Evolution: Biology's next top model?

Some say it could have been a global cooling event brought on by falling greenhouse-gas levels. Others argue that it was the drifting continents, which opened up Drake Passage and kick-started frigid, circumpolar currents. Whatever the reason, to the fish that lived on the Antarctic shelf about 34 million years ago, all that mattered was that it was getting cold, very cold. As the water temperatures plunged by about 5 °C to below zero over the next few million years, most fish became extinct or moved on to warmer climes. But one group, the Notothenioidei, remained. Thanks to some extraordinary evolutionary innovations, these bottom-dwellers radiated, speciated and ultimately dominated. Crucial proteins shifted shape so they could work at cold temperatures, and a digestive enzyme fragment took on a new role as antifreeze. Because colder waters hold more dissolved oxygen, red blood cells became dispensable, and the 16 species of the Channichthyidae family no longer make them at all. These are the icefish of the Antarctic: clear blooded, up to almost a metre long and with eerie, crocodilian features. To John Postlethwait, though, the icefish are “beautiful”. It’s not just their haunting looks that captivated Postlethwait, a developmental biologist at the University of Oregon, Eugene. It’s what they might do for the study of osteoporosis. A quirk of their evolution, he says, may make icefish a valuable model animal for discovering genetic controls on bone density. For decades, developmental biology has been dominated by an established A-list of models including the mouse (Mus musculus), fruitfly (Drosophila melanogaster), nematode (Caenorhabditis elegans), zebrafish (Danio rerio), African clawed frog (Xenopus laevis), chicken (Gallus gallus) and mustard weed (Arabidopsis thaliana). It has been rare for From Antarctic icefish to Galapagos finches, there are some interesting characters at the fringes of developmental biology. Brendan Maher explores a world of alternative model organisms. BIOLOGY’S NEXT TOP MODEL?