Decoding Dinosaurs: Insights from Chicken Feet

Most breeds of hens move around barefoot, without feathers on their claws. But a new study has discovered a simple genetic method to reverse this, neatly transforming the scales into something else. Even the smallest changes in gene expression can affect embryonic development.

The researchers’ methodcarried out via the so-called Sonic Hedgehog pathway, may also help explain how scaly dinosaurs eventually evolved into lineages of small, feathered birds.

Illuminating eggs

The study took 11-day-old incubating hen eggs and exposed them to bright light, a process called “egg illumination,” which illuminated the embryo inside.

This allowed the researchers to inject a chemical into the still-developing chicken that stimulated the Sonic Hedgehog pathway without changing the chicken’s DNA.

Read more: Pterosaur fossil reveals evolutionary origins of feathers

From scales to feathers

When the chicks hatched, they had downy feathers on their legs, feet and toes, and these feathers later developed into adults, which grew into clusters of skin.

Somehow Sonic Hedgehog’s setup caused the entire system to change, from scales to conventional feathers. And if one falls out, it grows back like the ones on the rest of the chicken’s body.

“Our results show that the evolutionary leap from scales to feathers did not require major changes in genome composition or expression,” says Michel Milinkovic, professor in the Department of Genetics and Evolution at the University of Geneva, in press release. “Instead, a transient change in the expression of a gene, [Sonic Hedgehog]can lead to 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’, others include scales, spines, hair, teeth and claws. They typically 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 on its own, 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 leg feathers.

What is the Sonic Hedgehog gene?

This time plays a critical role in vertebrate development, including the formation of the neural tube that forms the brain and spinal cord in humans, and the so-called “limb buds” that become the arms and legs.

SHH plays such a key role in human embryonic development that defects in the SHH protein or pathway can cause a disorder, holoprosencephaly (HPE). The right and 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 an early age or live to adulthood.

In 1980, biologists Christine Nusslein-Volhard and Eric Wiechaus first identified the SHH gene in fruit flies – although back then he was called the Hedgehog. They had noticed that flies with malfunctioning Hedgehog developed abnormal segments and bristles in prickly clumps, like a hedgehog.

A postdoctoral researcher named Robert Riddle later chose the name “Sonic Hedgehog” for the mammalian version of the gene after recently seeing his daughter reading a Sonic comic.

Read more: Chicken Feet and Dinosaurs and  Dermestid beetles feast on dinosaur feathers