Some mammals, like the Australian marsupial sugar glider, have special adaptations to glide through the air or even move about using powered flight like bats. How did this ability evolve?
“We don’t quite understand how novel traits and adaptations originate from a molecular and genetic perspective,” says Princeton University assistant professor Ricardo Mallarino. “We wanted to investigate how an evolutionary novelty arises.”
Mallarino is corresponding author on a paper published in Nature which looks at the genetic origins of the patagium – the thin skin membrane that allows some mammal species to take to the air.
The team sequenced the genomes of 15 marsupial species, looking at the DNA of both gliding and non-gliding animals.
Comparing the two groups revealed accelerated evolution near a gene called Emx2.
“What’s interesting is that the sequence of the gene itself doesn’t seem to be where the most relevant changes are taking place,” says corresponding author Erez Lieberman Aiden from Baylor College of Medicine in the US. “Instead, the key changes are in short DNA sequences, called ‘enhancers,’ that lie nearby in the genome. It’s those changing enhancers that alter how and where in the body Emx2 is active, and that drives the evolution of gliding.”
The researchers were able to test the evolutionary path taken at the genomic level by using a central characteristic of marsupials – their pouch.
“Marsupial joeys are born at a much earlier stage in development than typical mammals,” says co-first author Olga Dudchenko, also at Baylor and at Rice University. “Instead of continuing development in their mother’s womb, they crawl into her pouch, and stay there until they are ready to take on the world independently. The fact that they are right there in the pouch makes it much easier to study how individual genes, like Emx2, affect the marsupial’s development.”
Their tests show that Emx2 gives rise to patagia in marsupials using a developmental program which is probably present in all mammals.
Emx2 is active in the skin on the sides of mice as well as sugar gliders. But in sugar gliders, it is expressed for far longer.
“By modifying those critical Emx2 enhancers, one species after another has tapped into this universal program in order to develop the ability to glide,” Dudchenko says.
