The expression of beta (beta) keratins in the epidermal appendages of reptiles and birds

The integuments of extant vertebrates display a variety of epidermal append ages whose patterns, morphology and terminal differentiation (epidermal ker atins) depend upon interactions between ectodermal (epidermis) and mesoderm al (dermis) tissues, In reptiles and birds, appendage morphogenesis precede s terminal differentiation. Studies have demonstrated that appendage morpho genesis 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 appe ndage morphogenesis influence terminal differentiation of reptilian appenda ges is not known. The epidermal appendages of reptiles and birds are characterized by the pre sence of both alpha (alpha) and beta (beta) type keratin proteins, Studies have focused on the genes of avian beta keratins because they are the major structural proteins of feathers. The occurrence of beta keratin proteins i n the scales and claws of both birds and reptiles and their immunological c ross-reactivity suggest that the genes for reptilian beta keratins may be h omologous with those of birds, In bird appendages, the beta keratins are th e 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 beta 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 beta keratins of the chicken (Gallus domesticus) react with the e pidermal beta keratins of American alligators (Alligator mississippiensis), Furthermore, a peptide sequence (20 amino acids) from an alligator claw be ta keratin is similar to a highly conserved region of avian claw scale, fea ther, and feather-like beta keratins. These observations suggest that the b eta keratin genes of avian epidermal appendages have homologues in the Amer ican alligator, Understanding the origin and evolution of the beta keratin gene families in reptiles and birds will undoubtedly add to our understandi ng of the evolution of skin appendages such as scales and feathers.