Bowfin Genome Sheds New Light on Evolution of Ray-Finned Fishes

by johnsmith

The bowfin (Amia calva) is a species of ray-finned fish native to North America. Also known as freshwater dogfish, grinnel, and mud pike, this species is an evolutionary enigma because it embodies a unique combination of ancestral and advanced fish features. In a new study, an international team of researchers led by Michigan State University has begun to unravel the enigma by sequencing the bowfin genome.

The bowfin (Amia calva). Image credit: U.S. Fish and Wildlife Services.

The bowfin (Amia calva). Image credit: U.S. Fish and Wildlife Services.

The bowfin is the sole surviving member of a once large lineage of many species that are now known only from fossils.

Scientists have long been fascinated with the bowfin because it bears a combination of ancestral features, such as lung-like air breathing and a robust fin skeleton, and derived features like simplified scales and a reduced tail.

The bowfin also occupies a key position in the fish family tree, where it sits between the teleosts, a large and diverse group that arose recently, and more ancient branches that include sturgeons, paddlefish, and bichirs.

Due to this special position in the fish family tree, the bowfin can help scientists understand how aspects of modern fishes evolved from their ancient antecedents.

“For the first time, we have what’s called a chromosome-level genome assembly for the bowfin,” said senior author Dr. Ingo Braasch, a researcher in the Department of Integrative Biology at Michigan State University.

“If you think of the genome like a book, what we had in the past was like having all the pages ripped out in pieces. Now, we’ve put them back in the book.”

“This is really important information for a few reasons, and it starts with the bowfin being what Charles Darwin referred to as a ‘living fossil’.”

“The bowfin looks like an ancient fish. This doesn’t mean that the bowfin hasn’t evolved since ancient times, but it has evolved more slowly than most fishes.”

“This means that the bowfin has more in common with the last ancestor shared by fish and humans, hundreds of millions of years ago, than, say, today’s zebrafish.”

Zebrafish, which are modern teleost fishes, are a notable example because they’re widely used by scientists as a model to test and develop theories about human health. Having more genetic information about the bowfin helps make the zebrafish a better model.

“A lot of research on human health and disease is done on model organisms, like mice and zebrafish,” said Dr. Andrew Thompson, also from the Department of Integrative Biology at Michigan State University.

“But once you identify important genes and the elements that regulate those genes in zebrafish, it can be hard to find their equivalents in humans. It’s easier to go from zebrafish to bowfin to human.”

“For example, one particularly interesting gene is one that’s used in developing the bowfin’s gas bladder, an organ the fish uses to breathe and store air.”

Scientists believe that the last common ancestor shared by fish and humans had air-filled organs like these that were evolutionary predecessors to human lungs.

The study authors found that a certain genetic process in the bowfin’s gas bladder development bore striking similarities to what’s known about human lung development.

A similar process is also present in the modern teleost fishes, but it’s been obscured by eons of evolution.

“When you looked for the human genetic elements of this organ development in zebrafish, you couldn’t find it because teleost fishes have higher rates of evolution,” Dr. Thompson said.

“It’s there in modern fishes, but it’s hidden from view until you see it in bowfin and gar.”

With both the gar and bowfin genomes, the researchers were able to show where these genetic elements linked to gas bladder and lung formation were hiding out in the modern teleost fishes.

“You don’t want to base that bridge on one species. This finding also strengthens the implications for evolutionary history,” Dr. Braasch said.

“This is another piece of the puzzle that suggests the common ancestor of fish and humans had an air-filled organ and used it for breathing at the water surface, quite similar to what you see in bowfin and gar.”

The study appears in the journal Nature Genetics.


A.W. Thompson et al. The bowfin genome illuminates the developmental evolution of ray-finned fishes. Nat Genet, published online August 30, 2021; doi: 10.1038/s41588-021-00914-y

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