Evolution: study finds commonality in fish and mammal development

Evolution can appear to be a very complex process. How biodiversity developed over millions of years, producing thousands of various animal and plant species, is continually being studied and surprises seem to crop up with every new study. Whether piecing together the many branches and various dead ends that ultimately resulted in homo sapiens or deciphering the genetic code that determines who has a tail or who has wings, scientists are, piece by piece, assembling the puzzle that makes up nature's grand experiment in life on Earth.

And yet, from time to time, they discover within the puzzle a point of commonality - a puzzle piece that is being used over and over again - and the end result in diversity becomes simply a matter of timing. A recent study, published in the Proceedings of the National Academy of Sciences, highlights a genetic process that determines gill structures in elephant fish and sharks and its similarities with the development of limbs in lizards and mammals.

The elephant fish is a distant relative to sharks and rays, sharing the same type of cartilage-based skeletal system and also an outgrowth called a branchial ray - an appendage that extends from the skeleton and forms a supporting structure for the gills. Somewhere in the development process, the elephant develops one set of branchial ray while sharks develop several. To determine how or when this takes place requires studying things at the embryonic level. And for the scientists involved in the study, from Cambridge and the University of Chicago, this was a challenge as elephant fish embryos are difficult to find. Elephant fish lay their eggs in cold, muddy ocean bottoms, so the researchers spent months diving and searching possible breeding sites in Australia and New Zealand, gathering the needed embryos.

The researchers traced the impact of a genetic factor called Shh - the sonic hedgehog gene. It is common to both the elephant fish and sharks but when it expresses itself in the early developmental process determines whether there's one branchial ray set or more. This same process appears in the development of lizards and mammals, helping to determine outgrowths like limbs and number of toes for different species.

"The research highlights how evolution is extremely efficient, taking advantage of preexisting mechanisms, rather than inventing new ones," said Dr. Andrew Gillis of Cambridge University. "By simply tinkering with the timing of when or where a gene is expressed in an embryo, you can get very different anatomical outcomes in adults."

"It's basically showing that the limb story is part of a much more general narrative, which is the story of outgrowths," said Dr. Neil Shubin, University of Chicago. "There's a common development toolkit for all the outgrowths that we know in the body; they're all versions of one another in a developmental sense."

While analyzing all of the minute components found within the evolutionary process might seem a little esoteric or obscure to some, one of the advantages in understanding species development is to then be able to consider how or what might change that process. What environmental factors might come into play to alter or disrupt embryonic development, producing an evolutionary course correction or a tragic mutation? How easily can an evolutionary process, millions of years in the making, be altered by pollution, climate change, or other shifts in the norm?

As we study and learn more about both the complexity and the commonality or simplicity of evolutionary development, we can begin to see nature's wondrous puzzle of life and how the pieces can possibly be rearranged for better or for worse.

Read about the study in EurekAlert!
Read more in a Cambridge University press release.
Read the entire report published in Proceedings of the National Academy of Sciences.