Genetic Basis for the Evolution of Vertebrate Mineralized Tissue

Many proteins critical to vertebrate tissue mineralization arose from a common ancestor by gene duplication, and these constitute the Secretory Calcium-binding Phospho-Protein (SCPP) family. A bird eggshell matrix protein, mammalian milk caseins, and salivary proteins are also members of this family. The last common ancestor of these genes was created by a large segmental duplication, or whole genome duplication, early in vertebrate evolution. The SCPP family subsequently arose principally by tandem gene duplication, thereby generating many physiologically similar but functionally specialized proteins. It is notable that the repertoire of SCPP genes is significantly different in tetrapods and teleost fish, reflecting independent parallel gene duplication histories. Thus, while mineralized tissues have remained as an important adaptive trait, the underlying genetic basis has drifted in these two major vertebrate clades. The evolution of the SCPP gene family illustrates the initial co-option of available raw materials used to make the mineralized skeletal system of vertebrates with subsequent tinkering.