Gene duplication is a fundamental process in genome evolution. However, most young duplicates are degraded into pseudogenes by loss-of-function mutations, and the factors that allow some duplicate pairs to survive long-term remain controversial. One class of models to explain duplicate retention invokes sub- or neofunctionalization, especially through evolution of gene expression, while other models focus on sharing of gene dosage. While studies of whole genome duplications tend to support dosage sharing, the primary mechanisms in mammals-where duplications are small-scale and thus disrupt dosage balance-are unclear. Using RNA-seq data from 46 human and 26 mouse tissues we find that subfunctionalization of expression evolves slowly, and is rare among duplicates that arose within the placental mammals. A major impediment to subfunctionalization is that tandem duplicates tend to be co-regulated by shared genomic elements, in contrast to the standard assumption of modularity of gene expression. Instead, consistent with the dosage-sharing hypothesis, most young duplicates are down-regulated to match expression of outgroup singleton genes. Our data suggest that dosage sharing of expression is a key factor in the initial survival of mammalian duplicates, followed by slower functional adaptation enabling long-term preservation.
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