Atmospheric oxygen leads to the development of fingers over webbed digits
The presence of webbed fingers on an animal typically indicate that the species either lives in an aquatic environment, or gets around by gliding through the air, as these are where webbed fingers have an advantage. On land, free fingers allow us and many other species to grasp objects and move around easily. But how does the developing body decide whether its fingers should be webbed or free?
New research published in the journal Developmental Cell has found that during embryo development, some animal species detect the presence of atmospheric oxygen, which causes the removal of interdigital webbing.
Amphibians and amniotes – which includes mammals, reptiles, and birds – can have webbed digits. Amniotes rely on interdigital cell death to form fingers, while amphibians have differential growth patterns between the digits and the areas between them, known as interdigital regions. The researchers found that removal of the interdigital membrane through cell death by amniotes depends on the production of reactive oxygen species (ROS). Embryos are only able to produce ROS when they are exposed to sufficient oxygen levels during their development.
Because high oxygen levels are able to cause cell death in frogs, the researchers believe that this mechanism is shared by both amphibians and amniotes.
“But amphibians do not employ cell death to shape their interdigital regions; it is the difference in growth rate between the digital and interdigital regions that will determine their final proportions,” explains Mikiko Tanaka of the Tokyo Institute of Technology. “We think that interdigital cell death appeared in amphibians only as a by-product of the high oxygen levels, a first step in this evolutionary process. This new step eventually was integrated to the limb development and became essential to shape the limbs of modern amniotes.”
The study led by Tanaka involved analyzing embryos from a number of species. When oxygen levels were changed for chicken embryos – an amniote with interdigital cell death – this directly affected the number of dying cells. When oxygen levels were increased for African clawed frog embryos, interdigital cell death was induced, which is out of the norm for this amphibian.
In an effort to expand their evolutionary perspective, the research team also studied cell death and ROS in the Japanese fire-bellied newt and the coqui frog. Similar to the African clawed frog, the Japanese fire-bellied newt had not interdigital cell death, but coqui frogs did have cell death in their interdigital regions. But one important distinction is that, unlike the other two amphibians, coqui frogs grow as terrestrial eggs and breathe oxygen from the air. “This way, we show both experimentally and comparatively that interdigital cell death is correlated with life history strategy and oxygen availability in tetrapods (four-legged vertebrates),” Tanaka says.
While these findings being related to ROS is interesting, particularly because ROS is usually considered a detriment in aging and fertility, Tanaka also explains that “while both amniotes and amphibians can have either free or webbed fingers, they are formed in different ways in these two groups. The new developmental mechanism acquired by amniotes – interdigital cell death – allowed for the evolution of a great variety of limb shapes, such as the lobed fingers of coots, and even removal of some fingers in horses and camels.”
For future research, the researchers want to figure out exactly which pathways are regulated by ROS during development. They also are looking to discover how cell death became a key part of the limb development of amniotes. Furthering our understanding of these processes could help us gain insight into how drugs that combat excessive ROS production (i.e. ethanol, phenytoin, and thalidomide) may cause development defects.
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