Placentas and parturition: Adaptive molecular evolution and primate reproduction Derek Wildman1,2 1Center for Molecular Medicine and Genetics, Wayne State University School of Medicine, 2NIH Perinatology Research Branch/NICHD Abstract Our research group is conducting studies on the molecular evolution of labor and birth in primates and other mammals. Two recent studies from our laboratories are described: 1) The molecular evolution of primate progesterone receptors, and 2) the evolution of mammalian placental morphology. The steroid hormone progesterone is required for the maintenance of pregnancy. Withdrawal of progesterone ends pregnancy, but the mechanism for this withdrawal varies. We sequenced the gene encoding receptors of this hormone. In primates, during descent from the last common catarrhine ancestor, the progesterone receptor has evolved rapidly. This is shown by maximum likelihood nucleotide sequence analyses in which the rate of nonsynonymous substitution/nonsynonymous site exceeds the rate of synonymous substitution/synonymous site (i.e., dN/dS > 1). Catarrhine clades and lineages in which this rate increase is demonstrated (for all or for part of the gene) include the human-chimpanzee clade, the stem lineage of the Papionina [=Papionini], and the Colobini. These results suggest that progesterone receptor coding sequence changes had functional consequences in multiple primate lineages. Primate placental phenotypes include the epitheliochorial placental interface found in strepsirrhines and the hemochorial placental interface characteristic of haplorhines. Additional morphological features include the shape and form of placental maternofetal interdigitation. Using phylogenetic methods we demonstrated that the last common ancestor of extant primates had a hemochorial, discoid, and villous placenta. The epitheliochorial and diffuse placentas characteristic of stepsirrhine primates are shared derived features that evolved on the stepsirrhine stem lineage. The hemochorial type of placentation is the most invasive, and this implies that the contact between the maternal and fetal units has been intense throughout the descent of humans from a common primate ancestor.