An article in SCIAM
Bats are wonderfully weird and diverse mammals. The only ones to have mastered powered flight, they underwent an incredible adaptive radiation--to the point where they now constitute one of every five mammal species. The evolution of the bat wing--a membrane of skin supported by three highly elongated "fingers"--was critical to their success. But exactly how it took shape has long eluded scientists. The problem is, the wings of the earliest bat fossils, which are some 50 million years old, look pretty much like modern ones. Which is to say, paleontologists have yet to unearth fossils transitional between bats and their terrestrial forebears.
Perhaps, the new findings suggest, that's partly because the bat wing evolved quite rapidly. Lee A. Niswander of the Howard Hughs Medical Center and her colleagues studied the embryonic development of the bat forelimb, comparing it to that of the hindlimb (which has much shorter digits) and to the mouse forelimb. They found that during development, the bat's third, fourth and fifth digits (corresponding to our middle, ring and pinky fingers) are initially similar in size to the same digits in mice, but that the bat digits subsequently attain their spindly proportions.
It turns out that in bat forelimb digits, cartilage cells known as chondrocytes undergo relatively high rates of proliferation and differentiation, which would seem to account for the elongation of the digits. A single gene, known as Bmp2 (for bone morphogenetic protein 2), appears to underly this increased proliferation and differentiation. The researchers determined that the bat forelimb digits express more Bmp2
than bat hindlimbs or mouse forelimbs. They do not, however, express more Bmp4 or Bmp7--two other genes associated with chondrocyte maturation. And when they added Bmp2 protein to cultured bat forelimbs, they observed further elongation of the digits. The addition of a protein that blocks Bmp, in contrast, stunted digit growth.
Regulation of the Bmp pathway, Niswander and her collaborators conclude, plays a key role in the elongation of the bat digits during development. It may have been likewise critical in their elongation over the course of evolution. The authors put it thusly in their paper detailing the findings, published online this week by the Proceedings of the National Academy of Sciences:
By linking a simple change in a single developmental pathway to dramatically different morphologies, we provide a potential explanation as to how bats were able to achieve powered flight soon after they diverged from other animals nearly 65 million years ago.
Cool. I note that Harvard geneticist Cliff Tabin was the guest editor for this paper. Tabin and Robert Riddle wrote an article for Scientific American back in 1999 that described their important work on limb development. It's available here.
