The evolution of gill covers in fish (and humans)

New research has uncovered the genetic origin of gill covers in fish, which occurred over 430 million years ago. It sheds light on one of the most important evolutionary developments for fish, as well as over vertebrates such as humans.

All bony fish (or teleosts) have gill covers (or operculum) regardless of size or family

If you ask a palaeontologist what the most important change in vertebrate evolution was, they will probably tell you that it was the evolution of jaws in fish over 430 million years ago, which has been attributed to the rapid expansion in vertebrates we see today. However, a close second is likely to be the evolution of gill covers which occurred shortly after. Without this key evolutionary step fish would not have developed anywhere near as quickly and may not have ever made the transition onto land at all. Yet despite the importance of this landmark adaptation very little is known about how it came about, especially compared to other breakthrough moments like the evolution of the jaw. Fortunately, new research has now uncovered the genetic origin of the gill cover as well as an important piece of vertebrate history.

Importance of gill covers

The fact that fish breath water through gills is one of the most universally understood things about them. Yet exactly how they work and have developed is something that a lot fewer people truly understand. An important part of the gills is their hard covers, also known as operculum, which protect the vital organs located either side of the fish’s throat. However, they also play a secondary role which is arguably more important. The gill covers are also responsible for helping to pump a continuous flow of water over the gills beneath, which maximise the amount of oxygen they can breathe and as a result the rate they can create energy. Therefore the evolution of gill covers allowed fish to become more active and grow larger. It is also believed that being able to store water within the gill covers may also be how the first fish ventured onto land and evolved into the first amphibians. Therefore gill covers are ultimately what allowed fish to evolve into the diverse range of vertebrates we see today.

A diagram of the basic parts of a teleost fish, including the operculum located between the eye and pectoral fin which covers the gills

Genetic discovery

Despite the importance of gill covers, the exact mechanism behind how they developed has so far remained a mystery. That is until Lindsey Barske, a researcher at the Keck School of Medicine at the University of South Carolina, and her colleagues began genetic experiments in zebrafish. After altering the levels of a gene known as Pou3f3 they noted some striking mutations in the fish’s gills. Those that had their Pou3f3 removed failed to develop any gill covers at all, whereas those which had extra copies of the gene inserted grew extra rudimentary gill covers. Intrigued by their findings the team decided to look at the expression of this gene in other fish to see if Pou3f3 was responsible for their gill covers too.

Even the whale shark, the biggest fish in the world, relies on gill covers to help pump water over their gills

In the much older (evolutionary speaking) jawless fish such as hagfish and lampreys, which have gills but no external slits or covers, they discovered unsurprisingly that the Pou3f3 gene was not present and had therefore never developed. Whereas in cartilaginous fish, such as sharks and rays, they found that the Pou3f3 gene was active in every gill. In addition to this they also isolated the control element that is needed to activate this gene thanks to the fact that cartilaginous fish have two covers for their gills. They then later isolated the gene and its control element in every bony fish they looked at, which strongly suggests not only that this gene is responsible for gill covers, but also it is likely to be the same gene which evolved in fish over 430 million years ago. The team’s findings were recently published in a new paper in the journal PNAS.  

The human link

In addition to finding the Pou3f3 gene in almost all fish, the team also highlighted that it remains in our own genetic make-up and that it is potentially responsible for operculum-like structures during embryonic development. This means that not only do we still retain the genes for gills from our fish ancestors, but we may not be entirely done with them either. This is bound to conjure up sci-fi imagery of humans swimming underwater with gills either die of their necks, but it actually highlights something much more worth thinking about. It shows us that our underwater ancestors are still a part of us and that our connection to the oceans runs much deeper than we often realise. You could therefore argue that we have something of an ancestral responsibility to take care of our oceans, which is obviously something we need reminding of today.

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