The expression "When chickens have teeth" is widely recognized, at least amongst French speakers. The English language, on the other hand, has singled out a different animal: "When pigs fly," goes the saying. The prospect of pigs flying is equally impossible, of course. While most mammals, including humans, have remained confined to the ground, the ability to fly, or rather glide, has made an appearance several times throughout this class of vertebrates. Bats, experts in sustained flight, come to mind. But other mammals, such as flying squirrels, colugos and three species of marsupial are skillful gliders.

A team of American researchers has taken a particular interest in these gliders in an effort to understand how the patagium − the membrane that stretches between the legs and flank − came to be, giving them their famous superpowers. Why them and not any of the 1,400 bat species? "For two reasons," said Ricardo Mallarino, senior lecturer at Princeton University and coordinator of a study published on April 24 in the scientific journal Nature. "On the one hand, because the patagium arose independently in these three closely related species, which has allowed us to make comparisons. And second, because this membrane develops not in the uterus but in the maternal pouch, making it accessible to experimentation." Marsupials, including large kangaroos, give birth to embryos weighing around 1 gram that crawl into the ventral pouch, attach themselves to a teat and complete their development there.

The team began by sequencing the genomes of these three species, as well as 11 other marsupial species lacking the famous membrane. No gene mutations were found to explain the phenomenon. On the other hand, in the three gliding species, the team observed accelerated evolution in the vicinity of the EMX2 gene. The modifications were varied, reflecting the unique history of each species, but all were located in the same region.

This gene is no stranger to those who study the development of major anatomical structures in the embryo. Present in all mammals, it is notably involved in the development of the brain and pelvic girdle. Could it be that it also plays an important role in the development of the patagium?

So the Princeton geneticists put on their second hat, that of developmental biologists. They set up a breeding program for flying phalangers, small marsupials less than 20 centimeters long with a flat tail and the famous patagium. And they took advantage of the embryos' invaluable accessibility. They took the embryos out of their mothers' pouch, injected them with an agent capable of inhibiting the expression of the Emx2 gene on the skin of their flanks and then returned them to their warm cocoon. "A few days later, we measured the membrane: It was considerably smaller," said the researcher.

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