Fish living in waters near the North and South Poles separately evolved nearly identical antifreeze proteins to keep their blood and organs from freezing. Moreover, the Antarctic species apparently acquired its gene for the antifreeze protein at just about the time that average temperatures in the Antarctic Ocean plunged below freezing, about 14 million years ago. The findings, reported in two papers in tomorrow's Proceedings of the National Academy of Sciences, are being hailed as a textbook example of tracing the molecular steps behind an evolutionary innovation.
A team led by molecular biologists Chi-Hing Cheng and Arthur DeVries of the University of Illinois found that the gene for antifreeze glycoprotein, produced in Antarctic notothenioid fish to thwart ice-crystal formation, was nearly identical in stretches to the fish's gene for trypsinogen, a digestive enzyme. But they discovered that the antifreeze is no glorified gut grunt: The DNA stretch coding for the antifreeze protein's key novel attribute--its ice-binding region--is derived from a useless, noncoding segment of the trypsinogen gene. As biochemist John Logsdon Jr. of Dalhousie University in Halifax puts it, it's like "making sense from nonsense."
Cheng's group estimates that the two genes diverged between 5 million and 14 million years ago--suggesting that the fish evolved the antifreeze protein at about the time that Antarctic Ocean temperatures cooled to below 0 degrees Celsius. In a second paper, the scientists report that although the antifreeze protein in Arctic cod is nearly identical to the one in the notothenioids, the gene that codes for it isn't. In fact, they say, the ice-binding coding regions in the Arctic cod's gene "are not identifiably similar to any known sequence." Says Logsdon, it's "a striking case of convergent evolution."