Placental genes have ancient origins. Evolutionary origins of preferentially expressed genes in the placenta (410 genes), decidua (347 genes), or in any tissue (3055 genes). 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). The placenta demonstrates significant enrichment for genes with eukaryotic orthology according to both Homologene (P < 1 × 10−19) and Ensembl (P < 1 × 10−29). The decidua demonstrates significant enrichment for genes with vertebrate orthology according to both Homologene (P < 0.05) and Ensembl (P < 0.01).
