Reptilian ancestors, sonic the hedgehog and genetic alchemy combine to manipulate genes

Manipulating a single gene pathway could turn scales into feathers in a surprising genetic discovery.

Swiss researchers toying with the expression of a single gene – dubbed Shh (for Sonic Hedgehog) –have found its influence on a specific signalling pathway between cells leads to the development of a range of structures within animals.

These include the neural tube, which is a precursor to the brain and spinal cord, limbs and skin appendages like hair, scales and feathers.

Professor Michel Milinkovitch from the University of Geneva led the study, which modified a chicken embryo to investigate how the Shh pathway worked to grow skin appendages.

Injecting an Shh-specific molecule into the developing embryo’s bloodstream, the researchers successfully triggered the growth of down feathers on the juvenile chicken’s skin.

This includes areas that would usually be scaled, such as the chicken’s feet.

“Our results indicate that an evolutionary leap – from scales to feathers – does not require large changes in genome composition or expression,” says Professor Michel Milinkovitch, who led the study.

“Instead, a transient change in expression of one gene, Shh, can produce a cascade of developmental events leading to the formation of feathers instead of scales.”

Bearded dragons, reptilian ancestors and a pair of mutants

Milinkovitch’s work stems from an unusual moment when he spotted a scaleless bearded dragon at a pet market.

Bearded dragons, best known for their spiked and scaled appearance, hail from Australia, but can be selectively bred to elicit unusual characteristics.

Scale-free dragons are the result of inheriting two mutated ectodysplasin-A (EDA) genes which diminish the species’ scales and spiny tubercles. A bearded dragon with one EDA mutation will see the prominence of these characteristics reduce, while ‘silkbacks’ inherit one mutation from each parent and lose them entirely.

Mutations to the gene are also associated with hair loss and deformation of teeth and nails in some mammals, including humans.

Delving into the formation of scale-expressing genes, Milinkovitch’s lab found reptile embryos form placodes, the same precursor structures found in haired and feathered animals.

These placodes are homologous across different species, including birds and mammals and likely derived from a common reptilian ancestor that lived 300 million years ago.

Milinkovitch’s lab suggests research into gene expression pathways like Shh will help grow understanding of the processes that lead to species differentiation and evolution.