Most breeds of chicken go around barefoot, with no feathers on their talons. But a new study found a simple genetic method to reverse this, neatly transforming scales into something else. Even the smallest changes in gene expression can affect embryonic development.
The researchers’ method, carried out through the so-called Sonic Hedgehog pathway, could also help explain how scaly dinosaurs eventually evolved into lineages of small, feathery birds.
The study took 11-day-old incubating chicken eggs and held them up a bright light, a process called “egg candling” that illuminated the embryo inside.
This allowed researchers to inject a chemical into the still-developing chicken that stimulated the Sonic Hedgehog pathway, all without changing the chicken’s DNA.
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From Scales to Feathers
When the chickens hatched, they had downy feathers on their feet, on the footpads and digits, and these feathers later developed into adult ones that grew in clusters out of the skin.
Somehow, tweaking Sonic Hedgehog had caused the entire system to change, from scales to conventional feathers. And if one fell out, it grew back, as with those on the rest of the chicken’s body.
“Our results indicate that an evolutionary leap from scales to feathers does not require large changes in genome composition or expression,” says Michel Milinkovitch, a professor in the Department of Genetics and Evolution at the University of Geneva, in a press release. “Instead, a transient change in expression of one gene, [Sonic Hedgehog], can produce a cascade of developmental events leading to the formation of feathers instead of scales.”
The Genetics of Chicken Feathers
Chicken feathers are just one example of vertebrate “skin appendages,” with others including scales, spines, hair, teeth and nails. These tend to have “highly conserved early developmental processes,” the study says, meaning they come from a surprisingly narrow list of pathways, including Sonic Hedgehog (SHH).
To ensure that the chemical injected into the eggs activated SHH alone, the researchers performed RNA analysis on a series of eggs arrested at different stages of development. They found the chemical activated only Sonic Hedgehog, meaning it must have been responsible for the foot feathers.
What Is the Sonic Hedgehog Gene?
This pathway plays a critical role in vertebrate development, including the formation of the neural tube, which forms the brain and spinal cord in humans, and the so-called “limb buds,” which become the arms and legs.
SHH plays such a pivotal role in developing the human embryo that defects in the SHH protein or pathway can cause a disorder, holoprosencephaly (HPE). The right and the left sides of the brain fail to separate, causing facial deformities. Depending on the severity of the case, a child born with HPE may die at a young age or live to be an adult.
In 1980, biologists Christine Nusslein-Volhard and Eric Wiechaus first identified the SHH gene in fruit flies – though they called it Hedgehog at the time. They had noticed that flies with poor Hedgehog functioning grew unusual segments and bristles in spiny clumps, like a hedgehog.
Later, a post-doc researcher named Robert Riddle picked the name “Sonic Hedgehog” for the mammalian version of the gene, having recently seen his daughter read a Sonic comic book.
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