Fig. 6.

Schematic overview of hard keratin gene evolution. The presence of hard keratins in various vertebrate species was mapped onto a phylogenetic tree of vertebrates. The number of functional type I and type II keratins is shown on the right. The number of sauropsid-specific cysteine-rich type I keratins (aHAS1–4 and cHAS1–2) is indicated separately (+4 and + 2, respectively). Stars indicate gene innovation events and a “strike” symbol indicates the loss of the type I hair keratin-like gene(s) in the lineage leading to chicken.