Pregnancy is generally a co-operative interaction between mother and fetus in which the evolutionary genetic interests of both benefit from production of healthy offspring. While this view is largely supported by empirical data, Kinship Theory predicts that mother and fetus will disagree over the optimum level of maternal investment that maximises their respective fitnesses. This conflict will be more evident with polyandrous than monogamous mating systems, when resources are scarce and in late gestation when the fetus is growing maximally, particularly if conceptus mass is large relative to maternal mass. As the site of nutrient transfer, the placenta is pivotal in the tug-of-war between mother and fetus over resource allocation. It responds to both fetal signals of nutrient demand and maternal signals of nutrient availability and, by adapting its phenotype, regulates the distribution of available resources. These adaptations involve changes in placental size, morphology, transport characteristics, metabolism and hormone bioavailability. They are mediated by key growth regulatory, endocrine and nutrient supply genes responsive to mismatches between nutrient availability and the fetal genetic drive for growth. Indeed, evolution of genomic imprinting and placental secretion of hormones are believed to have been driven by maternal-fetal conflict over resource allocation. Although many of the specific mechanisms involved still have to be identified, the placenta confers optimal fitness on the offspring for its developmental environment by balancing conflict and cooperation in the allocation of resources through generation of nutrient transport phenotypes specific to the prevailing nutritional conditions and/or fetal genotype.
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