Genes representing the mature phase reveal recent evolution by duplication and divergence. Evolutionary origins of placental preferentially expressed genes during the developing (329 genes) and mature (70 genes) phases of placentation. The evolutionary origin of each gene was determined by finding the most distantly related species that has a one-to-one, reciprocal best-match ortholog identified in the Homologene database (A) or the Ensembl database (B). Asterisks (*) indicate significant enrichment over background (All Tissues; 3055 genes). The developing placenta is significantly enriched for genes with eukaryotic orthology according to both Homologene (P < 1.0 × 10−27) and Ensembl (P < 1.0 × 10−35). In contrast, the mature placenta is significantly enriched for genes with rodent orthology according to both Homologene (P < 1.0 × 10−7) and Ensembl (P < 1.0 × 10−11).
