Evolutionary differentiation of Cetartiodactyl placentae in the light of the viviparity-driven conflict hypothesis.

We analysed the evolution of placental traits in the novel mammalian clade Cetartiodactyla (Cetaceans and Artiodactyls) by a parsimony-based computer program (MacClade). A diffuse epitheliochorial placenta was identified as the stem species pattern of this clade. Trophoblast giant cells (TGCs) independently evolved in Camelids and Ruminants. The polycotyledonary placenta is an apomorphic character for Pecora (higher ruminants) and the oligocotyledonary placenta developed as a further step on the stem lineage of cervidae and moschidae. We interpret these findings by application of the "viviparity-driven conflict hypothesis", which states that divergent interests of mother and offspring lead to a rapid antagonistic coevolution, which might cause placental diversity. According to this hypothesis the evolution of camelid and ruminant TGCs can be interpreted as means to increase fetal endocrine influence on the maternal metabolism. The development of the cotyledonary placenta could be related to a diminished availability of glucose, which is associated with the evolution of forestomach fermentation in Pecora. An arms race, in which the mother tried to restrict and the fetus tried to increase transplacental glucose flow, might have promoted the evolution of the cotyledonary placenta, which has a high feto-maternal exchange area, but a low conductivity for glucose.