Heterogeneous patterns of gene-expression diversification in mammalian gene duplicates

Gene duplication is a major mechanism for molecular evolutionary innovation. Young gene duplicates typically exhibit elevated rates of protein evolution and, according to a number of recent studies, increased expression divergence. However, the nature of these changes is still poorly understood. To gain novel insights into the functional consequences of gene duplication, we have undertaken an in-depth analysis of a large data set of gene families containing primate- and/or rodent-specific gene duplicates. We have found a clear tendency toward an increase in protein, promoter, and expression divergence with increasing number of duplication events undergone by each gene since the human-mouse split. In addition, gene duplication is significantly associated with a reduction in expression breadth and intensity. Interestingly, it is possible to identify three main groups regarding the evolution of gene expression following gene duplication. The first group, which comprises around 25% of the families, shows patterns compatible with tissue-expression partitioning. The second and largest group, comprising 33-53% of the families, shows broad expression of one of the gene copies and reduced, overlapping, expression of the other copy or copies. This can be attributed, in most cases, to loss of expression in several tissues of one or more gene copies. Finally, a substantial number of families, 19-35%, maintain a very high level of tissue-expression overlap (>0.8) after tens of millions of years of evolution. These families may have been subject to selection for increased gene dosage.