NSF PR 97-29 - April 14, 1997
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Evolution of Fish Antifreeze Gene Sheds Light on Climate
History
New research shows that fish in the Antarctic and
Arctic oceans, at opposite ends of the earth, independently
evolved nearly identical antifreeze glycoproteins.
Studies by biologists have pinpointed the parent of
the Antarctic antifreeze gene: a digestive enzyme
called trypsinogen. The researchers also suggest that
the gene arose five to 14 million years ago, providing
a new line of evidence to confirm when the Southern
Ocean froze.
Two papers in the April 15, 1997 issue of the National
Academy of Sciences' Proceedings describe a rare,
direct link between the evolution of a protein, the
diversification of an animal and environmental change.
The authors, supported by National Science Foundation
(NSF) grants, are Liangbiao Chen, Arthur DeVries,
and Chi-Hing C. Cheng, all from the University of
Illinois.
Millions of years ago, fish in both northern and southern
polar waters adapted to a cooling climate by evolving
antifreeze proteins that kept them from freezing in
frigid oceans, and let them exploit new ecological
niches. The new research traces for the first time
the genetic process by which a novel protein evolved
to enable this adaptation.
The researchers show that the gene for antifreeze
glycoprotein (AFGP), found in the Antarctic family
of notothenioid fishes, evolved in a unique way: arising
"whole cloth" from trypsinogen, an enzyme produced
by the pancreas. New genes are usually created through
recycling of existing protein genes.
"This is the first clear example of how an old protein
gene spawned a gene for an entirely new protein with
a new function," said Cheng. It is also one of very
few newly-minted genes whose evolution can be so clearly
traced.
"Demonstrations of this sort at the molecular level
are rare and noteworthy," write John Logsdon and W.
Ford Doolittle in a commentary on the paper.
The AFGP gene differs very little from its parent
-- only 4 percent to 7 percent in the inherited gene
segments -- so in evolutionary terms, its molecular
clock began ticking quite recently. Segments at both
ends of the gene are nearly identical to the parent
trypsinogen gene. Applying the known rate at which
DNA evolves in salmon mitochondria to the amount the
AFGP gene has diverged from trypsinogen, the authors
have pegged the gene's origin at five to 14 million
years ago, close in time to the estimated freezing
of the Antarctic Ocean. The freezing date was deduced
independently, through studies of changing temperature
as recorded in plankton in ocean sediments.
Some biologists had argued that Arctic cod, which
produce very similar AFGPs, evolved from the same
stock as the Antarctic fish. But DeVries, who discovered
the first antifreeze gene in Antarctic fish thirty
years ago, says the new molecular evidence shows that
the two polar fishes, the Arctic cod and the Antarctic
notothenioid, developed their antifreeze genes separately.
By sequencing and analyzing -- essentially working
out the architecture -- of the Arctic AFGP gene, the
authors show that it does not resemble the gene for
trypsinogen, and differs from its southern counterpart
in gene structure and coding sequences as well.
The similar AFGPs in two unrelated fishes exemplify
convergent evolution -- the development of a similar
protein from different parents under similar environmental
pressure.
The notothenioid family now dominates Antarctica's
continental shelf, comprising more than half of the
species and 95% of the biomass, or weight, of fish
there. The fish arose in the deep ocean, but underwent
a burst of evolutionary radiation into different ecological
niches as the Southern Ocean cooled. In an evolutionary
pattern akin to Darwin's finches, they are the only
example of an oceanic fish to show this adaptive radiation.
The antifreeze protein was evidently a key mechanism
that let them colonize different depths of water.
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