The Expression of Beta (β) Keratins in the Epidermal Appendages of Reptiles and Birds1

Abstract

The integuments of extant vertebrates display a variety of epidermal appendages whose patterns, morphology and terminal differentiation (epidermal keratins) depend upon interactions between ectodermal (epidermis) and mesodermal (dermis) tissues. In reptiles and birds, appendage morphogenesis precedes terminal differentiation. Studies have demonstrated that appendage morphogenesis influences the expression of the appendage specific keratin genes. However, little is known about the nature of the structural genes expressed by the epidermal appendages of reptiles. How pattern formation and/or appendage morphogenesis influence terminal differentiation of reptilian appendages is not known.The epidermal appendages of reptiles and birds are characterized by the presence of both alpha (α) and beta (β) type keratin proteins. Studies have focused on the genes of avian β keratins because they are the major structural proteins of feathers. The occurrence of β keratin proteins in the scales and claws of both birds and reptiles and their immunological cross-reactivity suggest that the genes for reptilian β keratins may be homologous with those of birds. In bird appendages, the β keratins are the products of a large family of homologous genes. Specific members of this gene family are expressed during the development of each appendage. Recent sequence analyses of feather β keratins, from different orders of birds, demonstrate that there is more diversity at the DNA level than was implied by earlier protein sequencing studies.Immunological techniques show that the same antibodies that react with the epidermal β keratins of the chicken (Gallus domesticus) react with the epidermal β keratins of American alligators (Alligator mississippiensis). Furthermore, a peptide sequence (20 amino acids) from an alligator claw β keratin is similar to a highly conserved region of avian claw, scale, feather, and feather-like β keratins. These observations suggest that the β keratin genes of avian epidermal appendages have homologues in the American alligator. Understanding the origin and evolution of the β keratin gene families in reptiles and birds will undoubtedly add to our understanding of the evolution of skin appendages such as scales and feathers.