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Evolutionary Link Found Between Fish, Vertebrates


New research published in the Journal of Experimental Zoology (JEZB) and in Comparative Biochemistry and Physiology (CBP) reveals the evolutionary link between fish and other vertebrate animals.

Ingo Braasch, MSU integrative biologist, worked with a group under Pep Rotllant of the Spanish National Research Council, Department of Biotechnology and Aquaculture in Vigo, Spain, to co-author two recent papers that reveal how the spotted gar system can be useful to clarify this evolutionary connection.

The JEZB article, “BAC Recombineering of the Agouti Loci from Spotted Gar and Zebrafish Reveals the Evolutionary Ancestry of Dorsal-Ventral Pigment Asymmetry in Fish,” fosters a greater appreciation for gar because it bridges the gap between the disconnect detected among fish and land-dwelling tetrapods.

Braasch calls the agouti gene analysis the “Gars and Stripes” project.

Juvenile spotted gars showing a pattern of stripes that, later in life, will turn into spots. The back of the fish is darker than their bellies, which has been analyzed at the genetic-molecular level in recent studies by Braasch and his colleagues. Image courtesy of Solomon David, a postdoc in Braasch’s lab.

“When you see a zebrafish, you immediately recognize the horizontal stripes that give a zebrafish its name,” said Braasch, an assistant professor in the Department of Integrative Biology in the MSU College of Natural Science. “Not so obvious is another aspect of zebrafish’s pigment pattern, which is that the back (dorsal) side is rather dark, while the belly (ventral) side is lighter. So the zebrafish pigment pattern is like two paintings on top of one another.”

The significance of the painting analogy is that it offers a connection between certain characteristics—some evolutionarily newer (the stripes) and some older (the dorso-ventral dark to light gradient). Braasch’s research addresses whether the molecular-genetic basis of the dorso-ventral gradient seen in zebrafish is also present in gar, a distantly related fish that is considered an “ancient” type of fish.

On the molecular level, the zebrafish agouti gene sets up the dorso-ventral gradient as part of the cellular signaling machinery called the melanocortin system. The JEZB article presents new evidence that the agouti gene activity in gar shows a very similar dorso-ventral gradient when compared to agoutigene in zebrafish. This supports research findings that the agouti gradient is at least as old as the last common ancestor of gar and zebrafish.

“Our studies are also significant because—in order to test the function of the gar agouti gene—we introduced, for the first time, a very long chromosomal piece from the gar genome into genetically engineered zebrafish (a method called BAC recombineering),” Braasch said.

As detailed in the accompanying CBP article, the agouti gene activity gradient is even conserved between fish and mammals, but the cellular mechanisms to achieve dorso-ventral pigmentation gradients differ between fish and mammals.

In terms of how the results of the studies influence future projects, Braasch is considering tracing the evolutionary patterns in more distantly related fish lineages such as sharks and lungfishes as well as analyzing other components of the melanocortin signaling system in gar and other fish.

Additionally, Braasch and his team are working toward achieving the inverse genetic experiment of the JEZB study—to introduce long parts of the zebrafish genome into transgenic gar fish.

“This will be a big step forward in gar functional genomics,” Braasch said. 

Banner image courtesy of Andrew Thompson, a postdoc in Braasch’s lab.

  • via the College of Natural Science website

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