Most chicken breeds walk barefoot, with no feathers on their talons. But a new study has found a simple genetic method to reverse that, carefully transforming the scales into something else. Even the smallest changes in gene expression can affect embryonic development.
The researchers’ methodrealized through the so-called Sonic Hedgehog pathway, could also help explain how scaly dinosaurs eventually evolved into lineages of small, feathered birds.
The study took 11-day-old incubating chicken eggs and held them under bright light, a process called “egg candling” which illuminated the embryo inside.
This allowed the researchers to inject a chemical into the still-developing chicken that stimulated the Sonic Hedgehog pathway, all without altering the chicken’s DNA.
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From scales to feathers
When the chickens hatched, they had fluffy feathers on their feet, footpads, and fingers, and these feathers later developed into adult feathers that grew in clusters out of the skin.
Somehow, the tweaking of Sonic Hedgehog had caused the whole system to change, from scales to conventional feathers. And if one fell, it grew back, like those in 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, professor in the Department of Genetics and Evolution at the University of Geneva, in a Press release. “Instead, a transient change in the expression of a 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 that the chemical only activated Sonic Hedgehog, meaning it must be responsible for the foot feathers.
What is the Sonic Hedgehog Gene?
This way plays an essential 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 arms and legs.
SHH plays such a central role in the development of the human embryo that defects in the SHH protein or pathway can cause the disorder, holoprosencephaly (HPE). The right and left sides of the brain do not 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 adulthood.
In 1980, biologists Christine Nusslein-Volhard and Eric Wiechaus first identified the SHH gene in fruit flies – although they called him Hedgehog at the time. They had noticed that flies with poor hedgehog functioning developed unusual segments and hairs in spiny tufts, like a hedgehog.
Later, a post-doc researcher named Robert Riddle chose the name “Sonic Hedgehog” for the mammalian version of the gene, after recently watching his daughter read a Sonic comic book.
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