Living-Fossil Fish’s Genome Gives Clues of Landlubbers
The fish known as a living fossil for its close resemblance to ancient predecessors had its genome sequenced by scientists who say it offers clues to how animal ancestors crawled out of the sea.
African coelacanth, discovered alive in 1938 after having been presumed extinct for 70 million years, was found to have genes that reveal how land animals’ hands, feet, immune systems and other body features may have evolved, according to a report in the journal Nature.
The coelacanth can grow to two meters (six feet) and live as many as 60 years in the wild, according to National Geographic. Their front fins are fleshy, resembling the limbs of four-legged land animals. The gene sequencing enabled researchers to see changes between water and air, to the sense of smell, the immune system, and the body’s ability to eliminate waste.
“People had these romantic views of the coelacanth, that it was a fish lost in time or something like that,” said Jessica Alfoldi, a research scientist at the Broad Institute of MIT and Harvard in Cambridge, Massachusetts, and a study author. “We found that actually, the vast majority of the genome isn’t slow-evolving.”
The genes, or the minority of the genome that codes proteins, haven’t changed much, the report found. That’s because the deep-sea caves where the coelacanth live likely don’t have many predators or much competition for resources, Alfoldi said.
“Evolution acts strongly on genes when they need to change,” Alfoldi said. “That’s why we don’t look like fish anymore.”
The group did an analysis of which genes and regulatory DNA were lost, gained or altered. They found changes to regulatory DNA, which influenced genes involved in smell, suggesting a change from underwater smelling to detecting airborne odors. Also, a large number of immune system regulatory DNA changed, possibly as a response to the different types of pathogens found on land.
Several genetic regions may have been used to form limbs, fingers, toes, and, in mammals, the placenta. Of note is a region called HOXD, which is shared between coelacanths and four-legged land animals. This sequence is probably what altered to allow for hands and feet, the researchers said.
Waste elimination changed when the ancestors of landlubbers first crawled out of the sea. Protein is broken into sugars and nitrogen; fish get rid of the nitrogen by excreting ammonia into the water. Humans and other land animals convert the ammonia to urea in the liver; urea and water together are known as urine. One of the key genes in this cycle was modified in land animals, when compared to the fish.
The research was supported by grants from the National Human Genome Research Institution.
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