Michael R. McGowen1, Offer Erez2, Roberto Romero3,4,5 and Derek E. Wildman*,1,3,4,6
1Center for Molecular Medicine and Genetics, Wayne State University School of Medicine, Detroit, MI, USA, 2Department of Obstetrics and Gynecology, Soroka University Medical Center, Ben-Gurion University of the Negev, Beer-Sheva, Israel, 3Perinatology Research Branch, Program for Perinatal Research and Obstetrics, Division of Intramural Research, Eunice Kennedy Shriver National Institute of Child Health and Human Development, NIH, Bethesda, MD and Detroit, MI USA, 4Department of Obstetrics and Gynecology, University of Michigan, Ann Arbor, MI, USA, 5Department of Epidemiology and Biostatistics, Michigan State University, East Lansing, MI, USA and 6Department of Obstetrics and Gynecology, Wayne State University School of Medicine, Detroit, MI, USA
Embryo implantation varies widely in placental mammals. We review this variation in mammals with a special focus on two features: the depth of implantation and embryonic diapause. We discuss the two major types of implantation depth, superficial and interstitial, and map this character on a well-resolved molecular phylogenetic tree of placental mammals. We infer that relatively deep interstitial implantation has independently evolved at least eight times within placental mammals. Moreover, the superficial type of implantation represents the ancestral state for placental mammals. In addition, we review the genes involved in various phases of implantation, and suggest a future direction in investigating the molecular evolution of implantation-related genes.