DURHAM, N.C. Long a staple of nature documentaries, the somewhat bizarre development of a grub-like pink marsupial embryo outside the mother's womb is curious in another way.
Duke University researchers have found that the developmental program executed by the marsupial embryo runs in a different order than the program executed by virtually every other vertebrate animal.
"The limbs are at a different place in the entire timeline," said Anna Keyte, a postdoctoral biology researcher at Duke who did this work as part of her doctoral dissertation. "They begin development before almost any other structure in the body."
Biologists have been pursuing the notion that limb development is triggered by other organ systems coming on line first, but this study shows the marsupial's limbs begin development without such triggers.
"Development is probably more flexible than we might have known otherwise," said biology professor Kathleen Smith. Their study animals were gray short-tailed opossums (Monodelphis domestica) native to Brazil and Bolivia, but the same should hold true for any marsupial, Smith said.
For the undeveloped embryo to be able to drag itself across the mother's belly from the birth canal to the teat, it needs a formidable pair of forelimbs. To get them, its developmental program has been rearranged to start building the forelimbs much sooner.
"A lot of these genes were turned on earlier than you'd see in a mouse or a chick," Keyte said. The researchers were also able to show that the forelimbs received cells from a much larger part of the developing embryo than is normally seen in other vertebrates. What surprised the researchers was that the genetic program to establish the hind limbs also appeared to be turned on early.
Gene expression sets up the pattern of where each of the four limbs will be, but the marsupial's forelimbs grow much faster than the hind limbs because the embryo devotes mor
|Contact: Karl Leif Bates|