CHAMPAIGN, Ill. - Scientists studying antifreeze protein in Antarctic fish have traced the protein's genetic origin and historical roots. They've also provided an example of convergent evolution by ruling out the suspected common ancestry with an antifreeze-bearing Arctic fish.
Almost identical antifreeze glycoproteins are produced independently by two unrelated fish groups Antarctic notothenioids and Arctic cods in response to freezing conditions. The antifreeze protein in Antarctic notothenioids, which make up the majority of fishes in the Antarctic, appears to have arisen between 5 million to 14 million years ago, a period already established as when the Antarctic Ocean froze.
The discoveries were reported by University of Illinois researchers in two reports published in the April 15 issue of Proceedings of the National Academy of Sciences. The first paper, focusing on the Antarctic fish, gave the first look at the genetic process of creating a new protein for species survival.
"Most new protein genes evolve from the recycling of existing genes or gene segments," said
Chi-Hing C. (Chris) Cheng of the U. of I. department of molecular and integrative physiology. "They are taken and reassembled into a new gene, with nothing really new being created.
"The antifreeze glycoprotein came about differently, taking two parts of its ancestor, trypsinogen [a digestive enzyme produced by the pancreas], but those two parts did not form the new protein," she said. "The new protein came from building from the ground up a very short sequence in the middle, and going through repeated duplications to form the building blocks of the antifreeze glycoprotein."
Coauthor Arthur L. DeVries, who discovered the first antifreeze protein in Antarctic fish in the 1960s, said the second article clearly shows that the antifreeze gene in Arctic cod is not related in gene structure nor in coding sequences to the Antarctic fish. "We found tremendous differences," he said.
In an accompanying journal commentary, John M. Logsdon Jr. and W. Ford Doolittle of the Program in Evolutionary Biology at Dalhousie University in Canada wrote: "The strong message from this work is the clear link between a new function that has arisen out of strong selective pressure and an abrupt switch in environmental conditions adaptive molecular evolution. Demonstrations of this sort at the molecular level are rare and noteworthy."
Evolution of the antifreeze gene in the Antarctic fishes had a great impact on the ecology of the thermally isolated Antarctic Ocean that exists today, DeVries said. "If they hadn't evolved this gene, then the Antarctic Ocean may be quite devoid of fishes, and the food chain and marine ecosystem would be very different."
Joining DeVries and Cheng in the research, which was funded by the National
Science Foundation, was Liangbiao Chen, who has since earned his doctorate
from the U. of I. and is now a postdoctoral researcher at the National Institutes