A test of the neutral model of expression change in natural populations of house mouse subspecies.

Changes in expression of genes are thought to contribute significantly to evolutionary divergence. To study the relative role of selection and neutrality in shaping expression changes, we analyzed 24 genes in three different tissues of the house mouse (Mus musculus). Samples from two natural populations of the subspecies M. m. domesticus and M. m. musculus were investigated using quantitative PCR assays and sequencing of the upstream region. We have developed an approach to quantify expression polymorphism within such populations and to disentangle technical from biological variation in the data. We found a correlation between expression polymorphism within populations and divergence between populations. Furthermore, we found a correlation between expression polymorphism and sequence polymorphism of the respective genes. These data are most easily interpreted within a framework of a predominantly neutral model of gene expression change, where only a fraction of the changes may have been driven by positive selection. Although most genes investigated were expressed in all three tissues analyzed, significant changes of expression levels occurred predominantly in a single tissue only. This adds to the notion that enhancer-specific effects or transregulatory effects can modulate the evolution of gene expression in a tissue-specific way.