermore, the two regions are controlled by different groups of genes.

"Even though the posterior and anterior regions of a skate fin look similar, gene expression is totally different," Tetsuya Nakamura, study leader and a postdoctoral fellow at the University of Chicago, explained. "This indicates that they have very different mechanisms that control growth of the anterior portion, which are not found in tetrapods or other fish. I think these genes are very important for the evolution of skate or stingray shapes."

The next step, researchers say, is exploring other distinctive fin shapes found in other fish. 

"Skates appear to have redeployed a preexisting genetic module to the anterior of their fins, which enabled the evolution of their unique shape," Nakamura added. "A better understanding of these molecular mechanisms will help us answer questions about the general diversity of fish fins -- how they changed shapes, what shapes are possible, what molecular kits can be recruited, and ultimately how fins transitioned into tetrapod limbs."

Their study was recently published in the Proceedings of the National Academy of Sciences. 

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