Abstract

Evolution of mammalian reproductive success has witnessed a strong dependence on maternal energetics through placental in-utero development, the provisioning of post-natal milk and maternal care. The co-existence of three matrilineal generations as one (mother, offspring and post-meiotic oocytes) have provided a maternal niche for transgenerational co-adaptive selection pressure to operate. In-utero foetal growth has required increased maternal feeding in advance of foetal energetic demands; the mammary glands are primed for milk production in advance of birth, while the maternal hypothalamus is hormonally primed by the foetal placenta for nest building and post-natal care. Such forward planning resulted from mother-infant co-adaptation facilitated by co-expression of genes under matrilineal control in the developing hypothalamus and placenta. This foetal co-expression is concurrent with the placenta interacting with the maternal hypothalamus thereby providing a transgenerational template on which selection pressures can operate ensuring optimal maternalism in the next generation. Pivotal to these mammalian evolutionary developments, genomic imprinting emerged as a gene dosage regulatory mechanism, thought to have co-evolved with placentation, and providing genetic stability while increasing heritable epigenetic variance and phenotypic heterochrony.