Placenta-enrichment by duplication age in three therian species: human, mouse, and opossum. Significant global χ2 tests indicate differences in the number of placenta-specific expressed genes by age group in each of the studied species (placenta-specific expressed genes are defined as having more than twice uniform expression level in the placenta), after which a post hoc procedure was applied, as described in Methods. (+) Significant overrepresentation of placenta-specific expressed genes, (–) significant underrepresentation of placenta-specific expressed genes, (NS) not different from expectation. Overall, results obtained with the alternative definition of tissue specificity (tau) were the same, with the exception of mouse and human, where lineage-specific paralogs showed more placenta-specific expressed genes than expected.
