Hairs, feathers and scales have a lot in common

Dr Nicolas Di-Poï and Professor Michel C. Milinkovitch demonstrate that hairs in mammals, feathers in birds and scales in reptiles share a common ancestry. On the basis of new analyses of embryonic development, the Finnish and Swiss biologists evidenced molecular and micro-anatomical signatures that are identical between hairs, feathers, and scales at their early developmental stages. These new observations indicate that the three structures evolved from their common reptilian ancestor.

Mammalian hairs and avian feathers develop from a similar primordial structure called a 'placode': a local thickening of the epidermis with columnar cells that reduce their rate of proliferation and express very specific genes.

This observation has puzzled evolutionary and developmental biologists for many years because birds and mammals are not sister groups: they evolved from different reptilian lineages. According to previous studies, reptiles' scales however do not develop from an anatomical placode. This would imply that birds and mammals have independently 'invented' placodes during their evolution.

Michel C. Milinkovitch and Nicolas Di-Poï, group leader at the Institute of Biotechnology put this long controversy to rest by demonstrating that scales in reptiles develop from a placode with all the anatomical and molecular signatures of avian and mammalian placodes.

This indicates that the three types of skin appendages are homologous: the reptilian scales, the avian feathers and the mammalian hairs, despite their very different final shapes, evolved from the scales of their reptilian common ancestor.

During their new study, the researchers also investigated a mutant form of the bearded dragon lizard that lacks all scales. By analyzing the genome of this mutant, Di-Poï et Milinkovitch have discovered that the peculiar look of these naked lizards is due to the disruption of the ectodysplasin-A (EDA), a gene whose mutations in humans and mice are known to generate substantial abnormalities in the development of teeth, glands, nails and hairs.

The researchers have demonstrated that, when EDA is malfunctioning in lizards, they fail to develop a proper scale placode, exactly as mammals or birds affected with similar mutations in that same gene cannot develop proper hairs or feathers placodes. These data all coherently indicate the common ancestry between scales, feathers and hairs.