A New Study Traces Development of Gill Covers in Fishes

Researchers have reportedly identified a genome change that led to the evolution of gill covers more than 430 million years ago.

Stem cell scientists from the University of Southern California have traced the development of covers over gills, protecting the breathing mechanism for most early vertebrates as well as helping to pump water and reduce backflow. This evolutionary adaptation is considered just as important as the development of jaws in primitive fishes, allowing them to become predators and ensuring their survival.

In fact, jaws are present in almost 99 percent of modern-day vertebrates. However, the development of gill covers is most likely concurrent with the development of jaws and might have been equally critical to their dominance of the early vertebrates.

The details of their study are published in the Proceedings of the National Academy of Science (PNAS).

Tracing the Gene Responsible for Gill Covers

Researchers began the investigation by introducing mutations in the gene called Pou3f3 among zebrafishes. Surprisingly, fishes that did not have the gene or the DNA element responsible for controlling gill activity, failed to develop gill covers down the line. On the other hand, zebrafish that had overly active Pou3f3 later developed additional and rudimentary gill covers.

Cage Crump and Lindsey Barske, corresponding authors in the study, soon enlisted the help of other researchers from other institutions to further examine the effect of modifications made in the Pou3f3 gene. Specifically, they inquired whether this gene is responsible for the variation of gill covers among different vertebrates.

Crump serves as a stem cell biology and regenerative medicine professor at the USC Keck School of Medicine, while Barske is an assistant professor at Cincinnati Children's Hospital Medical Center.

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Driving Evolutionary Differences Between Cartilaginous and Bony Fishes

The team behind the study then observed that the DNA element that controls the production of the Pou3f3 gene in the gill area was not available in jawless fishes like sea lampreys. Conversely, cartilaginous fishes like skates and sharks have the control element for the Pou3f3 gene active in all of their gill regions.

Most of the cartilaginous fishes have separate covers for each gill. For bony fishes like zebrafish, the same control element synthesizes the gene in only one region, which creates a single cover overall gills.

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"Remarkably, we have identified not only a gene responsible for gill cover formation," Crump said in a statement, adding the ancient control element that created Pou3f3 to generate gill covers and drive diverse results in cartilaginous and bony fish species.

The researchers also discovered a "deeply conserved Pou3f3 arch enhancer present in humans through sharks," but not found in jawless fishes. A previous study noted that humans and sharks shared a common ancestry some 440 million years ago.

Although humans no longer have functional gills, the presence of conserved Pou3f3 genes is characterized by the growth of the hyoid - a second pharyngeal arch - during embryonic stages that grows over the posterior arches. This hyoid soon merges with the trunk, closing off the pharyngeal cavity.

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