PLACENTAL HORMONES, GENOMIC IMPRINTING, AND MATERNAL-FETAL COMMUNICATION

Placental hormones are produced by one genetic individual (the fetus) to act on the receptors of another genetic individual (the mother). Mo thers are probably able to extract some information from placental hor mones, but this information may be limited to a crude measure of fetal vigor. Placental hormones are most easily interpreted as fetal attemp ts to manipulate maternal metabolism for fetal benefit. An evolutionar y model is presented for a hypothetical hormone that increases the nut rient content of maternal blood. The model predicts that, at an evolut ionary equilibrium, the hormone will be produced solely by the mother or solely by the placenta, but not by both. If the gene for the hormon e is subject to genomic imprinting, the paternally-derived allele will be active and the maternally-derived allele will be silent. Hormone p roduction benefits the members of the mother's current litter at some cost to future litters. Therefore, paternity changes between litters i ncrease the level of hormone production. On the other hand, offspring that produce less of the hormone than litter-mates share the benefits but have lower costs. Therefore, multiple paternity within litters red uces the level of hormone production.