The Origin and Early Evolution of SCPP Genes and Tissue Mineralization in Vertebrates

Abstract

Various secretory calcium-binding phosphoprotein (SCPP) genes are involved in the formation of the bone, dentin, enamel, and enameloid in bony vertebrates. By contrast, no SCPP gene is found in cartilaginous vertebrates. In order to explain this difference, I investigated the origin and early evolution of SCPP genes. First, I examined the phylogeny of SPARC-family genes that include evolutionary precursors of SCPP genes. Then, I analyzed the genomic arrangement of the SCPP genes and three SPARC-family genes, SPARCL1, SPARCL1L1, and SPARCR1. The results are consistent with our previous hypothesis that an SCPP gene-like structure arose in the 5′ half of SPARCL1L1 in a common ancestor of jawed vertebrates, at about the same time as the origin of mineralized skeleton. It is possible that carti-laginous vertebrates secondarily lost early SCPP genes, while bony vertebrates gained various new SCPP genes. Some of these new SCPP genes appear to have specifically involved in scale formation; however, these scale genes were lost in tetrapods.