Salivary Gene Evolution within the Secretory Calcium-Binding Phosphoprotein Gene Family

Objectives: Previously, we had investigated the evolution of salivary mucin MUC7 in the mammalian phylogeny, and found that the gene for MUC7 evolved rapidly in the primate lineage. MUC7 is a member of the secretory calcium-binding phosphoprotein (SCPP) family of genes. This locus harbors 16 genes in humans encoding for various proteins related to bone, teeth, milk, and saliva. These genes have likely evolved through a series of gene duplications throughout vertebrate evolution. However, the evolutionary dynamics that shaped this locus in mammals are mostly unknown.
Methods: We bioinformatically documented gene loss and gain events in the SCPP locus among the genomes of humans and 48 other mammalian species, 4 birds, 4 reptiles, and 6 fish species. In parallel, we searched for the evolutionary origins of human SCPP genes and conducted evolutionary genetics analysis to test if these genes are evolving under adaptive pressures in different mammalian lineages.
Results: Our findings indicate that the SCCP locus has been adaptively evolving among mammals through the accumulation of nonsynonymous mutations as well as through gene gain and loss events. We could document at least three rapid gene duplication and diversification events coinciding with evolution of calcium transport in bony fish, milk production in placental mammals, and expansion of salivary proteins in primates.
Conclusions: Our results show that in the primate phylogeny, salivary genes evolved at a more rapid pace than genes related to bone, teeth, or milk, suggesting that dietary diversification in primates, including humans, may have driven evolutionary adaptation of their saliva.