Evolutionary biologists looking for the genetic mechanisms behind these anatomical differences have long sought to study the embryos of holocephalans and sharks. But the experiment was easier said than done, because of the inconvenient location where holocephalan eggs are laid: at the bottom of the ocean floor.
Undeterred, Dr. Gillis, now a postdoctoral researcher at Cambridge, used accounts from local fishermen and fishery biologists to find potentially accessible areas of elephant fish breeding in Australia and New Zealand. By conducting SCUBA surveys of those regions, Gillis and colleagues were able to collect a precious supply of elephant fish embryos to bring back to laboratories in Chicago and Cambridge for further experimental analysis.
"Diving for elephant fish eggs was not always a pleasure trip," says Dr Gillis. "Unfortunately, elephant fish like to lay their eggs in cold, muddy, shark-infested bays, so we spent months seeking out sites like this in southeastern Australia and New Zealand. When you finally find a few eggs in the muck, it feels like winning the lottery."
The embryos of elephant fish and dogfish, a kind of shark, were stained at different ages for the sonic hedgehog (Shh) gene, a factor first isolated in the late 1970's in fruit flies for its ability to control body development. At early stages of development, researchers detected Shh expression at the hyoid arch and four of the gill arches in both species.
But within a few weeks, Shh is only expressed in the hyoid arch of the elephant fish embryo, while the dogfish embryo continues to express the gene along all five arches. Therefore, the different patterns of Shh expression match the eventual anatomical differences in the growth of branchial rays.
"It's a real feat because we had a limited number of eggs,
|Contact: Robert Mitchum|
University of Chicago Medical Center