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Unrelated animals share blueprints for flight

Mammals have always been considered as creatures that roam on the ground, swim in the water, or climb trees, but not fly. However, according to a newly published paper in Science Advances, it appears that mammals have actually evolved flight more often than birds, and unrelated animals may even have used the same blueprints for building their "wings." The study reveals that as many as seven different groups of mammals living today have taken to the air independently of each other, including flying squirrels, marsupial possums, and the colugo. These animals have one thing in common, a special skin structure between their limbs called a patagium, or flight membrane.


The Evolutionary Experiments of Flight


The fact that similar structures, such as patagia, have arisen so many times (a process called convergent evolution) suggests that the genetic underpinnings of patagia might predate flight. This implies that patagia could be shared by all mammals, even those living on the ground. If this is true, studying patagia can help us to better understand the incredible adaptability of mammals, and we might even discover previously unknown aspects of human genetics.


Patagia: A Deceptively Simple Membrane


Despite being seemingly simple skin structures, patagia contain several tissues, including hair, a rich array of touch-sensitive neurons, connective tissue, and even thin sheets of muscle. In the earliest stages of formation, these membranes hardly differ from neighboring skin. However, at some point, the skin on the animal's sides starts to rapidly change or differentiate. The dermis undergoes a process called condensation, where cells bunch up, and the tissue becomes very dense. Meanwhile, the epidermis thickens in a process called hyperplasia.


The sugar glider: A Main Model Species


In some mammals, this differentiation happens when they are still an embryo in the uterus. However, in the marsupial sugar glider (Petaurus breviceps), the process begins after birth, while they are in the mother's pouch. This provides researchers with an incredible window into patagium formation. Researchers have examined the behaviors of thousands of genes active during the early development of the patagium in sugar gliders to try and figure out how this chain of events is kicked off.


From Gliders to Bats: Similarities in Wnt5a


Researchers have discovered that levels of a gene called Wnt5a are strongly correlated with the onset of those early skin changes—condensation and hyperplasia. Through a series of experiments involving cultured skin tissues and genetically engineered laboratory mice, they showed that adding extra Wnt5a was all it took to drive both of these early hallmarks of patagium formation.


When researchers extended their work to bats, they found extremely similar patterns of Wnt5a activity in their developing lateral patagia to that in sugar gliders. This was surprising since bats (placental mammals) last shared a common ancestor with the marsupial sugar glider around 160 million years ago. Moreover, researchers found a nearly identical pattern in the outer ear (or pinna) of lab mice, a nearly universal trait among mammals, including innumerable species with no flying ancestry.


A Molecular Toolkit: Implications for Human Genetics

Together, these results suggest something profound. Wnt5a's role in ushering in the skin changes needed for a patagium likely evolved long before the first mammal ever took to the air. Originally, the gene had nothing to do with flight, instead contributing to the development of seemingly unrelated traits. However, because of shared ancestry, most living mammals today inherited this Wnt5a-driven program that contributes to the development of flight membranes. As a result, the research team suggests that studying patagia can help us understand the adaptability of mammals and may even reveal unknown aspects of human genetics.


The Research Process


The study, published in Science Advances, involved analyzing the behaviors of thousands of genes active during the early development of the patagium in the marsupial sugar glider. The researchers aimed to figure out how this process is initiated.


The study showed that the onset of early skin changes, such as condensation and hyperplasia, is strongly correlated with levels of a gene called Wnt5a. The team then conducted a series of experiments using genetically engineered laboratory mice and cultured skin tissues. The experiments showed that adding extra Wnt5a was enough to drive both of these early hallmarks of patagium formation.


Surprisingly, when the team extended their work to bats, they found similar patterns of Wnt5a activity in their developing lateral patagia to that in sugar gliders. This was surprising because bats, which are placental mammals, last shared a common ancestor with the marsupial sugar glider around 160 million years ago.


Moreover, the team found a nearly identical pattern in the outer ear (or pinna) of lab mice. The pinna is a universal trait among mammals, including innumerable species with no flying ancestry.


Implications of the Study


The research suggests that the genetic underpinnings of patagia may predate flight and could be shared by all mammals, even those living on the ground. The study implies that the same molecular toolkit is likely present in humans and is working in ways we don't fully understand yet.


The research also highlights the incredible adaptability of mammals and how they can evolve to take to the air independently of each other. The fact that similar skin structures have arisen so many times through convergent evolution suggests that studying patagia can help us better understand the evolutionary history of mammals.


Limitations of the Study


Despite these groundbreaking findings, the study has its limitations. For instance, the research team did not make a flying mouse, which demonstrates that we still don't fully understand how a region of dense, thick skin becomes a thin and wide flight membrane. Furthermore, while the study shows a cause-and-effect relationship between Wnt5a and patagium skin differentiation, we don't know precisely how Wnt5a does it.


Journal Information: Charles Y. Feigin et al, Convergent deployment of ancestral functions during the evolution of mammalian flight membranes, Science Advances (2023). DOI: 10.1126/sciadv.ade7511
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