Hair & feathers evolved from reptile scales

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A normal bearded dragon, showing a full array of pointy scales.
Credit: Michel C. Milinkovitch 2016

What can naked lizards teach you? Plenty, it turns out, and it appears mammal hair, bird feathers and reptile scales all share common ancestry.

Biologists in Switzerland examined bearded dragons that lack scales, and found they have a specific genetic mutation that renders them scale-less. The same mutation causes feathers and hairs on birds and mammals to develop abnormally.

The work was published in Science Advances.

At the root of it all lies the “placode” – a thickening in an animal’s skin with specific cells that express very specific genes that give rise to feathers and hairs.

According to previous studies, reptiles’ scales don’t develop from placodes, so birds and mammals must have evolved the structure separately (called convergent evolution). To add weight to the argument, no fossilised remains have been found with any kind of intermediate forms of scales and hairs.

But Nicolas Di-Poï and Michel Milinkovitch from the University of Geneva had their doubts. So they examined the developing skin of snake, crocodile and lizard embryos and sure enough, found placodes – thickened patches of skin, with the right cells and genes switched on, just like in birds and mammals.

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Placodes (dark blue spots) are visible before the development of hair, scales and feathers in (from left to right) a mouse, snake, chicken and crocodile.
Credit: UNIGE 2016 / Tzika, Di-Poï, Milinkovitch

There are a couple of reasons they’d been undiscovered so long, Di-Poï and Milinkovitch write – most likely because they only appear for a short period during a reptile’s development, and seemingly sporadically across the body.

So where do the naked lizards come in?

Essentially, they served to show how a genetic mutation in reptiles can disrupt a placode, just as it does in birds and mammals.

The central bearded dragon (Pogona vitticeps), an Australian native lizard, is known for its fearsome display of spiny scales. But some individuals lack them entirely.

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From bottom to top: a normal bearded dragon, a heterozygous mutant (with only one copy of the mutated ectodysplasin-A gene) and a homozygous mutant (with two copies of the mutated ectodysplasin-A gene). The heterozygous mutant has smaller scales than the normal animal, whereas the homozygous mutant has no scales whatsoever.
Credit: Michel C. Milinkovitch 2016

By comparing the genome of naked vs scaled (vs partially scaled) bearded dragons, the pair discovered a gene – ectodysplasin-A – that stops scale development by disrupting placodes.

A bearded dragon carrying one form of the mutated gene had smaller scales than a “normal” one, whereas those with two mutated forms of the gene were completely smooth and scale-free.

The next challenge, Di-Poï and Milinkovitch say, is to untangle the diversity of feathers, hair and scales. Knowing they stem from a common ancestor is just the start.

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