A signature of evolutionary constraint on a subset of ectopically expressed olfactory receptor genes

Abstract

Olfactory receptor (OR) genes constitute the basis for the sense of smell. It has long been observed that a subset of mammalian OR genes are expressed in nonolfactory tissues, in addition to their expression in the olfactory epithelium. However, it is unknown whether OR genes have alternative functions in the nonolfactory tissues. Using a dedicated microarray, we surveyed OR gene expression in olfactory epithelium as well as a number of nonolfactory tissues, in human and chimpanzee. Our observations suggest that ectopically expressed OR orthologous genes are expressed in the same nonolfactory tissues in human and chimpanzee more often than expected by chance alone. Moreover, we found that the subset of orthologous OR genes with conserved ectopic expression evolve under stronger evolutionary constraint than OR genes expressed exclusively in the olfactory epithelium. Thus, although we cannot provide direct functional data, our observations are consistent with the notion that a subset of ectopically expressed OR genes have additional functions in nonolfactory tissues.

FIG. 1.—

Cumulative distributions of d_N/d_S values (x-axis) of ectopically expressed OR genes with conserved expression pattern (blue), OR genes that are expressed in at least one nonolfactory tissues, but do not have a conserved expression pattern (red), and OR genes that are expressed exclusively in olfactory epitheliums (green). The smaller panel shows the d_N/d_S medians in the three groups. The error bars represent 95% CIs calculated using bootstrapping (1,000 repetitions).