Emergence of saliva protein genes in the secretory calcium-binding phosphoprotein (SCPP) locus and accelerated evolution in primates

Our new pre-print on the evolution of the SCPP gene family.

Genes within the secretory calcium-binding phosphoprotein (SCPP) family evolved in
conjunction with major evolutionary milestones: the formation of a calcified skeleton in
vertebrates, the emergence of tooth enamel in fish, and the introduction of lactation in
mammals. The SCPP gene family also contains genes expressed primarily and abundantly in
human saliva. Here, we explored the evolution of the saliva-related SCPP genes by harnessing
currently available genomic and transcriptomic resources. Our findings provide insights into the
expansion and diversification of SCPP genes, notably identifying previously undocumented
convergent gene duplications. In primate genomes, we found additional duplication and
diversification events that affected genes coding for proteins secreted in saliva. These
saliva-related SCPP genes exhibit signatures of positive selection in the primate lineage while
the other genes in the same locus remain conserved. We found that regulatory shifts and gene
turnover events facilitated the accelerated gain of salivary expression. Collectively, our results
position the SCPP gene family as a hotbed of evolutionary innovation, suggesting the potential
role of dietary and pathogenic pressures in the adaptive diversification of the saliva composition
in primates, including humans.

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