title: The evolution of pregnancy.
abstract: Although viviparity has evolved many times in the animal kingdom, it remains relatively uncommon-scorpions and therian mammals being rare examples of entirely viviparous major taxa. Viviparity is a specialised form of intra-species parasitism which biases parental investment towards fertilised eggs, temporally spreads that investment, and also temporarily protects offspring from many selection pressures. Importantly, the mammalian viviparity appeared at a relatively late stage in the process of vertebrate evolution. Because of this, viviparity was 'superimposed' on complex pre-existing cardiovascular, respiratory, metabolic and immune systems, and has altered them dramatically. Also, pregnancy has exerted pervasive effects on gene expression in mammals, including genetic imprinting, X inactivation, sex determination, and the ectopic expression in the extra-embryonic membranes of many genes previously expressed in the gonads, brain, pituitary and immune system. Finally, although lactation probably pre-dated viviparity in mammalian evolution, the two have co-evolved as alternative strategies of offspring nutrition ever since.
authors.affiliation: Veterinary Anatomy Programme, Department of Physiology, Development and Neuroscience, Cambridge University, Downing Street CB2 3DY, UK. Electronic address: db125@cam.ac.uk.
authors.forename: David R J
authors.initials: DR
authors.lastname: Bainbridge
mesh_terms: D000818:Animals; D019143:Evolution, Molecular; D005260:Female; D006801:Humans; D011247:Pregnancy / Q000235:genetics* / Q000502:physiology; D052286:Viviparity, Nonmammalian / Q000235:genetics / Q000502:physiology
keywords: Evolution; Lactation; Mammals; Non-mammals; Pregnancy; Viviparity