Primate transcript and protein expression levels evolve under compensatory selection pressures

Variation in gene regulation is thought to have played an important role in the evolution of primates, and many studies have documented differences in mRNA expression levels across primate species. However, it is not yet known to what extent measurements of divergence in mRNA levels reflect divergence in protein expression levels, which are more directly tied to phenotypic differences. To address this question, we used high-resolution, quantitative mass spectrometry to collect thousands of protein expression measurements from human, chimpanzee, and rhesus macaque lymphoblastoid cell lines (LCLs). We also used RNA sequencing to collect transcript expression data from the same samples. Considering the two datasets jointly we found that there is much more inter-species divergence at the mRNA level than at the protein level. Remarkably, we found dozens of genes with significant expression differences between species at the mRNA level yet little or no difference in protein expression. Overall, our data suggest a much stronger evolutionary constraint on protein expression levels than on mRNA levels. We conclude that inter-species mRNA expression differences may often have limited functional consequences due to either buffering or compensatory changes in post-transcriptional or posttranslational regulation of proteins. Gene expression patterns were compared between human, chimpanzee, and rhesus macaque lymphoblastoid cell lines (5 individuals from each species) using RNA-Seq. These gene expression patterns were also compared to protein expression patterns based on mass-spec data, which are available separately (see publication for details).