Evolutionary conflicts in pregnancy and calcium metabolism--a review

The maternal-fetal unit contains three distinct haplotypes at each locus: the maternally derived fetal haplotype (MDFH) that is shared by the mother and fetus, the paternally derived fetal haplotype (PDFH), and the non-inherited maternal haplotype (NIMH). The evolutionary forces acting on these haplotypes are distinct. The NIMH is absent from the offspring and could benefit from early abortion if this enhances the probability of the mother conceiving again and producing an offspring that inherits the NIMH. This raises the possibility that some forms of recurrent spontaneous abortion may be caused by non-inherited haplotypes. Such 'selfish' behaviour would be opposed by other components of the maternal genome. Natural selection acting on genes expressed in fetuses (or their placentae) favours greater maternal investment in the fetus than does natural selection acting on genes expressed in mothers. Furthermore, in the presence of genomic imprinting, the PDFH favours greater levels of investment in the fetus than does the MDFH. These conflicts are illustrated using the example of maternal-fetal conflicts over the supply of calcium. Inactivation of the paternal copy of GNAS in proximal renal tubule is interpreted as a measure to maintain fetal bone mineralization in times of calcium stress at the expense of the maternal skeleton.