Based on extensive analysis of molecular and micro-anatomical signatures, researchers have settled an age old controversy surrounding the origins of feathers, hair and scales.
Scientists have published a study in Science Advances wherein they have analyzed embryonic developed to conclude that says that all these three skin appendages are homologous i.e. they share a common ancestry. The evidence of the common ancestry was obtained through analysis of molecular and micro-anatomical signatures that are identical between hairs, feathers and scales at their early developmental stages.
For those of you who are not aware, 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 has been a puzzle that evolutionary and developmental biologists haven’t been able to find answer to as birds and mammals are not sister groups: they evolved from different reptilian lineages.
However, authors of the latest study found the answer to this puzzle and demonstrated that scales in reptiles develop from a placode with all the anatomical and molecular signatures of avian and mammalian placodes. The two scientists finely observed and analysed the skin morphological and molecular characteristics during embryonic development in crocodiles, snakes and lizards.
For their study researchers also investigated the bearded dragon, a species of lizard that comes in three variants. The first is the normal wild-type form. The second has scales of reduced size because it bears one copy of a natural genetic mutation. The third has two copies of the mutation … and lacks all scales. By comparing the genome of these three variants, Di-Poï and Milinkovitch have discovered the gene affected by this mutation.
‘We identified 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’, says Michel Milinkovitch.
The Swiss 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.